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A 

TEXT-BOOK  OF  HUMAN   PHYSIOLOGY, 


BRUBAKER 


TEXT-BOOK 


OF 


HUMAN  PHYSIOLOGY 


INCLUDING  A  SECTION  ON 


PHYSIOLOGIC  APPARATUS. 


BY 
ALBERT  P.  BRUBAKER,  A.M.,  M.D., 

PROFESSOR   OF   PHYSIOLOGY   AND   HYGIENE   IN   THE  JEFFERSON   MEDICAL   COLLEGE;   PROFESSOR 

OF    PHYSIOLOGY    IN    THE    PENNSYLVANIA    COLLEGE    OF    DENTAL    SURGERY; 

LECTURER   ON   PHYSIOLOGY   AND  HYGIENE  IN   THE   DREXEL 

INSTITUTE   OF   ART,    SCIENCE,    AND   INDUSTRY. 


THIRD  EDITION.     REVISED  AND  ENLARGED. 
WITH    COLORED    PLATES  AND  383  ILLUSTRATIONS. 


PHILADELPHIA: 

P.  BLAKISTON'S  SON  &  CO., 

1012  WALNUT  STREET. 
1908. 


Copyright,  1908,  by  P.  Blakiston's  Son  &  Co. 


Printed  by 

The  Maple  Press 

York  Pa. 


TO 

HENRY  C.  CHAPMAN,  M.D., 

PROFESSOR    OF   INSTITUTES    OF    MEDICINE    AND    MEDICAL   JURISPRUDENCE    IN 
THE  JEFFERSON   MEDICAL   COLLEGE. 

IN  GRATEFUL  RECOGNITION  OF  THE  MANY  KINDNESSES  RECEIVED  FROM 

HIM,  DURING  A  PERIOD  OF  TWENTY-FIVE  YEARS,  THIS 

VOLUME  IS  RESPECTFULLY  DEDICATED  BY 

THE  AUTHOR. 


PREFACE  TO  THIRD  EDITION 


The  publishers  having  requested  the  preparation  of  a  Third  Edition 
of  the  Text-Book  of  Physiology,  an  attempt  has  been  made  to  still 
further  increase  its  value  to  those  for  •whom  it  was  primarily  intended 
by  subjecting  the  whole  text  to  a  careful  revision,  by  the  insertion  of 
a  number  of  new  diagrams  and  by  the  incorporation  of  new  material. 
In  this  attempt  the  author  has  been  assisted  by  the  suggestions  con- 
tained in  reviews  and  in  letters  received  from  teachers  and  students. 
Altogether  some  fifty  additional  pages  have  been  added  to  the  body 
of  the  text.  This  material  will  be  found  in  the  chapters  on  the 
chemistry  of  the  proteids,  the  physiology  of  muscle  tissue,  absorption, 
the  physiology  of  the  heart  and  vascular  apparatus,  the  nerve  system 
and  vision. 

Once  again  I  wish  to  express  my  sincere  thanks  to  those  teachers 
and  students  who  have  used  and  recommended  this  book  and  for  the 
generous  praise  they  have  bestowed  on  it  verbally  and  by  letter  and 
trust  that  in  its  present  form  it  will  still  further  meet  their  approval. 

I  am  indebted  to  Dr.  George  Bachmann  for  the  new  diagrams  in 
the  text  to  which  his  name  is  appended. 

To  Mr.  L  A.  Hagy  my  thanks  are  due  for  invaluable  assistance  in 
seeing  the  work  through  the  press. 

To  Messrs.  P.  Blakiston's  Son  &  Co.  I  am  greatly  indebted  for 
their  encouragement  and  generosity  in  the  promotion  of  every  thing 
that  pertains  to  the  material  value  of  this  work. 

A.  P.  B. 


vii 


PREFACE 


The  object  in  view  in  the  preparation  of  this  volume  was  the  selec- 
tion and  presentation  of  the  more  important  facts  of  physiology,  in  a 
form  which  it  is  believed  will  be  helpful  to  students  and  to  practi- 
tioners of  medicine.  Inasmuch  as  the  majority  of  students  in  a 
medical  college  are  preparing  for  the  practical  duties  of  professional 
life,  such  facts  have  been  selected  as  will  not  only  elucidate  the 
normal  functions  of  the  tissues  and  organs  of  the  body,  but  which 
will  be  of  assistance  in  understanding  their  abnormal  manifestations 
as  they  present  themselves  in  hospital  and  private  work.  Both  in 
the  selection  of  facts  and  in  the  method  of  presentation  the  author 
has  been  guided  by  an  experience  gained  during  twenty  years  of 
active  teaching. 

The  description  of  physiologic  apparatus  and  the  methods  of 
investigation,  other  than  those  having  a  clinical  interest,  have  been 
largely  excluded  from  the  text,  for  the  reason  that  both  are  more 
appropriately  considered  in  works  devoted  to  laboratory  methods 
and  laboratory  instruction,  and  for  the  further  reason  that  the  student 
receives  this  information  while  engaged  in  the  practical  study  of 
physiology  in  the  laboratory,  now  an  established  feature  in  the 
curriculum  of  the  majority  of  medical  colleges.  For  those  who  have 
not  had  laboratory  opportunities  a  brief  account  of  some  essential 
forms  of  apparatus  and  the  purposes  for  which  they  are  intended  will 
be  found  in  an  appendix. 

I  wish  to  acknowledge  my  indebtedness  to  Professor  Colin  C. 
Stewart  for  many  valuable  suggestions  in  the  preparation  of  different 
sections  of  the  volume;  to  Dr.  Carl  Weiland  for  assistance  in  the 
chapter  on  vision;  to  Dr.  Joseph  P.  Bolton  for  excellent  suggestions 
on  questions  relating  to  physiologic  chemistry. 


TABLE  OF  CONTENTS. 


CHAPTER  I.  page 

Introduction i 

CHAPTER  II. 
Chemic  Composition  of  the  Human   Body 7 

CHAPTER  III. 
Physiology  of  the  Cell „ 26 

CHAPTER  IV.    . 
Histology  of  the  Epithelial  and  Connective  Tissues 33 

CHAPTER  V. 
The  Physiology  of  Movement 43 

CHAPTER  VI. 
The  Physiology  of  the  Skeleton 48 

CHAPTER  VII. 
General  Physiology  of  Muscle-tissue 53 

CHAPTER  VIII. 
The  General  Physiology  of  Nerve-tissue 96 

CHAPTER  IX. 
Foods 127 

CHAPTER  X. 
Digestion 145 

CHAPTER  XI. 
Absorption 213 

CHAPTER  XII. 
The  Blood ' 236 

CHAPTER  XIII. 
The  Circulation  of  the  Blood 271 

CHAPTER  XIV. 
The  Circulation  of  the  Blood  (Continued)   326 

CHAPTER  XV. 
Respiration 3S2 

CHAPTER  XVI. 

Animal  Heat 436 

xi 


xii  TABLE  OF  CONTENTS. 

CHAPTER  XVII.  page 

Secretion 446 

CHAPTER  XVIII. 
Excretion 472 

CHAPTER  XIX. 
The  Central  Organs  of  the  Nerve  System  and  their  Nerves 491 

CHAPTER  XX. 

The   Medulla   Oblongata;  the   Isthmus   of  the  Encephalon;  the  Basal 

Ganglia 519 

CHAPTER  XXI. 
The  Cerebrum  537 

CHAPTER  XXII. 
The  Cerebellum  569 

CHAPTER  XXIII. 
The  Cranial  Nerves 577 

CHAPTER  XXIV. 
The  Sympathetic  Nerve  System 616 

CHAPTER  XXV. 
Phonation;  Articulate  Speech 626 

CHAPTER  XXVI. 
The  Special  Senses 638 

CHAPTER  XXVII. 
The  Sense  of  Sight 650 

CHAPTER  XXVIII. 
The  Sense  of  Hearing   689 

CHAPTER  XXIX. 
Reproduction 701 

APPENDIX. 
Physiologic  Apparatus   721 


Index 745 


TEXT-BOOK  OF  PHYSIOLOGY. 


CHAPTER  I. 
INTRODUCTION. 


An  animal  organism  in  the  living  condition  exhibits  a  series  of 
phenomena  which  relate  to  growth,  movement,  mentality,  and  re- 
production. During  the  period  preceding  birth,  as  well  as  during 
the  period  included  between  birth  and  adult  life,  the  individual  grows 
in  size  and  complexity  from  the  introduction  and  assimilation  of 
material  from  without.  Throughout  its  life  the  animal  exhibits 
a  series  of  movements,  in  virtue  of  which  it  not  only  changes  the  relation 
of  one  part  of  its  body  to  another,  but  also  changes  its  position  relatively 
to  its  environment.  If,  in  the  execution  of  these  movements,  the  parts 
are  directed  to  the  overcoming  of  opposing  forces,  such  as  gravity, 
friction,  cohesion,  elasticity,  etc.,  the  animal  may  be  said  to  be  doing 
work.  The  result  of  normal  growth  is  the  attainment  of  a  physical 
development  that  will  enable  the  animal,  and,  more  especially,  man, 
to  perform  the  work  necessitated  by  the  nature  of  its  environment  and 
the  character  of  its  organization.  In  man,  and  probably  in  lower 
animals  as  well,  mentality  manifests  itself  as  intellect,  feeling,  and 
volition.  At  a  definite  period  in  the  life  of  the  animal  it  reproduces 
itself,  in  consequence  of  which  the  species  to  which  it  belongs  is  per- 
petuated. 

The  study  of  the  phenomena  of  growth,  movement,  mentality, 
and  reproduction  constitutes  the  science  of  animal  physiology. 
But  as  these  general  activities  are  the  resultant  of  and  dependent 
on  the  special  activities  of  the  individual  structures  of  which  an  animal 
body  is  composed,  physiology  in  its  more  restricted  and  generally 
accepted  sense  is  the  science  which  investigates  the  actions  or  functions 
of  the  individual  organs  and  tissues  of  the  body  and  the  physical  and 
chemic  conditions  which  underlie  and  determine  them. 

This  may  naturally  be  divided  into: 
i.    Special    physiology,   the   object    of  which  is  a  study  of  the  vital 

phenomena  or  functions  exhibited  by  the  organs  of  any  individual 

animal. 


2  TEXT-BOOK  OF  PHYSIOLOGY. 

2.  Comparative  physiology,  the  object  of  which  is  a  comparison  of 
the  vital  phenomena  or  functions  exhibited  by  the  organs  of 
two  or  more  animals  of  different  species,  with  a  view  to  unfold- 
ing their  points  of  resemblance  or  dissimilarity. 

Human  physiology  is  that  department  of  physiologic  science 
which  has  for  its  object  the  study  of  the  functions  of  the  organs  of 
the  human  body  in  a  state  of  health. 

Inasmuch  as  the  study  of  function,  or  physiology,  is  associated 
with  and  dependent  on  a  knowledge  of  structure,  or  anatomy,  it  is 
essential  that  the  student  should  have  a  general  acquaintance  not 
only  with  the  structure  of  man,  but  with  that  of  typical  forms  of  lower 
animal  life  as  well. 

If  the  body  of  any  animal  be  dissected,  it  will  be  found  to  be  com- 
posed of  a  number  of  well-defined  structures,  such  as  heart,  lungs, 
stomach,  brain,  eye,  etc.,  to  which  the  term  organ  was  originally  ap- 
plied, for  the  reason  that  they  were  supposed  to  be  instruments  capable 
of  performing  some  important  act  or  function  in  the  general  activities 
of  the  body.  Though  the  term  organ  is  usually  employed  to  designate 
the  larger  and  more  familiar  structures  just  mentioned,  it  is  equally 
applicable  to  a  large  number  of  other  structures  which,  though  possibly 
less  obvious,  are  equally  important  in  maintaining  the  life  of  the  in- 
dividual— e.  g.,  bones,  muscles,  nerves,  skin,  teeth,  glands,  blood- 
vessels, etc.  Indeed,  any  complexly  organized  structure  capable  of 
performing  a  given  function  may  be  described  as  an  organ.  A  descrip- 
tion of  the  various  organs  which  make  up  the  body  of  an  animal,  their 
external  form,  their  internal  arrangement,  their  relations  to  one  another, 
constitutes  the  science  of  animal  anatomy. 

This  may  naturally  be  divided  into : 
i.    Special  anatomy,   the  object  of  which  is  the  investigation  of  the 
construction,  form,  and    arrangement  of   the   organs  of   any  in- 
dividual animal. 
2.    Comparative  anatomy,  the  object  of  which  is  a  comparison  of  the 
organs   of  two   or  more   animals  of  different  species,  with  a  view 
to  determining  their  points  of  resemblance  or  dissimilarity. 
If  the  organs,  however,  are  subjected  to  a  further  analysis,  they 
can  be  resolved  into  simple  structures,  apparently  homogeneous,  to 
which  the  name  tissue  has  been  given — e.  g.,  epithelial,  connective, 
muscle,  and  nerve  tissue.     When  the  tissues  are  subjected  to  a  micro- 
scopic analysis,  it  is  found  that  they  are  not  homogeneous  in  structure, 
but  composed  of  still  simpler  elements,  termed  cells  and  fibers.     The 
investigation  of  the  internal  structure  of  the  organs,  the  physical  prop- 
erties and  structure  of  the  tissues,  as  well  as  the  structure  of  their 
component  elements,  the  cells  and  fibers,  constitutes  a  department  of 
anatomic  science  known  as  histology,  or  as  it  is  prosecuted  largely 
with  the  microscope,  microscopic  anatomy. 


INTRODUCTION.  3 

Human  anatomy  is  that  department  of  anatomic  science  which 
has  for  its  object  the  investigation  of  the  construction  of  the  human 
body. 

GENERAL  STRUCTURE  OF  THE  ANIMAL  BODY. 

The  body  of  every  animal,  from  fish  to  man,  may  be  divided  into — ■ 

1.  An  axial  portion,  consisting  of  the  head,  neck,  and  trunk;  and — 

2.  An  appendicular  portion,  consisting  of  the  anterior  and  posterior 

limbs  or  extremities. 

The  axial  portion  of  all  mammals,  to  which  class  man  zoologic- 
ally belongs,  as  well  as  of  all  birds,  reptiles,  amphibians,  and  osseous 
fish,  is  characterized  by  the  presence  of  a  bony,  segmented  axis,  which 
extends  in  a  longitudinal  direction  from  before  backward,  and  which 
is  known  as  the  vertebral  column  or  backbone.  In  virtue  of  the  ex- 
istence of  this  column  all  the  classes  of  animals  just  mentioned  form 
one  great  division  of  the  animal  kingdom,  the  Vertebraia. 

Each  segment,  or  vertebra,  of  this  axis  consists  of — 

1.  A  solid  portion,  known  as  the  body  or  centrum,  and 

2.  A  bony  arch  arising  from  the  dorsal  aspect  and  surmounted  by  a 

spine-like  process. 

At  the  anterior  extremity  of  the  body  of  the  animal  the  vertebra; 
are  variously  modified  and  expanded,  and,  with  the  addition  of  new 
elements,  form  the  skull;  at  the  posterior  extremity  they  rapidly  di- 
minish in  size,  and  terminate  in  man  in  a  short,  tail-like  process.  In 
many  animals,  however,  the  vertebral  column  extends  for  a  consider- 
able distance  beyond  the  trunk  into  the  tail.  The  vertebral  column 
may  be  regarded  as  the  foundation  element  in  the  plan  of  organization 
of  all  the  higher  animals  and  the  center  around  which  the  rest  of  the 
body  is  developed  and  arranged  with  a  certain  degree  of  conformity. 
In  all  vertebrate  animals  the  bodies  of  the  segments  of  the  vertebral 
column  form  a  partition  which  serves  to  divide  the  trunk  of  the  body 
into  two  cavities — viz.,  the  dorsal  and  the  ventral.     (See  Fig.  1.) 

The  dorsal  cavity  is  found  not  only  in  the  trunk,  but  also  in  the 
head.  Its  walls  are  formed  partly  by  the  arches  which  arise  from  the 
posterior  or  dorsal  surface  of  the  vertebras  and  partly  by  the  bones  of 
the  skull.  If  a  longitudinal  section  be  made  through  the  center  of  the 
vertebral  column,  and  including  the  head,  the  dorsal  cavity  will  be 
observed  running  through  its  entire  extent.  Though  for  the  most 
part  it  is  quite  narrow,  at  the  anterior  extremity  it  is  enlarged  and 
forms  the  cavity  of  the  skull.  This  cavity  is  lined  by  a  membranous 
canal,  the  neural  canal,  in  which  are  contained  the  brain  and  the  neural 
or  spinal  cord.  Through  openings  in  the  sides  of  the  dorsal  cavitv 
nerves  pass  out  which  connect  the  brain  and  spinal  cord  with  all  the 
structures  of  the  body. 

The  ventral  cavity  is  confined  mainly  to  the  trunk  of  the  body. 
Its  walls  are  formed  bv  muscles  and  skin,  strengthened  in  most  animals 


TEXT-BOOK  OF  PHYSIOLOGY. 


by  bony  arches,  the  ribs.  Within  the  ventral  cavity  is  contained  a 
musculo-membranous  tube  or  canal  known  as  the  alimentary  or  food 
canal,  which  begins  at  the  mouth  on  the  ventral  side  of  the  head,  and, 

after  passing  through  the  neck  and 
trunk,  terminates  at  the  posterior  ex- 
tremity of  the  trunk  at  the  anus.  It 
may  be  divided  into  mouth,  pharynx, 
esophagus,  stomach,  small  and  large 
intestines. 

In  all  mammals  the  ventral  cavity 
is  divided  by  a  musculo-membranous 
partition  into  two  smaller  cavities,  the 
thorax  and  abdomen.  The  former 
contains  the  lungs,  heart  and  its  great 
blood-vessels,  and  the  anterior  part  of 
the  alimentary  canal,  the  gullet  or 
esophagus;  the  latter  contains  the 
continuation  of  the  alimentary  canal 
— that  is,  the  stomach  and  intestines 
— and  the  glands  in  connection  with 
it,  the  liver  and  pancreas.  In  the 
posterior  portion  of  the  abdominal 
cavity  are  found  the  kidneys,  ureters, 
and  bladder,  and  in  the  female  the 
organs  of  reproduction.  The  thoracic 
and  abdominal  cavities  are  each  lined 
by  a  thin  serous  membrane,  known, 
respectively,  as  the  pleural  and  perit- 
oneal membranes,  which,  in  addition, 
are  reflected  over  the  surfaces  of  the 
organs  contained  within  them.  The 
alimentary  canal  and  the  various  cav- 
ities connected  with  it  are  lined 
throughout  by  a  mucous  membrane. 

The  surface  of  the  body  is 
covered  by  the  skin.  This  is  com- 
posed of  an  inner  portion,  the  derma, 
and  an  outer  portion,  the  epidermis. 
The  former  consists  of  fibers,  blood- 
vessels, nerves,  etc.;  the  latter  of  layers 
of  scales  or  cells.  Embedded  within 
the  skin  are  numbers  of  glands;  which 
exude,  in  the  different  classes  of  ani- 
mals, sweat,  oily  matter,  etc.  Project- 
ing from  the  surface  of  the  skin  are 
Beneath  the  skin  are  found  muscles, 


Fig.  i. — Diagrammatic  Longit- 
udinal Section  of  the  Body.  V, 
V.  Bodies  of  the  vertebrae  which 
divide  the  body  into  the  dorsal  and 
ventral  cavities,  a,  a'.  The  dorsal 
cavity.  C,  p'.  The  abdominal  and 
thoracic  divisions  of  the  ventral  cavity, 
separated  from  each  other  by  a  trans- 
verse muscular  partition,  the  dia- 
phragm d.  B.  The  brain.  Sp.C. 
The  spinal  cord.  e.  The  esophagus. 
S.  The  stomach,  from  which  con- 
tinues the  intestine  to  the  opening  at 
the  posterior  portion  of  the  body. 
/.  The  liver,  p'.  The  pancreas,  k. 
The  kidney,  o.  The  bladder.  I'.  The 
lungs,     h.  The  heart. 

hairs,    bristles,  feathers,  claws, 
bones,  blood-vessels,  nerves,  etc. 


INTRODUCTION.  5 

The  appendicular  portion  of  the  body  consists  of  two  pairs  of 
symmetric  limbs,  which  project  from  the  sides  of  the  trunk,  and 
which  bear  a  determinate  relation  to  the  vertebral  column.  They 
consist  fundamentally  of  bones,  surrounded  by  muscles,  blood-vessels, 
nerves  and  lymphatics.  The  limbs,  though  having  a  common  plan 
of  organization,  are  modified  in  form  and  adapted  for  prehension 
and  locomotion  in  accordance  with  the  needs  of  the  animal. 

Anatomic  Systems. — All  the  organs  of  the  body  which  have 
certain  peculiarities  of  structure  in  common  are  classified  by  anato- 
mists into  systems — e.  g.,  the  bones,  collectively,  constitute  the  bony 
or  osseous  system;  the  muscles,  the  nerves,  the  skin,  constitute,  respec- 
tively, the  muscle,  the  nerve,  and  the  tegumentary  systems. 

Physiologic  Apparatus. — More  important  from  a  physiologic 
point  of  view  than  a  classification  of  organs  based  on  similarities  of 
structure  is  the  natural  association  of  two  or  more  organs  acting  together 
for  the  accomplishment  of  some  definite  object,  and  to  which  the  term 
physiologic  apparatus  has  been  applied.  While  in  the  community  of 
organs  which  together  constitute  the  animal  body  each  one  performs 
some  definite  function,  and  the  harmonious  cooperation  of  all  is  neces- 
sary to  the  life  of  the  individual,  everywhere  it  is  found  that  two  or 
more  organs,  though  performing  totally  distinct  functions,  are  cooperat- 
ing for  the  accomplishment  of  some  larger  or  compound  function  in 
which  their  individual  functions  are  blended — e.  g.,  the  mouth,  stom- 
ach, and  intestines,  with  the  glands  connected  with  them,  constitute  the 
digestive  apparatus,  the  object  or  function  of  which  is  the  complete 
digestion  of  the  food.  The  capillary  blood-vessels  and  lymphatic 
vessels  of  the  body,  and  especially  those  in  relation  to  the  villi  of  the 
small  intestine,  constitute  the  absorptive  apparatus,  the  function  of 
which  is  the  introduction  of  new  material  into  the  blood.  The  heart 
and  blood-vessels  constitute  the  circulatory  apparatus,  the  function  of 
which  is  the  distribution  of  blood  to  all  portions  of  the  body.  The 
lungs  and  trachea,  together  with  the  diaphragm  and  the  walls  of  the 
chest,  constitute  the  respiratory  apparatus,  the  function  of  which  is  the 
introduction  of  oxygen  into  the  blood  and  the  elimination  from  it  of 
carbon  dioxid  and  other  injurious  products.  The  kidneys,  the  ureters, 
and  the  bladder  constitute  the  urinary  apparatus.  The  skin,  with  its 
sweat-glands,  constitutes  the  perspiratory  apparatus,  the  functions  of 
both  being  the  excretion  of  waste  products  from  the  body.  The  liver, 
the  pancreas,  the  mammary  glands,  as  well  as  other  glands,  each  form 
a  secretory  apparatus  which  elaborates  some  specific  material  necessary 
to  the  nutrition  of  the  individual.  The  functions  of  these  different 
physiologic  apparatus — e.  g.,  digestion,  absorption  of  food,  elaboration 
of  blood,  circulation  of  blood,  respiration,  production  of  heat,  secretion, 
and  excretion — are  classified  as  nutritive  junctions,  and  have  for  their 
final  object  the  preservation  of  the  individual. 


6  TEXT-BOOK  OF  PHYSIOLOGY. 

The  nerves  and  muscles  constitute  the  nervo-muscle  apparatus, 
the  function  of  which  is  the  production  of  motion.  The  eye,  the  ear, 
the  nose,  the  tongue,  and  the  skin,  with  their  related  structures,  con- 
stitute, respectively,  the  visual,  auditory,  olfactory,  gustatory,  and 
tactile  apparatus,  the  function  of  which,  as  a  whole,  is  the  reception 
of  impressions  and  the  transmission  of  nerve  impulses  to  the  brain, 
where  they  give  rise  to  visual,  auditory,  olfactory,  gustatory,  and  tactile 
sensations  and  volitional  impulses. 

The  brain,  in  association  with  the  sense  organs,  forms  an  appa- 
ratus related  to  mental  processes.  The  larynx  and  its  accessory  organs 
— the  lungs,  trachea,  respiratory  muscles,  the  mouth  and  resonant 
cavities  of  the  face — form  the  vocal  and  articulating  apparatus,  by 
means  of  which  voice  and  articulate  speech  are  produced.  The  func- 
tions exhibited  by  the  apparatus  just  mentioned — viz.,  motion,  sensa- 
tion, language,  mental  and  moral  manifestations — are  classified  as 
functions  of  relation,  as  they  serve  to  bring  the  individual  into  con- 
scious relationship  with  the  external  world. 

The  ovaries  and  the  testes  are  the  essential  reproductive  organs, 
the  former  producing  the  germ-cell,  the  latter  the  sperm  element. 
Together  with  their  related  structures — the  fallopian  tubes,  uterus, 
and  vagina  in  the  female,  and  the  urogenital  canal  in  the  male — 
they  constitute  the  reproductive  apparatus  characteristic  of  the  two 
sexes.  Their  cooperation  results  in  the  union  of  the  germ-cell  and 
sperm  element  and  the  consequent  development  of  a  new  being.  The 
function  of  reproduction  serves  to  perpetuate  the  species  to  which  the 
individual  belongs. 

The  animal  body  is  therefore  not  a  homogeneous  organism,  but 
one  composed  of  a  large  number  of  widely  dissimilar  but  related  organs. 
As  all  vertebrate  animals  have  the  same  general  plan  of  organization, 
there  is  a  marked  similarity  both  in  form  and  structure  among  corre- 
sponding parts  of  different  animals.  Hence  it  is  that  in  the  study  of 
human  anatomy  a  knowledge  of  the  form,  construction,  and  arrange- 
ment of  the  organs  in  different  types  of  animal  life  is  essential  to  its 
correct  interpretation;  it  follows  also  that  in  the  investigation  and  com- 
prehension of  the  complex  problems  of  human  physiology  a  knowledge 
of  the  functions  of  the  organs  as  they  manifest  themselves  in  the  differ- 
ent types  of  animal  life  is  indispensable.  As  many  of  the  functions  of 
the  human  body  are  not  only  complex,  but  the  organs  exhibiting  them 
are  practically  inaccessible  to  investigation,  we  must  supplement  our 
knowledge  and  judge  of  their  functions  by  analogy,  by  attributing  to 
them,  within  certain  limits,  the  functions  revealed  by  experimentation 
upon  the  corresponding  organs  of  lower  animals.  This  experimental 
knowledge,  corrected  by  a  study  of  the  clinical  phenomena  of  disease 
and  the  results  of  post-mortem  investigations,  forms  the  basis  of  modern 
human  physiology. 


CHAPTER  II. 
CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY. 

Since  it  has  been  demonstrated  that  every  exhibition  of  functional 
activity  is  associated  with  changes  of  structure,  it  has  been  apparent 
that  a  knowledge  of  the  chemic  composition  of  the  body,  not  only 
when  in  a  state  of  rest,  but  to  a  far  greater  degree  when  in  a  state  of 
activity,  is  necessary  to  a  correct  understanding  of  the  intimate  nature 
of  physiologic  processes.  Though  the  analysis  of  the  dead  body  is 
comparatively  easy,  the  determination  of  the  successive  changes  in  com- 
position of  the  living  body  is  attended  with  many  difficulties.  The 
living  material,  the  bioplasm,  is  not  only  complex  and  unstable  in  com- 
position, but  extremely  sensitive  to  all  physical  and  chemic  influences. 
The  methods,  therefore,  which  are  employed  for  analysis  destroy  its 
composition  and  vitality,  and  the  products  which  are  obtained  are 
peculiar  to  dead  rather  than  to  living  material. 

Chemic  analysis,  therefore,  may  be  directed — 
i.  To  the  determination  of  the  composition  of  the  dead  body. 
2.  To  the  determination   of  the   successive  changes  in  composition 
which  the  living  bioplasm  undergoes  during  functional  activity. 

A  chemic  analysis  of  the  dead  body,  with  a  view  to  disclosing  the 
substances  of  which  it  is  composed,  their  properties,  their  intimate 
structure,  their  relationship  to  one  another,  constitutes  what  might 
be  termed  chemic  anatomy.  An  investigation  of  the  living  ma- 
terial and  of  the  successive  changes  it  undergoes  in  the  performance 
of  its  functions  constitutes  what  has  been  termed  chemic  physiology 
or  physiologic  chemistry. 

By  chemic  analysis  the  animal  body  can  be  reduced  to  a  number 
of  liquid  and  solid  compounds  which  belong  to  both  the  inorganic 
and  organic  worlds.  These  compounds,  resulting  from  a  proximate 
analysis,  have  been  termed  proximate  principles.  That  they  may 
merit  this  term,  however,  they  must  be  obtained  in  the  form  under 
which  they  exist  in  the  living  condition.  The  organic  compounds 
consist  of  representatives  of  the  carbohydrate,  fatty,  and  protein  groups 
of  organic  bodies;  the  inorganic  compounds  consist  of  water,  various 
acids,  and  inorganic  salts. 

The  compounds  or  proximate  principles  thus  obtained  can  be 
further  resolved  by  an  ultimate  analysis  into  a  small  number  of  chemic 
elements  which  are  identical  with  elements  found  in  many  other  organic 
as  well  as  inorganic  compounds.  The  different  chemic  elements 
which  are  thus  obtained,  and  the  percentages  in  which  they  exist  in  the 
body,  are  as  follows — viz.,  oxygen,  72  percent.;  hydrogen,  9.10;  nitrogen, 


8  TEXT-BOOK  OF  PHYSIOLOGY. 

2.5;  carbon,  13.50;  phosphorus,  1.15;  calcium,  1.30;  sulphur,  0.147; 
sodium,  0.10;  potassium,  0.026;  chlorin,  0.085;  fluorin,  iron,  silicon, 
magnesium,  in  small  and  variable  amounts. 

THE  CARBOHYDRATES. 

The  carbohydrate  compounds,  which  enter  into  the  composition 
of  the  animal  body,  are  mainly  starches  and  sugar.  In  many  respects 
they  are  closely  related,  and  by  appropriate  means  are  readily  con- 
verted into  one  another.  In  composition  they  consist  of  the  elements 
carbon,  hydrogen,  and  oxygen.  As  their  name  implies,  the  hydrogen 
and  oxygen  are  present  in  these  compounds  in  the  proportion  in  which 
they  exist  in  water,  or  as  2  :  1.  The  molecule  of  the  carbohydrates 
above  mentioned  consists  of  either  six  atoms  of  carbon  or  a  multiple 
of  six;  in  the  latter  case  the  quantity  of  hydrogen  and  oxygen  taken  up 
by  the  carbon  is  increased,  though  the  ratio  remains  unchanged. 

The  carbohydrates  may  be  divided  into  three  groups — viz.:  (1) 
amyloses,  including  starch,  dextrin,  glycogen,  and  cellulose;  (2)  dex- 
troses, including  dextrose,  levulose,  galactose;  (3)  saccharoses,  including 
saccharose,  lactose,  and  maltose.  According  to  the  number  of  carbon 
atoms  entering  into  the  second  group  (six),  they  are  frequently  termed 
monosaccharids;  those  of  the  third  group,  disaccharids — twice  six; 
those  of  the  first  group,  polysaccharids — multiples  of  six. 

Though  but  few  of  the  members  of  the  carbohydrate  group  are 
constituents  of  the  human  body,  many  are  constituents  of  the  foods; 
on  account  of  their  importance  in  this  respect,  and  their  relation  to 
one  another,  the  chemic  features  of  the  more  generally  consumed  car- 
bohydrates will  be  stated  in  this  connection. 

1.  AMYLOSES,  (C6H10O5)n. 

Starch  is  widely  distributed  in  the  vegetable  world,  being  abundant 
in  the  seeds  of  the  cereals,  leguminous  plants,  and  in  the  tubers  and 
roots  of  many  vegetables.  It  occurs  in  the  form  of  microscopic  granules 
which  vary  in  size,  shape,  and  appearance,  according  to  the  plant  from 
which  they  are  obtained.  Each  granule  presents  a  nucleus,  or  hilum, 
around  which  is  arranged  a  series  of  eccentric  rings,  alternately  light  and 
dark.  The  granule  consists  of  an  envelope  and  stroma  of  cellulose, 
containing  in  its  meshes  the  true  starch  material — granulose.  Starch 
is  insoluble  in  cold  water  and  alcohol.  When  heated  with  water  up  to 
70  °  C,  the  granules  swell,  rupture,  and  liberate  the  granulose,  which 
forms  an  apparent  solution;  if  present  in  sufficient  quantity,  it  forms  a 
gelatinous  mass  termed  starch  paste.  On  the  addition  of  iodin,  starch 
strikes  a  characteristic  deep  blue  color;  the  compound  formed — iodid 
of  starch — is  weak,  the  color  disappearing  on  heating,  but  reappearing 
on  cooling. 

Boiling  starch  with  dilute  sulphuric  acid  (twenty-five  per  cent.) 
converts  it  into  dextrose.     In  the  presence  of  vegetable  diastase  or 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  9 

animal  ferments,  starch  is  converted  into  maltose  and  dextrose,  two 
forms  of  sugar. 

Dextrin  is  a  substance  formed  as  an  intermediate  product  in  the 
transformation  of  starch  into  sugar.  There  are  at  least  two  principal 
varieties — erythrodextrin,  which  strikes  a  red  color  with  iodin,  and 
achr  00  dextrin,  which  is  without  color  when  treated  with  this  reagent. 
In  the  pure  state  dextrin  is  a  yellow-white  powder,  soluble  in  water. 
In  the  presence  of  vegetable  ferments  erythrodextrin  is  converted  into 
maltose. 

Glycogen  is  a  constituent  of  the  animal  liver,  and,  to  a  slight  ex- 
tent, of  muscles  and  of  tissues  generally.  In  the  tissues  of  the  embryo 
it  is  especially  abundant.  When  obtained  in  a  pure  state  it  is  an 
amorphous,  white  powder.  It  is  soluble  in  water,  forming  an  opales- 
cent solution.  With  iodin  it  strikes  a  port- wine  color.  In  some  respects 
it  resembles  starch,  in  others  dextrin.  Like  vegetable  starch,  glycogen 
or  animal  starch  can  be  converted  by  dilute  acids  and  ferments  into 
sugar  (dextrose). 

Cellulose  is  the  basic  material  of  the  more  or  less  solid  framework 
of  plants.  It  is  soluble  in  ammoniacal  solution  of  cupric  oxid,  from 
which  it  can  be  precipitated  by  acids.  It  is  an  amorphous  powder; 
dilute  acids  can  convert  it  into  dextrose. 

2.  DEXTROSES,  C6H12Oe. 

Dextrose,  glucose,  or  grape-sugar  is  found  in  grapes,  most 
sweet  fruits,  and  honey,  and  as  a  normal  constituent  of  liver,  blood, 
muscles,  and  other  animal  tissues.  In  the  disease  diabetes  mellitus 
it  is  found  also  in  the  urine. 

When  obtained  from  any  source,  it  is  soluble  in  wTater  and  in  hot 
alcohol,  from  which  it  crystallizes  in  six-sided  tables  or  prisms.  As 
usually  met  with,  it  is  in  the  form  of  irregular,  warty  masses.  It  is 
sweet  to  the  taste.  When  examined  with  the  polariscope,  dextrose 
turns  the  plane  of  polarized  light  to  the  right.  It  is  therefore  termed 
dextro-rotatory.  It  has  for  a  long  time  been  known  that  when  sugar, 
cupric  hydroxid,  and  an  alkali — e.  g.,  sodium  or  potassium — are 
present  in  solution,  the  sugar  will  abstract  from  the  cupric  hydroxid 
a  portion  of  its  oxygen,  thus  reducing  it  to  a  lower  stage  of  oxidation 
giving  rise  to  cuprous  oxid.  Sugar  has  a  similar  action  on  both  silver 
and  bismuth.  On  this  property  of  sugar  a  standard  solution  of 
cupric  hydroxid  was  suggested  by  Fehling  which  may  be  employed 
for  both  qualitative  and  quantitative  tests  for  the  presence  of  sugar  in 
solution. 

Fehling s  Test  Solution. — This  is  a  solution  of  cupric  hydroxid 
made  alkaline  by  an  excess  of  sodium  or  potassium  hydroxid  with 
the  addition  of  sodium  and  potassium  tartrate.  It  is  made  by  dissolv- 
ing cupric  sulphate  34.64  grams,  potassium  hydroxid  125  grams,  so- 
dium and  potassium  tartrate  173  grams,  in  distilled  water  sufficient  to 
make  one  liter. 


io  TEXT-BOOK  OF  PHYSIOLOGY. 

The  reaction  is  expressed  by  the  following  equation : 
CuS04  +  2KOH  =  Cu(OH)2  +  K2S04. 

The  object  of  the  sodium  and  potassium  tartrate  is  to  dissolve 
the  cupric  hydroxid  and  hold  it  in  solution. 

For  qualitative  analysis  it  is  only  necessary  to  boil  a  few  cubic  centi- 
meters of  this  solution  in  a  test-tube;  then  add  the  suspected  solution 
and  again  heat  to  the  boiling-point.  If  sugar  be  present,  the  cupric 
hydroxid  is  reduced  to  the  condition  of  a  cuprous  oxid,  which  shows 
itself  as  a  red  or  orange-yellow  precipitate.  The  color  of  the  precipitate 
depends  on  the  relative  excess  of  either  copper  or  sugar,  being  red  with 
the  former,  orange  or  yellow  with  the  latter.  The  delicacy  of  this  test  is 
shown  by  the  fact  that  a  few  minims  of  this  solution  will  detect  in  1  c.c. 
of  water  the  TV  of  a  milligram  of  sugar. 

For  quantitative  analysis,  10  c.c.  of  Fehling's  solution,  diluted  with 
40  c.c.  of  water,  are  heated  in  a  porcelain  capsule,  to  which  the  sus- 
pected solution  is  cautiously  added  from  a  buret  until  the  blue  color 
entirely  disappears.  The  strength  of  this  solution  is  such  that  1  c.c. 
is  decolorized  by  5  milligrams  of  sugar  {dextrose),  from  which  the  per- 
centage of  sugar  in  any  solution  can  be  determined. 

All  the  sugars,  with  the  exception  of  chemically  pure  saccharose, 
may  be  tested  for  with  this  solution. 

The  Fermentation  Test. — All  the  sugars  with  the  exception  of 
lactose  undergo  reduction  to  simpler  compounds,  mainly  alcohol  and 
carbon  dioxid,  under  the  action  of  the  yeast  plant,  Saccharomyces 
cerevisice.  The  change  with  dextrose  is  expressed  in  the  following 
equation :  . 

C6HI206  =  2C2H60  +  2C02. 
Dextrose    =    Alcohol     +  Carbon  Dioxid. 

About  95  per  cent,  of  the  dextrose  is  so  changed,  the  remaining 
5  per  cent,  yielding  secondary  products — succinic  acid,  glycerin,  etc. 
As  a  means  of  testing  any  solution  for  the  presence  of  sugar  this  method 
may  be  adopted.  It  is  generally  very  satisfactory.  From  the  quantity 
of  carbon  dioxid  and  alcohol  thus  produced  the  quantity  of  sugar  in  the 
solution  may  be  determined. 

Levulose,  or  fruit-sugar,  is  found  in  association  with  dextrose 
as  a  constituent  of  many  fruits.  It  is  sweeter  than  dextrose  and  more 
soluble  in  both  water  and  dilute  alcohol.  From  alcoholic  solutions  it 
crystallizes  in  fine,  silky  needles,  though  it  usually  occurs  in  the  form 
of  a  syrup. 

Levulose  is  distinguished  from  dextrose  by  its  property  of  turning 
the  plane  of  polarized  light  to  the  left;  the  extent  to  which  it  does  so, 
however,  varies  with  the  temperature  and  concentration  of  the  solution. 
For  this  reason  it  is  turned  levulo-rotatory. 

Under  the  influence  of  the  yeast  plant  it  slowly  undergoes  fermen- 
tation, yielding  the  same  products  as  dextrose.  It  also  has  a  reducing 
action  on  cupric  hydroxid. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  n 

Galactose  is  obtained  by  boiling  milk-sugar  (lactose)  with  dilute 
sulphuric  acid.  In  many  chemic  relations  it  resembles  dextrose.  It 
is  less  soluble  in  water,  however,  crystallizes  more  easily,  and  has  a 
greater  dextro-rotatory  power.  It  also  undergoes  fermentation  with 
the  yeast  plant. 

3.  SACCHAROSES,  C12H22Ou. 

Saccharose,  or  cane-sugar,  is  widely  distributed  throughout 
the  vegetable  world,  but  is  especially  abundant  in  sugar-cane,  sor- 
ghum cane,  sugar-beet,  Indian  corn,  etc.  It  crystallizes  in  large  mono- 
clinic  prisms.  It  is  soluble  in  water  and  in  dilute  alcohol.  Saccharose 
has  no  reducing  power  on  cupric  hydroxid,  and  hence  its  presence  can 
not  be  detected  by  Fehling's  solution.  It  is  dextro-rotatory.  Boiled 
with  dilute  mineral,  as  well  as  with  organic  acids,  saccharose  combines 
with  water  and  undergoes  a  change  in  virtue  of  which  it  rotates  the 
plane  of  polarized  light  to  the  left,  and  hence  the  product  was  termed 
invert  sugar.  This  latter  has  been  shown  to  be  a  mixture  of  equal 
quantities  of  levulose  and  dextrose.  This  inversion  of  saccharose 
through  hydration  and  decomposition  is  expressed  in  the  following 
equation : 

CI2H22OXI  +  H20  =  C6HI206  +  C6HI206 
Saccharose    +  Water  =    Levulose    +    Dextrose 


Invert  Sugar. 

Saccharose  is  not  directly  fermentable  by  yeast,  but  through  the 
specific  action  of  a  ferment,  invertin  or  invertase,  secreted  by  the  yeast 
plant,  or  the  inverting  ferment  of  the  small  intestine,  it  undergoes  in- 
version, as  previously  stated,  after  which  it  is  readily  fermented,  yield- 
ing alcohol  and  carbon  dioxid. 

Lactose  is  the  form  of  sugar  found  exclusively  in  the  milk  of  the 
mammalia,  from  which  it  can  be  obtained  in  the  form  of  hard,  white, 
rhombic  prisms  united  with  one  molecule  of  water.  It  is  soluble  in 
water,  insoluble  in  alcohol  and  ether.  It  is  dextro-rotatory.  It  reduces 
cupric  hydroxid,  but  to  a  less  extent  than  dextrose.  Dilute  acids 
decompose  it  into  equal  quantities  of  dextrose  and  galactose.  Lactose 
is  not  fermentable  with  yeast,  but  in  the  presence  of  the  lactic  acid 
.bacillus  it  is  decomposed  into  lactic  acid,  and  finally  into  butyric  acid, 
as  expressed  in  the  following  equation: 

CI2H220„     +     H20     =     4C3H6O3 
Lactose  +    Water      =      Lactic  Acid. 

2C3H60.,     =     C4H802     +     2C02     +       2H, 
Lactic  Acid    =     Butyric  Acid  -r     Carbon    +      Free 

Dioxid        Hydrogen. 

Maltose  is  a  transformation  product  of  starch,  and  arises  when- 
ever the  latter  is  acted  on  by  malt  extract  or  the  diastatic  ferments 
in  saliva  and  pancreatic  juice.  The  change  is  expressed  by  the  fol- 
lowing equation : 

2C6HioOs     +     H.O     =     CI2H220„ 
Starch.  Water.  Maltose. 


12  TEXT-BOOK  OF  PHYSIOLOGY. 

Maltose  crystallizes  in  the  form  of  white  needles,  which  are  soluble 
in  water  and  in  dilute  alcohol.  It  is  dextro-rotatory.  In  the  presence 
of  ferments  and  dilute  acids  maltose  undergoes  hydration  and  decom- 
position, giving  rise  to  two  molecules  of  dextrose.  It  has  a  reducing 
action  on  cupric  hydroxid.  Fermentation  is  readily  caused  by  yeast, 
but  whether  directly  or  indirectly  by  inversion  is  somewhat  uncertain. 

Osazones. — All  the  sugars  which  possess  the  power  of  reducing 
cupric  hydroxid  are  capable  of  combining  with  phenyl-hydrazin,  with 
the  formation  of  compounds  termed  osazones.  The  osazones  so 
formed  are  crystalline  in  structure,  but  have  different  melting-points, 
varying  degrees  of  solubility  and  optic  properties,  all  of  which  serve  to 
detect  the  various  sugars  and  to  distinguish  one  from  the  other.  Of 
the  different  osazones,  phenyl-glucosazone  is  the  most  characteristic, 
and  occurs  in  the  form  of  long,  yellow  needles.  It  may  be  obtained 
from  dextrose  by  the  following  method :  To  50  c.c.  of  a  dextrose  solu- 
tion add  2  gm.  of  phenyl-hydrazin  and  2  gm.  of  sodium  acetate,  and 
boil  for  an  hour.  On  cooling,  the  osazone  crystallizes  in  the  form  of 
long,  yellow  needles. 

THE  FATS. 

The  fats  constitute  a  group  of  organic  bodies  found  in  the  tissues 
of  both  vegetables  and  animals.  In  the  vegetable  world  they  are 
largely  found  in  fruits,  seeds,  and  nuts,  where  they  probably  originate 
from  a  transformation  of  the  carbohydrates.  In  the  animal  body 
the  fats  are  found  largely  in  the  subcutaneous  tissue,  in  the  marrow 
of  bones,  in  and  around  various  internal  organs  and  in  milk.  In  these 
situations  fat  is  contained  in  small,  round  or  polygon-shaped  vesicles, 
which  are  united  by  areolar  tissue  and  surrounded  by  blood-vessels. 
At  the  temperature  of  the  body  the  fat  is  liquid,  but  after  death  it  soon 
solidifies  from  the  loss  of  heat. 

The  fats  are  compounds  consisting  of  carbon,  hydrogen,  and  oxygen, 
of  which  the  first  is  the  chief  ingredient,  forming  by  weight  about  75 
per  cent.,  while  the  last  is  present  only  in  small  quantity.  The  fat 
found  in  animals  is  a  mixture,  in  varying  proportions  in  different  ani- 
mals, of  three  neutral  fats — stearin,  palmitin,  and  olein.  Each  fat  is  a 
derivative  of  glycerin  and  the  particular  acid  indicated  by  its  name — 
e.  g.,  stearic  acid,  in  the  case  of  stearin,  etc.  The  reaction  which  takes 
place  in  the  combination  of  glycerin  and  the  acid  is  expressed  in  the 
following  equation: 

C,H5(HO)3     +     (HCl8H3502)3     =     C3Hs(Cl8H3SOfl)3     +     3H.O. 
Glycerin.  Stearic  Acid.  Stearin.  Water. 

Hence,  strictly  speaking,  the  fats  are  compound  ethers,  in  which 
the  hydrogen  of  the  organic  acid  is  replaced  by  the  trivalent  radicle, 
tritenyl,  C3HS. 

Stearin,  C3Hs(Cl8H3S02)3,  is  the  chief  constituent  of  the  more 
solid  fats.     It  is  solid  at  ordinary  temperatures,  melting  at  55 °  C, 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  13 

then  solidifying  again  as  the  temperature  rises,  until  at  71  °  C.  it  melts 
permanently.     It  crystallizes  in  square  tables. 

Palmitin,  C3H5(Cl6H3102)3  is  a  semifluid  fat,  solid  at  45 °  C. 
and  melting  at  62  °  C.  It  crystallizes  in  fine  needles,  and  is  soluble 
in  ether. 

Olein,  C3H.(Cl8H3302)3,  is  a  colorless,  transparent  fluid,  liquid  at 
ordinary  temperatures,  only  solidifying  at  o°  C.  It  possesses  marked 
solvent  powers,  and  holds  stearin  and  palmitin  in  solution  at  the  tem- 
perature of  the  body. 

Saponification. — When  subjected  to  the  action  of  superheated 
steam,  a  neutral  fat  is  saponified — i.  e.,  decomposed  into  glycerin  and 
the  particular  acid  indicated  by  the  name  of  the  fat  used:  e.  g.,  stearic, 
palmitic,  or  oleic.     The  reaction  is  expressed  as  follows: 

C3Hs(Cl8H330?)3  +  3H20  =  C3H5(HO)3  +  3(ClSH3402) 
Olein.  Water.  Glycerin.  Oleic  Acid. 

The  fat  acids  thus  obtained  are  characterized  by  certain  chemic 
features,  as  follows: 

Stearic  acid  is  a  firm,  white  solid,  fusible  at  69  °  C.  It  is  soluble 
in  ether  and  alcohol,  but  not  in  water. 

Palmitic  acid  occurs  in  the  form  of  white,  glistening  scales  or 
needles,  melting  at  62  °  C. 

Oleic  acid  is  a  clear,  colorless  liquid,  tasteless  and  odorless  when 
pure.     It  crystallizes  in  white  needles  at  o°  C. 

If  this  saponification  takes  place  in  the  presence  of  an  alkali — 
e.  g.,  potassium  hydro xid  or  sodium  hydroxid — the  acid  produced 
combines  at  once  with  the  alkali  to  form  a  salt  known  as  a  soap,  while 
the  glycerin  remains  in  solution.     The  reaction  is  as  follows: 

3KHO    +  (ClSH3402)3  =  3(KCl8H3302)    +  3HaO 
Potassium.  Oleic  Acid.  Potassium  Oleate,  Water. 

All  soaps  are,  therefore,  salts  formed  by  the  union  of  alkalies  and 
fat  acids.  The  sodium  soaps  are  generally  hard,  while  the  potas- 
sium soaps  are  soft.  Those  made  with  stearin  and  palmitin  are  harder 
than  those  made  with  olein.  If  the  soap  is  composed  of  lead,  zinc, 
copper,  etc.,  it  is  insoluble  in  water. 

Emulsification. — When  a  neutral  oil  is  vigorously  shaken  with 
water  or  other  fluid,  it  is  broken  up  into  minute  globules  that  are  more 
or  less  permanently  suspended;  the  permanency  depending  on  the 
nature  of  the  liquid.  The  most  permanent  emulsions  are  those  made 
with  soap  solutions.  The  process  of  emulsification  and  the  part 
played  by  soap  can  be  readily  observed  by  placing  on  a  few  cubic  centi- 
meters of  a  solution  of  sodium  carbonate  (0.25  per  cent.)  a  small  quant- 
ity of  a  perfectly  neutral  oil  to  which  has  been  added  2  or  3  per  cent, 
of  a  fat  acid.  The  combination  of  the  acid  and  the  alkali  at  once 
forms  a  soap.  The  energy  set  free  by  this  combination  rapidly  divides 
the  oil  into  extremely  minute  globules.  A  spontaneous  emulsion  is 
thus  formed. 


i4  TEXT-BOOK  OF  PHYSIOLOGY. 

THE  PROTEINS. 

The  proteins  constitute  a  group  of  organic  bodies  which  are  found 
in  both  vegetable  and  animal  tissues.  Though  present  in  all  animal 
tissues,  they  are  especially  abundant  in  muscles  and  bones,  where 
they  constitute  20  per  cent,  and  30  per  cent,  respectively.  Though 
genetically  related,  and  possessing  many  features  in  common,  the 
different  members  of  the  protein  group  are  distinguished  by  character- 
istic physical  and  chemic  properties  which  serve  not  only  for  their 
identification,  but  for  their  classification  into  more  or  less  well-defined 
groups. 

Chemic  Composition. — A  chemic  analysis  of  proteins  shows  that 
they  consist  of  carbon,  hydrogen,  oxygen,  nitrogen  and  sulphur,  though 
the  percentage  of  each  of  these  elements  varies  somewhat  in  the 
different  proteins. 

A  certain  number  of  proteins  contain  phosphorus  while  almost 
all  of  them  contain  different  inorganic  salts  in  varying  amounts.  The 
average  percentage  composition  of  several  proteins  is  shown  in  the 
following  analyses: 

C.         H.         N.         O.  S. 

Egg-albumin, 52.9  7.2  15.6  23.9  0.4    (Wiirtz). 

Serum-albumin, 53.0  6.8  16.0  22.29  1.77  (Hammersten). 

Casein, 52.3  7.07  15.91  22.03  0.82  (Chittenden  and  Painter). 

Myosin, 52.82  7. 11  16.77  21.90  1.27  (Chittenden  and  Cummins). 

The  molecular  composition  of  the  proteins  is  not  definitely  known 
and  the  formulas  which  have  been  suggested  are  therefore  only  approxi- 
mative. Leow  assigns  to  albumin  the  formula  C72HII2Nl8022S, 
while  Schiitzenberger  raises  the  numbers  to  C240H392N6s07SS3,  either 
of  which  shows  that  the  proteid  molecule  is  extremely  complex. 

Structure  of  the  Protein  Molecule. — From  the  large  size  of  the 
protein  molecule  as  indicated  by  its  chemic  composition  it  might  be 
inferred  that  its  structure  was  equally  complex.  This  modern  inves- 
tigation has  shown  to  be  the  case. 

When  any  one  of  the  typical  proteins,  found  in  animal  or  vegetable 
tissues,  is  hydrolysed  by  acids,  alkalies  and  animal  ferments  under 
appropriate  conditions,  it  can  be  resolved  through  a  series  of  descending 
stages  into  relatively  simple  nitrogen-holding  bodies  termed  amino- 
acids  and  diamino- acids,  of  which  somewhat  more  than  twenty  have 
been  isolated  and  their  properties  determined.  The  principal  amino- 
acids  are  as  follows:  Glycocoll,  alanin,  leucin,  isoleucin,  amino-iso- 
valerianic  acid,  serin,  aspartic  acid,  glutamic  acid,  phenylalanin, 
tyrosin,  prolin,  tryptophan.  The  principal  diamino-acids  are  as 
follows:     Ornithin,  lysin,  histidin,  arginin,  cystin. 

The  protein  molecule  is  therefore  structurally  complex.  The 
manner  in  which  these  elementary  compounds  are  arranged,  united 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  15 

or  grouped  in  any  given  protein,  is  practically  unknown.  More  or  less 
successful  attempts  have  been  made  at  the  reconstruction  of  the 
protein  molecule  by  synthetic  methods,  by  the  union  of  two  or  more  of 
the  amino-acids.  A  number  of  such  compounds  have  been  formed 
by  the  union  of  from  two  to  ten  or  more  amino-acids,  all  of  them 
exhibiting  many  of  the  protein  reactions.  Such  bodies  are  termed 
polypeptids. 

Physical  Properties. — As  a  class  the  proteins  are  characterized 
by  the  following  properties: 

1 .  Indiffusibility. — None    of    the    proteins    normally    assume    the 

crystalline  form,  and  hence  they  are  not  capable  of  diffusing 
through  parchment  or  an  animal  membrane.  Peptone,  a  product 
of  the  digestion  of  proteins,  is  an  exception  as  regards  its  diffu- 
sibility.  As  met  with  in  the  body,  all  proteins  are  amorphous,  but 
vary  in  consistence  from  the  liquid  to  the  solid  state.  The  colloid 
character  of  the  proteins  permits  of  their  separation  and  purifi- 
cation from  crystalloid  diffusible  compounds  by  the  process  of 
dialysis. 

2.  Solubility. — Some  of  the  proteids  are  soluble  in  water,  others  in 

solutions  of  the  neutral  salts  of  varying  degrees  of  concentration, 
in  strong  acids  and  alkalies.  All  are  insoluble  in  alcohol  and 
ether. 

3.  Coagulability. — Under  the  influence  of  heat  and  animal  ferments, 

some  of  the  proteins  readily  pass  from  the  soluble  liquid  state  to 
the  insoluble  solid  state,  attended  by  a  permanent  alteration  in 
their  chemic  composition.  To  this  change  the  term  coagulation 
has  been  given.  The  various  proteins,  however,  coagulate  at 
different  temperatures.  Proteins  are  capable  of  precipitation 
without  losing  their  solubility  by  ammonium  sulphate,  sodium 
chlorid,  and  magnesium  sulphate. 

4.  Fermentability. — In    the  presence  of   specific  microorganisms- 

bacteria — the  proteins,  owing  to  their  complexity  and  instability, 
are  prone  to  undergo  disintegration  and  reduction  to  simpler 
compounds.  This  decomposition  or  putrefaction  occurs  most 
readily  when  the  conditions  most  favorable  to  the  growth  of 
bacteria  are  present — viz.,  a  temperature  varying  from  250  C. 
to  400  C,  moisture,  and  oxygen.  The  intermediate  as  well  as 
the  terminal  products  of  the  decomposition  of  the  proteins  are 
numerous,  and  vary  with  the  composition  of  the  protein  and 
the  specific  physiologic  action  of  the  bacteria.  Among  the 
intermediate  products  is  a  series  of  alkaloid  bodies,  some  of  which 
possess  marked  toxic  properties,  known  as  ptomains.  The  toxic 
symptoms  which  frequently  follow7  the  ingestion  of  foods  in 
various  stages  of  putrefaction  are  to  be  attributed  to  these  com- 
pounds. The  terminal  products  are  represented  by  hydrogen 
sulphid,  ammonia,  carbon  dioxid,  fats,  phosphates,  nitrates,  etc. 


16  TEXT-BOOK  OF  PHYSIOLOGY. 

Classification. — The  protein  compounds  by  virtue  of  their  struc- 
tural composition,  their  physical  and  chemic  properties,  permit  of  a 
provisional  arrangement  into  groups  as  follows: 

PROTAMINS. 

These  proteins  are  derived  for  the  most  part  from  the  heads 
of  the  spermatozoa  of  fish.  They  take  their  names  from  the 
species  of  fish  from  which  they  are  obtained,  e.  g.,  salmin  (salmon), 
sturin  (sturgeon),  scombrom,  (mackerel),  etc.  Inasmuch  as  they 
respond  to  Piotrowski's  test  in  a  characteristic  way  they  are  regarded 
as  true  proteins.  When  subjected  to  hydrolysis  they  can  be  resolved 
into  the  diamino  bodies,  lysin,  arginin  and  histidin,  of  which  they  con- 
stitute about  90  per  cent.,  and  a  small  number  of  the  mono-amino-acids. 
Because  of  the  fact  that  the  diamino  bodies,  lysin,  histidin  and  arginin 
contain  6  atoms  of  carbon  they  are  known  as  the  hexone  bases. 
Inasmuch  as  the  protamins  contain  practically  but  these  three  bodies, 
they  are  regarded  as  the  simplest  of  all  the  proteins.  Since  a  typical 
protein  always  yields  on  hydrolysis  the  hexone  bases,  in  addition  to  a 
variable  number  of  mono-amino-acids,  it  is  believed  that  the  usual 
protein  is  composed  of  a  nucleus  of  the  hexone  bases  to  which  is  attached 
a  variable  number  of  mono-amino-acids.  The  proportions  in  which 
the  bases  exist  in  the  nucleus  and  the  proportions  in  which  the  amino - 
acids  are  united  to  the  nucleus,  vary  in  different  proteins. 

HISTONS. 

The  proteins  embraced  in  this  class  comprise  a  series  of  compounds 
which  are  somewhat  more  complex  than  the  protamins  and  less  com- 
plex than  the  typical  proteins;  for  on  hydrolysis  they  not  only  yield 
the  hexone  bases  but  in  addition  a  certain  number  of  amino-acids. 
They  are,  therefore,  intermediate  in  structural  composition  between 
the  protamins  and  the  usual  proteins.  Their  protein  character  is 
indicated  by  their  reaction  to  Millon's  reagent  and  to  Piotrowski's 
test.  The  histons  are  usually  found  in  combination  with  nucleic  acid, 
in  the  spermatozoa  of  most  animals  and  especially  in  fish,  and  in  the 
coloring  matter  (the  hemoglobin)  of  the  red  corpuscles.  The  proteins, 
of  the  tissues  usually  contain  from  25  to  30  per  cent,  of  histons. 

ALBUMINS. 

The  members  of  this  group  are  soluble  in  water,  in  dilute  saline 
solutions,  and  in  saturated  solutions  of  sodium  chlorid  and  mag- 
nesium sulphate.  They  are  coagulated  by  heat,  and  when  dried  form 
an  amber-colored  mass. 

(a)  Serum-albumin. — This  most  important  protein  is  found  in 
blood,  lymph,  chyle,  and  some  tissue  fluids.  It  is  obtained 
readily  by  precipitation  from  blood-serum,  after  the  other 
proteins  have  been  removed,  on  the  addition  of  ammonium 
sulphate.  When  freed  from  saline  constituents,  it  presents 
itself  as  a  pale,  amorphous  substance,  soluble  in  water  and  in 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  17 

strong  nitric  acid.  It  is  coagulated  at  a  temperature  of  73  ° 
C,  as  well  as  by  various  acids — e.g.,  citric,  picric,  nitric,  etc. 
It  has  a  rotatory  power  of  — 62. 6°. 

(b)  Egg-albumin. — Though  not  a  constituent  of  the  human 
body,  egg-albumin  resembles  the  foregoing  in  many  respects. 
When  obtained  in  the  solid  form  from  the  white  of  the  egg, 
it  is  a  yellow  mass  without  taste  or  odor.  Though  similar 
to  serum-albumin,  it  differs  from  it  in  being  precipitated  by 
ether,  in  coagulating  at  540  C,  and  in  having  a  lower  rotatory 
power,— 35. 50. 

(c)  Lact-albumin. — As  its  name  implies,  this  protein  is  found 
in  milk.  It  can  be  precipitated  from  milk-plasma  by  sodium 
sulphate  after  the  precipitation  of  the  other  proteids  by  half 
saturation  with  ammonium  sulphate.  It  slowly  coagulates  at 
77°  C. 

(d)  Myo-albumin. — This  protein  is  found  in  muscle-plasma  from 
which  it  subjects  the  plasma  to  fractional  heat  coagulation. 
At  730  C.  myo-albumin  coagulates. 

GLOBULINS. 

(a)  Serum-globulin  or  Paraglobulin. — This  protein,  as  its  name 
implies,  is  found  in  blood-serum,  though  it  is  present  in  other 
animal  fluids.  When  precipitated  by  magnesium  sulphate 
or  carbon  dioxid,  it  presents  itself  as  a  flocculent  substance, 
insoluble  in  water,  soluble  in  dilute  acids  and  alkalies,  and 
coagulating  at  75 °  C. 

(b)  Fibrinogen. — This  protein  is  found  in  blood-plasma  in  asso- 
ciation with  serum-globulin  and  serum-albumin.  It  is  also 
present  in  lymph-tissue  fluids  and  in  pathologic  transudates. 
It  can  be  obtained  from  blood-plasma  which  has  been  pre- 
viously treated  with  magnesium  sulphate  on  the  addition  of 
a  saturated  solution  of  sodium  chlorid.  It  is  soluble  in  dilute 
acids  and  alkalies,  and  coagulates  at  56 °  C. 

(c)  Para-myosinogen. — This  protein  is  a  constituent  of  the 
muscle-plasma  from  which  it  can  be  precipitated  by  a  tem- 
perature of  470  C. 

(d)  Myosinogen. — This  protein  is  the  chief  constituent  of  the 
muscle-plasma  and  is  of  great  nutritive  value.  During  the 
living  condition  it  is  liquid,  but  after  death  it  readily  undergoes 
a  chemic  change  and  contributes  to  the  formation  of  an  insoluble 
protein  known  as  myosin.  It  is  soluble  in  dilute  hydrochloric 
acid  and  dilute  alkalies.     It  coagulates  at  56 °  C. 

(e)  Crystallin  or  Globulin. — This  is  obtained  by  passing  a  stream 
of  C02  through  a  watery  extract  of  the  crystalline  lens. 

SCLERO-PROTEINS  (ALBUMINOIDS). 

The  sclero-proteins  constitute  a  group  of  substances  similar  to  the 


18  TEXT-BOOK  OF  PHYSIOLOGY. 

proteins  in  many  respects,  though  differing  from  them  in  others. 
When  obtained  from  the  tissues,  in  which  they  form  an  organic  basis, 
they  are  found  to  be  amorphous,  colloid,  and  when  decomposed  yield 
products  similar  to  those  of  the  true  proteins.  The  principal  members 
of  this  group  are  as  follows: 

(a)  Collagen,  Ossein. — These  are  two  closely  allied,  if  not  identical, 
substances,  found  respectively  in  the  white  fibrous  connective 
tissue  and  in  bone.  When  the  tendons  of  muscles,  the  liga- 
ments, or  decalcified  bone  are  boiled  for  several  hours,  the 
collagen  and  ossein  are  converted  into  soluble  gelatin,  which, 
when  the  solution  cools  becomes  solid. 

(b)  Chondrigen. — This  is  supposed  to  be  the  organic  basis  of  the 
more  permanent  cartilages.  When  the  latter  are  boiled,  they 
yield  a  substance  which  gelatinizes  on  cooling,  and  to  which 
the  name  chondrin  has  been  given.  Chondrin,  however,  is 
not  a  pure  gelatin,  but  has  associated  with  it  a  compound 
proteid  known  as  chrondro-mucoid. 

(c)  Elastin  is  the  name  given  to  the  substance  composing  the  fibers 
of  the  yellow,  elastic  connective  tissue. 

(d)  Keratin  is  the  substance  found  in  all  horny  and  epidermic 
tissues,  such  as  hairs,  nails,  scales,  etc.  It  differs  from  most 
proteids  in  containing  a  high  percentage  of  sulphur. 

PHOSPHO-PROTEINS. 

The  two  members  of  this  group  are  distinguished  by  yielding  on 
decomposition  a  protein  which  contains  phosphorus.  It  was  formerly 
regarded  as  a  nuclein. 

(a)  Caseinogen. — This  is  the  principal  protein  of  milk,  in  which 
it  exists  in  association  with  an  alkali,  and  hence  was  formerly 
regarded  as  an  alkali-albumin.  It  is  precipitated  by  acetic 
acid  and  by  magnesium  sulphate.  It  is  coagulated  by  rennet — 
that  is,  separated  into  an  insoluble  protein,  casein  or  tyrein, 
and  a  soluble  albumin.  Calcium  phosphate  seems  to  be  the 
natural  alkali  necessary  to  this  process,  for  if  it  be  removed 
by  dialysis,  or  precipitated  by  the  addition  of  potassium  oxalate, 
coagulation  does  not  take  place. 

(b)  Vitellin. — Vitellin  is  a  constituent  of  the  vitellis  or  yolk  of  eggs. 
It  differs  from  other  proteins  in  the  fact  that  it  is  semicrystalline 
in  character.  Though  usually  regarded  as  a  nucleo-protein 
it  is  not  definitely  known  whether  or  not  it  contains  phosphorus 
in  its  composition. 

CONJUGATED  OR  COMBINED  PROTEINS. 

The  different  members  of  this  group  are  capable  of  being  decom- 
posed by  chemic  methods  into  a  protein  and  a  non-protein  substance; 
e.  g.,  a  coloring  matter,  a  carbohydrate,  or  a  nuclein.  The  chemic 
character  of  the  non-protein  substance  furnishes  the  basis  for  the 
following  classification : 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  19 

CHROMO-PROTEINS. 

(a)  Hemoglobin. — Hemoglobin  is  the  coloring  matter  of  the  red 
corpuscles,  of  which  it  constitutes  about  94  per  cent.  It 
possesses  the  power  of  absorbing  oxygen  as  it  passes  through 
the  lung  capillaries  and  of  yielding  it  up  to  the  tissues  as  it 
passes  through  the  tissue  capillaries.  In  the  arterial  blood 
it  is  known  as  oxyhemoglobin,  and  in  the  venous  blood  as 
deoxy-  or  reduced-hemoglobin.  When  hydrolysed  by  acids 
or  alkalies,  hemoglobin  undergoes  a  cleavage  into  a  protein, 
globin,  and  a  pigment  hematin. 

(b)  Myohematin.— Myohematin  is  a  protein  supposed  to  be 
present  in  muscle.  It  has  never  been  isolated,  hence  its  chemic 
features  are  unknown.  Spectroscopic  examination  indicates 
that  it  is  capable  of  absorbing  and  again  yielding  up  oxygen. 
For  this  reason  it  is  believed  to  be  a  derivative  of  hemoglobin. 

GLUCO-PROTEINS. 

(a)  Mucin.— Mucin  is  the  proteid  which  gives  the  mucus,  secreted 
by  the  epithelial  cells  of  the  mucous  membranes  and  related 
glands,  its  viscid,  tenacious  character.  It  is  also  a  constituent 
of  the  intercellular  substance  of  the  connective  tissues.  It 
is  readily  precipitated  by  acetic  acid.  When  heated  with 
dilute  acids,  mucin  undergoes  a  cleavage  into  a  simpler  proteid 
and  a  carbohydrate  termed  mucose,  which  is  capable  of  reducing 
Fehling's  solution. 

(b)  Mucoids. — The  mucoids  resemble  the  mucins  though  differ- 
ing from  them  in  solubility  and  in  not  being  precipitable  from 
alkaline  solutions  by  acetic  acid.  They  are  found  in  the 
vitreous  humor,  white  of  egg,  cartilage,  and  in  other  situations. 
They  differ  slightly  one  from  the  other  in  properties  and  chemic 
composition.     They  yield  on  decomposition  a  carbohydrate. 

NUCLEO-PROTEINS. 

The  nucleo-proteins  are  obtained  from  the  nuclei  and  cell-sub- 
stance of  tissue-cells.  Chemically  they  are  characterized  by 
the  presence  of  phosphorus  in  relatively  large  amounts.  When 
hydrolysed,  they  separate  into  a  protein  and  a  nuclein.  The 
nucleins  derived  from  cell  nuclei  can  be  still  further  separated 
into  a  simpler  protein  and  nucleic  acid,  which  latter  in  turn 
yields  phosphoric  acid  and  the  so-called  purin  bases,  xanthin, 
hypoxanthin,  adenin,  and  guanin.  All  nucleins  which  yield 
the  purin  bases  are  termed  true  nucleins. 

PROTEIN  DERIVATIVES. 

The  proteins  of  this  group  are  derived  from  both  albumins  and 
globulins  by  the  gradual  action  of  dilute  acids  and  alkalies,  and  may 
be  regarded  as  compounds  of  a  proteid  with  an  acid  or  an  alkali. 
Thev  have  been  designated. 


20  TEXT-BOOK  OF  PHYSIOLOGY. 

INFRA-PROTEINS. 

(a)  Acid-albumin. — This  is  formed  when  a  native  albumin  is 
digested  with  dilute  hydrochloric  acid  (0.2  per  cent.)  or  dilute 
sulphuric  acid  for  some  minutes.  It  is  precipitated  by  neu- 
tralization with  sodium  hydroxid  (0.1  per  cent,  solution). 
After  the  precipitate  is  washed,  it  is  found  to  be  insoluble 
in  distilled  water  and  in  neutral  saline  solutions.  In  acid 
solutions  it  is  not  coagulated  by  heat. 

(b)  Alkali-albumin. — This  is  formed  when  a  native  albumin  is 
treated  with  a  dilute  alkali — e.  g.,  0.1  per  cent,  of  sodium 
hydroxid — for  five  or  ten  minutes.  On  careful  neutralization 
with  dilute  hydrochloric  acid,  it  is  precipitated.  It  is  also 
insoluble  in  distilled  water  and  in  saline  solutions;  it  is  not 
coagulable  by  heat. 

PROTEOSES  AND  PEPTONES. 

During  the  progress  of  the  digestive  process,  as  it  takes  place  in 
the  stomach  and  intestines,  there  is  produced  by  the  action  of  the 
gastric  and  pancreatic  juices,  out  of  the  proteins  of  the  food,  a  series 
of  new  proteins,  known  as  proteoses  and  peptones.  The  chemic 
properties  of  these  substances  will  be  considered  in  connection  with 
the  process  of  digestion. 

COAGULATED  PROTEINS. 

Although  these  proteins  are  not  found  as  constituents  of  the  animal 
organism,  they  possess  much  interest  on  account  of  their  relation  to 
prepared  foods  and  to  the  digestive  process.  They  are  produced 
when  solutions  of  egg-albumin,  serum-albumin,  or  globulins  are 
subjected  to  a  temperature  of  100 °  C.  or  to  the  prolonged  action  of 
alcohol.  They  are  insoluble  in  water,  in  dilute  acids,  and  in  neutral 
saline  solutions. 

In  this  same  group  may  be  included  also  those  coagulated  pro- 
teins which  are  produced  by  the  action  of  animal  ferments  on  soluble 
proteins — e.  g.,  fibrin,  myosin,  casein. 

(a)  Fibrin. — Fibrin  is  derived  from  a  soluble  protein — fibrinogen 
— by  the  action  of  a  special  ferment.  It  is  not  present  under 
normal  circumstances  in  the  circulating  blood,  but  makes  its 
appearance  after  the  blood  is  withdrawn  from  the  vessels 
and  at  the  time  of  coagulation.  It  can  also  be  obtained  by 
whipping  the  blood  with  a  bundle  of  twigs,  on  which  it  accu- 
mulates. When  freed  from  blood  by  washing  under  water, 
it  is  seen  to  consist  of  bundles  of  white  elastic  fibers  or  threads. 
It  is  insoluble  in  water,  in  alcohol,  and  ether.  In  dilute  acids 
it  swells,  becomes  transparent,  and  finally  is  converted  into 
acid-albumin.  In  dilute  alkalies  a  similar  change  takes  place, 
but  the  resulting  product  is  an  alkali-albumin.  Fibrin  pos- 
sesses  the   property   of  decomposing   hydrogen   dioxid,   H202 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  21 

— i.  e.,  liberating  oxygen,  which  accumulates  in  the  form  of 
bubbles  on  the  fibrin.  On  incineration  fibrin  yields  an  ash 
which  contains  calcium  phosphate  and  magnesium  phosphate. 

(b)  Myosin. — Myosin  develops  in  muscles  after  death  and  is  the 
cause  of  the  stiffening  of  the  muscles.  It  has  been  regarded 
as  a  derivative  of  the  soluble  protein  myosinogen  alone,  but 
there  is  evidence  that  in  its  formation  both  paramyosinogen 
and  myosinogen  take  part.  It  is  not  definitely  known  whether 
this  is  the  result  of  the  action  of  a  special  ferment  or  not. 

(c)  Casein. — Casein  is  derived  from  the  chief  protein  of  milk — 
caseinogen — by  the  action  of  a  special  ferment  known  as 
rennin  or  chymosin.  This  ferment  is  a  constituent  of  gas- 
tric juice. 

The  Color  Reactions  of  Proteins. — When  proteins  are  present  in 
solution,  they  may  be  detected  by  the  following  color  reactions — viz. : 

1.  Xanthoproteic.     The  solution  is  boiled  with  nitric  acid  for  several 

minutes,  when  the  proteid  assumes  a  light  yellow  color.  After 
the  solution  has  cooled,  the  addition  of  ammonia  changes  the 
color  to  an  orange  or  amber-red  due  to  the  presence  of  phenyl- 
alanin  and  tyrosin. 

2.  The   rose-red  reaction.     The  solution  is  boiled  with  acid  nitrate 

of  mercury  (Millon's  reagent)  for  a  few  minutes,  when  the  co- 
agulated proteid  turns  a  purple-red  color.  This  color  is  attributed 
to  the  presence  of  tyrosin. 

3.  The  blue- violet  reaction.     A  few  drops  of  copper  sulphate  solution 

arc  first  added  to  the  protein  solution,  and  then  an  excess  of 
sodium  hydroxid.     A  blue- violet  color  is  produced,  which  deepens 
somewhat  on  heating,  but  no  further  change  ensues.     This  is 
also  known  as  Piotrowski's  test:     As  this  same  color  is  developed 
with  the  substance  biuret,  it  is  also  known  as  the  biuret  reaction. 
Biuret  is  formed  by  heating  urea  and  driving  off  ammonia. 
Precipitation  Tests. — Proteins  in  solution  may  be  precipitated  by 
nitric  acid,  acetic  acid  and  potassium  ferrocyanid,  picric  acid,  copper 
sulphate,  tannin,  alcohol,  etc.     As  stated  in  a  foregoing  paragraph, 
certain  of  the  proteins,  e.  g.,  fibrinogen,  caseinogen  and  myosinogen, 
will  undergo,  by  the  action  of  an  animal  ferment,  a  cleavage  into  a 
solid  and  a  soluble  portion.     To  this  process  the  term  ferment  coagu- 
lation is  applied.     The  solidification  of  proteins  by  the  action  of  heat, 
is  designated  heat  coagulation. 


INORGANIC  CONSTITUENTS. 

The  inorganic  compounds  and  mineral  constituents  obtained 
from  the  solids  and  fluids  of  the  body  are  very  numerous,  and,  in 
some  instances,  quite  abundant.  Though  many  of  the  compounds 
thus  obtained  are  undoubtedly  derivatives  of  the  tissues  and  necessary 
to  their  physical  and  physiologic  activity,  others,  in  all  probability, 


22  TEXT-BOOK  OF  PHYSIOLOGY. 

are  decomposition  products,  or  transitory  constituents  introduced 
with  the  food.  Of  the  inorganic  compounds,  the  following  are  the 
most  important: 

WATER. 

Water  is  the  most  important  of  the  inorganic  constituents,  as  it 
is  indispensable  to  life.  It  is  present  in  all  the  tissues  and  fluids 
without  exception,  varying  from  99  per  cent,  in  the  saliva  to  80  per 
cent,  in  the  blood,  75  per  cent,  in  the  muscles  to  2  per  cent  in  the 
enamel  of  the  teeth.  The  total  quantity  contained  in  a  body  weigh- 
ing 75  kilograms  (165  pounds)  is  52.5  kilograms  (115  pounds).  Much 
of  the  water  exists  in  a  free  condition,  and  forms  the  chief  part  of  the 
fluids,  giving  to  them  their  characteristic  degree  of  fluidity.  Possess- 
ing the  capability  of  holding  in  solution  a  large  number  of  inorganic 
as  well  as  some  organic  compounds,  and  being  at  the  same  time  diffus- 
ible, it  renders  an  interchange  of  materials  between  all  portions  of  the 
body  possible.  It  aids  in  the  absorption  of  new  material  into  the 
blood  and  tissues,  and  at  the  same  time  it  transfers  waste  products 
from  the  tissues  to  the  blood,  from  which  they  are  finally  eliminated, 
along  with  the  water  in  which  they  are  dissolved.  A  portion  of  the 
water  is  chemically  combined  with  other  tissue  constituents  and  gives 
to  the  tissues  their  characteristic  physical  properties.  The  consistency, 
elasticity,  and  pliability  are,  to  a  large  extent,  conditioned  by  the 
amount  of  water  they  contain.  The  total  quantity  of  water  eliminated 
by  the  kidneys,  lungs,  and  skin  amounts  to  about  3  kilograms  (6| 
pounds). 

CALCIUM  COMPOUNDS. 

Calcium  phosphate,  Ca3(P04)2,  has  a  very  extensive  distribu- 
tion throughout  the  body.  It  exists  largely  in  the  bones,  teeth,  and 
to  a  slight  extent  in  cartilage,  blood,  and  other  tissues.  Milk  con- 
tains 0.27  per  cent.  The  solidity  of  the  bones  and  teeth  is  almost 
entirely  due  to  the  presence  of  this  salt,  and  is,  therefore,  to  be  re- 
garded as  necessary  to  their  structure.  It  enters  into  chemic  union 
with  the  organic  matter,  as  shown  by  the  fact  that  it  can  not  be  separated 
from  it  except  by  chemic  means,  such  as  hydrochloric  acid.  Though 
insoluble  in  water,  it  is  held  in  solution  in  the  blood  and  milk  by  the 
protein  constituents,  and  in  the  urine  by  the  acid  phosphate  of  soda. 
The  total  quantity  of  calcium  phosphate  which  enters  into  the  forma- 
tion of  the  body  has  been  estimated  at  2.5  kilograms.  The  amount 
eliminated  daily  from  the  body  has  been  estimated  at  0.4  gm.,  a  fact 
which  indicates  that  nutritive  changes  do  not  take  place  with  much 
rapidity  in  those  tissues  in  which  it  is  contained. 

Calcium  carbonate,  CaC03,  is  present  in  practically  the  same 
situations  in  the  body  as  the  phosphate,  and  plays  essentially  the 
same  role.  It  is,  however,  found  in  the  crystalline  form,  aggregated 
in  small  masses  in  the  internal  ear,  forming  the  otoliths,  or  ear  stones. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  23 

Though  insoluble,  it  is  held  in  solution  by  the  carbonic  acid  diffused 
through  the  fluids. 

Calcium  fluorid,  CaF2,  is  found  in  bones  and  teeth. 

SODIUM  COMPOUNDS. 

Sodium  chlorid,  NaCl,  is  present  in  all  the  tissues  and  fluids 
of  the  body,  but  especially  in  the  blood,  0.6  per  cent.,  lymph,  0.5, 
and  pancreatic  juice,  0.25  per  cent.  The  entire  quantity  in  the  body 
has  been  estimated  at  about  200  gm.  Sodium  chlorid  is  of  much 
importance  in  the  body  as  it  determines  and  regulates  to  a  large 
extent  the  phenomena  of  diffusion  which  are  there  constantly  taking 
place.  This  is  illustrated  by  the  fact  that  a  solution  of  albumin 
placed  in  the  rectum  without  the  addition  of  this  salt  will  not  be 
absorbed.  When  the  salt  is  added,  absorption  takes  place.  The 
ingested  water  is  absorbed  into  the  blood  largely  in  consequence  of 
the  percentage  of  this  salt  which  it  contains.  The  normal  percentage 
of  sodium  chlorid  in  the  blood-plasma  assists  in  maintaining  the  shape 
and  structure  of  the  red  blood- corpuscles  by  determining  the  amount 
of  water  entering  into  their  composition.  The  same  is  true  of  other 
tissue  elements. 

Sodium  chlorid  also  influences  the  general  nutritive  process,  in- 
creasing the  disintegration  of  the  proteins,  as  shown  by  the  increased 
amount  of  urea  excreted.  During  its  existence  in  the  body  it  under- 
goes chemic  transformations  or  decompositions,  yielding  its  chlorid 
to  form  the  potassium  chlorid  of  the  blood-corpuscles  and  muscles 
and  to  form  the  hydrochloric  acid  of  the  gastric  juice. 

Sodium  phosphate,  Na2HP04,  is  found  in  all  solids  and  fluids 
of  the  body,  to  which,  with  but  few  exceptions,  it  imparts  an  alkaline 
reaction.  This  is  especially  true  of  blood,  lymph,  and  tissue  fluids 
generally.  It  is  essential  to  physiologic  action  that  all  tissue  elements 
should  be  bathed  by  an  alkaline  medium. 

Sodium  carbonate,  Na2C03,  is  generally  found  in  association 
with  the  preceding  salt.  As  it  is  an  alkaline  compound,  it  also  assists 
in  giving  to  the  blood  and  lymph  their  characteristic  alkalinity.  In 
carnivorous  animals  the  sodium  phosphate  is  the  more  abundant, 
while  in  the  herbivorous  animals  the  sodium  carbonate  is  the  more 
abundant. 

Sodium  sulphate,  Na2S04,  is  present  in  many  of  the  tissues  and 
fluids,  especially  in  the  urine.  Though  introduced  in  the  food,  it  is 
also,  in  all  probability,  formed  in  the  body  from  the  decomposition 
and  oxidation  of  the  proteids. 

POTASSIUM  COMPOUNDS. 

Potassium  chlorid,  KC1,  is  met  with  in  association  with  sodium 
chlorid  in  almost  all  situations  in  the  body.  It  preponderates,  how- 
ever, in  the  tissue  elements,  especially  in  the  muscle  tissue,  nerve 
tissue,  and  red  corpuscles.     The  plasma  with  which  these  structure? 


24  TEXT-BOOK  OF  PHYSIOLOGY. 

are  bathed  contains  but  a  very  small  amount  of  this  salt,  but,  as 
previously  stated,  a  relatively  large  quantity  of  sodium  chlorid.  Though 
introduced  to  some  extent  in  the  food,  it  is  very  likely  that  it  is  also 
formed  through  the  decomposition  of  the  sodium  chlorid. 

Potassium  phosphate,  K2HP04,  is  found  in  association  with 
sodium  phosphate  in  all  the  fluids  and  solids.  As  it  has  similar 
chemic  properties,  its  functions  are  practically  the  same. 

Potassium  carbonate,  K2C03,  is  generally  found  with  the  pre- 
ceding salt. 

MAGNESIUM  COMPOUNDS. 

Magnesium  phosphate,  Mg3(P04)2,  is  found  in  all  tissues,  in 
association  with  calcium  phosphate,  though  in  much  smaller  quantity. 

Magnesium  carbonate,  MgC03,  occurs  only  in  traces  in  the 
blood. 

Both  of  these  compounds  have  functions  similar  to  the  calcium 
compounds,  and  exist,  in  all  probability,  under  similar  conditions. 

IRON  COMPOUNDS. 

Iron  is  a  constituent  of  the  coloring-matter  of  the  blood.  Traces, 
however,  are  also  found  in  lymph,  bile,  gastric  juice,  and  in  the  pig- 
ment of  the  eyes,  skin  and  hair.  The  amount  of  iron  contained  in  a 
body  weighing  75  kilograms  is  about  3  gm.  It  exists  under  various 
forms — e.  g.,  ferric  oxid,  and  in  combination  with  organic  compounds. 

Chemic  analysis  thus  shows  that  the  chemic  elements  into  which 
the  compounds  may  be  resolved  by  an  ultimate  analysis  do  not  exist 
in  the  body  in  a  free  state,  but  only  in  combination,  and  in  char- 
acteristic proportions,  to  form  compounds  whose  properties  are  the 
resultant  of  those  of  the  elements.  Of  the  four  principal  elements 
which  make  up  97  per  cent,  of  the  body,  O,  H,  N  are  extremely  mobile, 
elastic,  and  possessed  of  great  atomic  heat.  C,  H,  N  are  distinguished 
for  the  narrow  range  of  their  affinities,  and  for  their  chemic  inertia. 
C  possesses  the  great  atomic  cohesion.  O  is  noted  for  the  number  and 
intensity  of  its  combinations. 

As  the  properties  of  the  compounds  formed  by  the  union  of  ele- 
ments must  be  the  resultants  of  the  properties  of  the  elements  them- 
selves, it  follows  that  the  ternary  compounds,  starches,  sugars,  and 
fats  must  possess  more  or  less  inertia,  and  at  the  same  time  instability; 
while  in  the  more  complex  proteids,  in  which  sulphur  and  phosphorus 
are  frequently  combined  with  the  four  principal  elements,  molecular 
instability  attains  its  maximum.  As  all  the  foregoing  compounds 
possess  in  varying  degrees  the  properties  of  inertia  and  instability, 
it  follows  that  living  matter  must  possess  corresponding  properties, 
and  the  capability  of  undergoing  unceasingly  a  series  of  chemic  changes, 
both  of  composition  and  decomposition,  in  response  to  the  chemic  and 
physical  influences  by  which  it  is  surrounded,  and  which  underlie  all 
the  phenomena  of  life. 


CHEMICAL  COMPOSITION  OF  THE  HUMAN  BODY. 


25 


PRINCIPLES  OF  DISSIMILATION. 

In  addition  to  the  previously  mentioned  compounds— viz.,  carbo- 
hydrates, fats,  proteids,  and  inorganic  salts— there  is  obtained  bv 
chemic  analysis  from  the  tissues  and  fluids  of  the  body: 

1.  A  number  of  organic  acids,  such  as  acetic,  lactic,  oxalic,  butyric, 

propionic,  etc.,  in  combination  with  alkaline  and  earthy  bases. 

2.  Organic  compounds,  such  as  alcohol,  glycerin,  cholesterin. 

3.  Pigments,  such  as  those  found  in  bile  and  urine. 

4.  Crystallizable  nitrogenized  bodies,  such  as  urea,  uric  acid,  xanthin, 

hippuric  acid,  creatin,  creatinin,  etc. 
While  some  few  of  these  compounds  may  possibly  be  regarded  as 
necessary  to  the  physiologic  integrity  of  the  tissues  and  fluids,  the 
majority  of  them  are  to  be  regarded  as  products  of  dissimilation  of 
the  tissues  and  foods  in  consequence  of  functional  activity,  and  rep- 
resent stages  in  their  reduction  to  simpler  forms  previous  to  bein<r 
eliminated  from  the  bod  v. 


CHAPTER  III. 
PHYSIOLOGY  OF  THE  CELL. 

A  microscopic  analysis  of  the  tissues  shows  that  they  can  be  re- 
solved into  ultimate  elements,  termed  cells,  which  may,  therefore, 
be  regarded  as  the  primary  units  of  structure.  Though  cells  vary 
considerably  in  shape,  size,  and  chemic  composition  in  the  different 
tissues  of  the  adult  body,  they  are,  nevertheless,  descendants  from 
typical  cells,  known  as  embryonic  or  undifferentiated  cells,  examples 
of  which  are  the  leukocytes  of  the  blood  and  lymph  and  the  first 
offspring  of  the  fertilized  ovum.  Ascending  the  fine  of  embryonic 
development,  it  will  be  found  that  every  organized  body  originates 
in  a  single  cell — the.  ovum.  As  the  cell  is  the  elementary  unit  of  all 
tissues,  the  function  of  each  tissue  must  be  referred  to  the  function 
of  the  cell.  Hence  the  cell  may  be  defined  as  the  primary  anatomic 
and  physiologic  unit  of  the  organic  world,  to  which  every  exhibition 
of  life,  whether  normal  or  abnormal,  is  to  be  referred. 

Structure  of  Cells. — Though  cells  vary  in  shape  and  size  and 
internal  structure  in  different  portions  of  the  body,  a  typical  cell  may 
be  said  to  consist  mainly  of  a  gelatinous  substance  forming  the  body 
of  the  cell,  termed  protoplasm  or  bioplasm,  in  which  is  embedded  a 
smaller  spheric  body,  the  nucleus.  The  shape  of  the  adult  cell  varies 
according  to  the  tissue  in  which  it  is  found;  when  young  and  free 
to  move  in  a  fluid  medium,  the  cell  assumes  a  spheric  form,  but  when 
subjected  to  pressure,  may  become  cylindric,  fusiform,  polygonal, 
or  stellate.  Cells  vary  in  size  within  wide  limits,  ranging  from  7.7^ 
(3TF0"  °f  an  inch,  the  diameter  of  a  red  blood-corpuscle),  to  135/*  (3-3-5- 
of  an  inch,  the  diameter  of  the  large  cells  in  the  gray  matter  of  the 
spinal  cord).     (See  Fig.  2.) 

The  cell  protoplasm  consists  of  a  soft,  semifluid,  gelatinous 
material,  varying  somewhat  in  appearance  in  different  tissues. 
Though  frequently  homogeneous,  it  often  exhibits  a  finely  granular 
appearance  under  medium  powers  of  the  microscope.  Young  cells 
consist  almost  entirely  of  clear  protoplasm.  Mature  cells  contain, 
according  to  the  tissue  in  which  they  are  found,  material  of  an  en- 
tirely different  character — e.  g.,  small  globules  of  fat,  granules  of 
glycogen,  mucigen,  pigments,  digestive  ferments,  etc.  Under  high 
powers  of  the  microscope  the  cell  protoplasm  is  found  to  be  pervaded 
by  a  network  of  fibers,  termed  spongioplasm,  in  the  meshes  of  which 
is  contained  a  clearer  and  more  fluent  substance,  the  hyaloplasm. 
The  relative  amount  of  these  two  constituents  varies  in  different 
cells,  the  proportion  of  hyaloplasm  being  usually  greater  in  young 

26 


PHYSIOLOGY  OF  THE  CELL. 


27 


cells.  The  arrangement  of  the  fibers  forming  the  spongioplasm  also 
varies,  the  fibers  having  sometimes  a  radial  direction,  in  others  a 
concentric  disposition,  but  most  frequently  being  distributed  evenly 
in  all  directions.  In  many  cells  the  outer  portion  of  the  cell  proto- 
plasm undergoes  chemic  changes  and  is  transformed  into  a  thin, 
transparent,  homogeneous  membrane— the  cell  membrane — which 
completely  incloses  the  cell  substance.  The  cell  membrane  is  per- 
meable to  water  and  watery  solutions  of  various  inorganic  and  organic 
substances.     It  is,  however,  not  an  essential  part  of  the  cell. 

The  nucleus  is  a  small  vesicular  body  embedded  in  the  proto- 
plasm near  the  center  of  the  cell.  In  the  resting  condition  of  the  cell 
it  consists  of  a  distinct  membrane,  composed  of  amphipyrenin,  in- 
closing the  nuclear  contents.     The  latter  consists  of  a  homogeneous 


Nuclear  membrane.    , 


Linin. 


Nuclear  fluid  (matrix).  - 


Nucleolus. 


Chromatin-cords 
(nuclear  network). 


Nodal  enlargements  _ 
o£  the  chromatin. 


"  — Cell  membrane. 
Exoplasm. 

Microsomes. 
Centrosoma. 

~  "  Spongioplasm. 
— -   Hyaloplasm. 

Foreign  inclosures. 


Fig.  2. — Diagram  of  a  Cell.     Microsomes  and  spongioplasm  are  onlv  partly  drawn. 
—{Stohr.) 


amorphous  substance — the  nuclear  matrix — in  which  is  embedded 
the  nuclear  network.  It  can  often  be  seen  that  a  portion  of  one 
side  of  the  nucleus,  called  the  pole,  is  free  from  this  network.  The 
main  cords  of  the  network  are  arranged  as  V-shaped  loops  about  it. 
These  main  cords  send  out  secondary  branches  or  twigs,  which, 
uniting  with  one  another,  complete  the  network.  The  nuclear  cords 
are  composed  of  granules  of  chromatin — so  called  because  of  its 
affinity  for  certain  staining  materials — held  together  by  an  achromatin 
substance  known  as  linin.  Besides  the  nuclear  network,  there  are 
embedded  in  the  nuclear  matrix  one  or  more  small  bodies  composed 
of  pyrenin,  known  as  nucleoli.  At  the  pole  of  the  nucleus,  either 
within  or  just  without  in  the  protoplasm,  is  a  small  body,  the  centro- 
som-e,  or  pole  corpuscle. 

Chemic  Composition  of   the  Cell. — The  composition  of  living 


28  TEXT-BOOK  OF  PHYSIOLOGY. 

bioplasm  is  difficult  of  determination,  for  the  reason  that  all  chemic 
and  physical  methods  employed  for  its  analysis  destroy  its  vitality, 
and  the  products  obtained  are  peculiar  to  dead  rather  than  to  living 
matter.  Moreover,  as  bioplasm  is  the  seat  of  extensive  chemic 
changes,  it  is  not  easy  to  determine  whether  the  products  of  analysis 
are  crude  food  constituents  or  cleavage  or  disintegration  products. 
Nevertheless,  chemic  investigations  have  shown  that  even  in  the  living 
condition  bioplasm  is  a  highly  complex  compound — the  resultant 
of  the  intimate  union  of  many  different  substances.  About  75  per 
cent,  of  bioplasm  consists  of  water  and  25  per  cent,  of  solids,  of  which  the 
more  important  compounds  are  various  nucleo-proteins  (characterized 
by  their  large  percentage  of  phosphorus),  globulins,  traces  of  lecithin, 
cholesterin,  and  possibly  fat  and  carbohydrates.  Inorganic  salts,  es- 
pecially the  potassium,  sodium,  and  calcium  chlorids  and  phosphates, 
are  almost  invariable  and  essential  constituents. 


MANIFESTATIONS  OF  CELL  LIFE. 

Growth,  Nutrition,  and  Reproduction. — All  cells  exhibit  the 
three  fundamental  properties  of  life — viz.,  growth,  nutrition,  repro- 
duction. Growth  is  an  increase  in  size.  When  newly  reproduced  all 
cells  are  extremely  small,  but  in  consequence  of  their  organization  and 
the  character  of  their  surrounding  medium,  they  gradually  grow 
until  they  attain  the  size  characteristic  of  the  adult  state. 

Nutrition  is  the  maintenance  of  the  physiologic  condition  of  the 
cell  and  includes  both  growth  and  repair.  So  long  as  this  is  accom- 
plished, the  cells  and  the  tissues  which  are  formed  by  them  continue 
to  exhibit  their  functions  or  their  characteristic  modes  of  activity. 
Both  growth  and  nutrition  are  dependent  on  the  power  which  living 
material  possesses  of  not  only  absorbing  nutritive  material  from  the 
surrounding  medium,  the  lymph,  but  of  subsequently  assimilating  it, 
organizing  it,  transforming  it  into  material  like  itself  and  endowing 
it  with  its  own  physiologic  properties. 

In  the  physiologic  condition  the  living  material  of  the  cell,  the 
bioplasm,  is  the  seat  of  a  series  of  chemic  changes  which  vary  in 
degree  from  moment  to  moment  in  accordance  with  the  degree  of 
functional  activity,  and  on  the  continuance  of  which  all  life  phenomena 
depend.  Some  of  these  chemic  changes  are  related  to  or  connected 
with  the  molecules  of  the  living  material,  while  others  are  connected 
with  the  food  material  supplied  to  them.  Of  the  chemic  changes 
occurring  within  the  molecules  some  are  destructive,  dissimilative  or 
disintegrative  in  character,  whereby  the  molecule  is  in  part  eventually 
reduced  through  a  series  of  descending  chemic  stages  to  simpler  com- 
pounds which,  apparently  of  no  use  in  the  cell,  are  eliminated  from 
it.  It  is,  therefore,  said  that  the  living  material  undergoes  molecular 
disintegration  as  a  result  of  functional  activity.  To  these  changes 
the  term  katabolism  is  also  applied.     Other  of  these  changes  are  con- 


PHYSIOLOGY  OF  THE  CELL.  29 

structive,  assimilative  or  integrative  in  character,  whereby  a  part  at 
least  of  the  food  material  is  transformed  through  a  series  of  ascending 
chemic  stages  into  living  material,  and  whereby  it  is  repaired  and  its 
former  physiologic  condition  restored.  It  is,  therefore,  said  that  the 
living  material  undergoes  molecular  integration  as  a  preparation  for 
functional  activity.  To  these  changes  the  term  anabolism  is  also 
applied.  The  sum  total  of  all  chemic  changes  which  go  on  in  the  cell, 
both  anabolic  and  katabolic,  is  embraced  in  the  general  term  metab- 
olism. During  the  course  of  its  physiologic  activities  the  cell  bioplasm 
produces  materials  of  an  entirely  different  character  which  vary  with 
the  cell,  such  as  fat,  glycogen,  mucigen,  pigments,  ferments,  etc., 
which  are  generally  spoken  of  as  metabolic  products. 

The  food  materials,  sugar,  fat  and  proteins,  which  are  constituents 
of  the  fluids  circulating  in  the  tissue  spaces,  as  above  stated,  are  also 
reduced  to  simpler  forms  and  the  energy  which  they  contain  liberated 
in  the  form  of  heat.  It  is,  therefore,  said  that  the  food  also  undergoes 
metabolism. 

There  is,  however,  much  difference  of  opinion  as  to  the  extent  to 
which  the  living  material  is  metabolized  and  to  the  actual  disposition  of 
the  food  materials,  and  especially  the  proteins.  Thus  Voit  contends 
that  the  tissue  molecules  are  comparatively  stable  in  composition  and 
under  ordinary  conditions  of  nutrition  do  not  undergo  any  material 
change  during  either  rest  or  activity,  and  that  metabolism  is  confined 
to  the  food  materials  occupying  spaces  in  and  around  the  living  cell. 
The  cause  which  initiates  this  metabolism  is  unknown,  but  is  supposed 
to  reside  in  the  cell,  if  it  is  not  a  property  of  the  cell  itself.  Because 
of  the  fact  that  but  a  very  small  amount,  if  any,  of  sugar  or  fat  enters 
into  the  composition  of  bioplasm  it  is  generally  admitted  that  these 
foods  are  metabolized  in  the  tissue  spaces  and  in  the  manner  just 
alluded  to.  The  problem,  however,  is  different  in  the  case  of  the 
proteins.  Voit  contends,  as  previously  stated,  that  the  proteins  of 
the  tissue  molecules,  which  he  distinguishes  as  tissue  proteins,  do 
not  metabolize  and  confines  all  protein  metabolism  to  the  food  pro- 
teins circulating  in  the  tissue  spaces  and  which  he  distinguishes  as 
circulating  proteins.  Even  in  starvation  the  tissue  proteins,  as  such, 
do  not  metabolize  until  they  have  been  dissolved  in  consequence  of 
chemic  changes  and  transformed  into  circulating  protein. 

Pfltiger,  however,  asserts  that  the  circulating  proteins  can  not 
be  metabolized  but  that  they  must  first  be  built  up  into  tissue  proteins. 
The  metabolism  of  protein  is,  therefore,  confined,  in  this  view,  to  the 
molecules  of  the  living  material.  It  is  possible,  however,  that  both 
views  are  correct  and  that  in  the  physiologic  condition  the  activity  of 
the  tissues  is  attended  by  a  partial  destruction  of  the  tissue  molecules 
which  is  followed  in  turn  by  construction  during  the  subsequent  rest, 
but  that  the  greater  part  of  the  protein  metabolism  takes  place  out- 
side the  cell,  though  in  contact  with  it. 

Though  the  cell  is,  therefore,  the  seat  of  two  opposing  processes, 


3o  TEXT-BOOK  OF  PHYSIOLOGY. 

assimilation  and  dissimilation,  it  retains  under  normal  conditions  an 
average  physiologic  state,  and  so  long  as  this  is  the  case  it  is  in  a  con- 
dition of  nutritive  equilibrium  and  capable  of  performing  its  various 
functions. 

Though  the  foregoing  statements  are  applied  to  the  individual  cell 
they  are  equally  applicable  to  the  body  as  a  whole,  inasmuch  as  the 
organs  and  tissues  of  which  it  consists  are  composed  of  cells.  The 
body  grows  in  size  and  maintains  its  nutrition,  by  the  introduction 
of  food  materials  which  are  utilized  in  part,  for  the  repair  of  the  tissues 
which  have  undergone  molecular  disintegration  in  consequence  of 
activity,  and  in  part  for  the  liberation  of  energy.  As  a  result  of  the 
disintegration  or  the  metabolism  of  tissue  and  food  materials,  products 
such  as  carbon  dioxid,  urea,  etc.,  are  formed  which,  apparently  of  no 
further  use,  are  discharged  from  the  body  by  eliminating  organs  as 
the  kidney,  lungs,  skin,  etc.  Assimilation  and  dissimilation  are 
constantly  taking  place.  If  the  food  assimilated  and  metabolized 
exactly  replaces  the  tissues  dissimilated  and  the  food  metabolized  the 
body  will  retain  a  condition  of  nutritive  equilibrium. 

Physiologic  Properties  of  Bioplasm. — All  living  bioplasm 
possesses  properties  winch  serve  to  distinguish  and  characterize  it — 
viz.,  irritability,  conductivity,  and  motility. 

Irritability,  or  the  power  of  reacting  in  a  definite  manner  to  some 
form  of  external  excitation,  whether  mechanic,  chemic,  or  electric, 
is  a  fundamental  property  of  all  living  bioplasm.  The  character 
and  extent  of  the  reaction  will  vary,  and  will  depend  both  on  the 
nature  of  the  bioplasm  and  the  character  and  strength  of  the  stimulus. 
If  the  bioplasm  be  muscle,  the  response  will  be  a  contraction ;  if  it  be 
gland,  the  response  will  be  a  secretion;  if  it  be  nerve,  the  response  will 
be  a  sensation  or  some  other  form  of  nerve  activity. 

Conductivity,  or  the  power  of  transmitting  molecular  disturbances 
arising  at  one  point  to  all  portions  of  the  irritable  material,  is  also 
a  characteristic  feature  of  all  bioplasm.  This  power,  however,  is 
best  developed  in  that  form  of  bioplasm  found  in  nerves,  which  serves 
to  transmit,  with  extreme  rapidity,  molecular  disturbances  arising  at 
the  periphery  to  the  brain,  as  well  as  from  the  brain  to  the  periphery. 
Muscle  bioplasm  also  possesses  the  same  power  in  a  high  degree. 

Motility,  or  the  power  of  executing  apparently  spontaneous  move- 
ments, is  exhibited  by  many  forms  of  cell  bioplasm.  In  addition  to 
the  molecular  movements  which  take  place  in  certain  cells,  other  forms 
of  movement  are  exhibited,  more  or  less  constantly,  by  many  cells 
in  the  animal  body — e.  g.,  the  waving  of  cilia,  the  ameboid  movements 
and  migrations  of  white  blood-corpuscles,  the  activities  of  spermato- 
zooids,  the  projection  of  pseudopodia,  etc.  These  movements, 
arising  without  any  recognizable  cause,  are  frequently  spoken  of  as 
spontaneous.  Strictly  speaking,  however,  all  protoplasmic  movement 
is  the  resultant  of  natural  causes,  the  true  nature  of  which  is  beyond 
the  reach  of  present  methods  of  investigation. 


PHYSIOLOGY  OF  THE  CELL. 


3i 


Reproduction. — Cells  reproduce  themselves  in  the  higher  ani- 
mals in  two  ways — by  direct  division  and  by  indirect  division,  or 
karyokinesis.  In  the  former  the  nucleus  becomes  constricted,  and 
divides  without  any  special  grouping  of  the  nuclear  elements.  It  is 
probable  that  this  occurs  only  in  disintegrating  cells,  and  never  in 
a  physiologic  multiplication.  In  division  by  karyokinesis  (Fig.  3) 
there  is  a  progressive  rearranging  and  definite  grouping  of  the  nucleus, 
the  result  of  which  changes  is  the  division  of  the  centrosome,  the 
chromatin,  and  the  rest  of  the  nucleus  into  two  equal  portions,  which 


Close  Skein  Loose  Skein  (viewed 

(viewed  from  from  above — i.  e.,  from 

the  side).  the  pole).  Mother  Stars  (viewed  from  the  side). 

Polar  field. 


Spindle. 


Mother  Star  (viewed      Daughter  Star 
from  above). 


?-r>v. 


IjPPJP 


Beginning  Completed 

Division  of  the  Protoplasm. 


Fig.  3. — Karyokinetic  Figures  Observed  in  the  Epithelium  of  the  Oral 
Cavity  of  a  Salamander.  The  picture  in  the  upper  right-hand  corner  is  from  a  section 
through  a  dividing  egg  of  Siredon  pisciformis.  Neither  the  centrosomes  nor  the  first 
stages  of  the  development  of  the  spindle  can  be  seen  by  this  magnification.  X  560. — 
(Stohr.) 


form  the  nuclei.  Following  the  division  of  the  nuclei,  the  protoplasm 
divides.  The  process  may  be  divided  into  three  phases: 
1.  Prophase. — The  centrosome,  at  first  small  and  lying  within  the 
nucleus,  increases  in  size  and  moves  into  the  protoplasm,  where 
it  lies  near  the  nucleus,  surrounded  by  a  clear  zone,  from  winch 
delicate  threads  radiate  through  an  area  known  as  the  attraction 
sphere.  The  nucleus  enlarges  and  becomes  richer  in  chromatin. 
The  lateral  twigs  of  the  chromatin  cords  are  drawn  in,  while 
the  main  cords  become  much  contorted.  These  cords  have  a 
general  direction  transverse  to  the  long  axis  of  the  cell,  and 
parallel  to  the  plane  of  future  cleavage.  They  are  seen  as  V- 
shaped  segments  or  loops,  chromosomes,  having  their  closed  ends 
directed  toward  a  common  center,  the  polar  field,  while  the  other 


32  TEXT-BOOK  OF  PHYSIOLOGY. 

ends  interdigitate  on  the  opposite  side  of  the  nucleus — the  anti- 
pole. The  polar  field  corresponds  to  the  area  occupied  by  the  cen- 
trosome.  This  arrangement  is  known  as  the  close  skein;  but  as 
the  process  goes  on,  the  chromosomes  become  thicker,  shorter 
and  less  contorted,  producing  a  much  looser  arrangement,  known 
as  the  loose  skein.  During  the  formation  of  the  loose  skein,  the 
centrosome  divides  into  two  portions,  which  move  apart  to  posi- 
tions at  the  opposite  ends  of  the  long  axis  of  the  nucleus.  At  the 
same  time  delicate  achromatin  fibers  make  their  appearance, 
arranged  in  the  form  of  a  double  cone,  the  apices  of  which  corre- 
spond in  position  to  the  centrosomes.  This  is  known  as  the 
nuclear  spindle.  During  the  prophase  the  nuclear  membrane  and 
the  nucleoli  disappear. 

2.  The   Metaphase. — The   two  centrosomes  are  at  opposite  ends  of 

the  long  axis  of  the  nucleus,  each  surrounded  by  an  attraction 
sphere,  now  called  the  polar  radiation.  The  chromosomes 
become  yet  shorter  and  thicker,  and  move  toward  the  equator 
of  the  nucleus,  where  they  lie  with  their  closed  ends  toward  the 
axis,  presenting  the  appearance,  when  seen  from  the  poles,  of 
a  star — the  so-called  mother  star,  or  monaster.  While  moving 
toward  the  equator  of  the  nucleus,  and  often  earlier,  each  chromo- 
some undergoes  longitudinal  cleavage,  the  sister  loops  remaining 
together  for  a  time.  Upon  the  completion  of  the  monaster,  one  loop 
of  each  pair  passes  to  each  pole  of  the  nucleus,  guided,  and  per- 
haps drawn  by  the  threads  of  the  nuclear  spindle.  The  separation 
of  the  sister  segments  begins  at  their  apices,  and  as  the  open  ends 
are  drawn  apart  they  remain  connected  by  delicate  achromatin 
filaments  drawn  out  from  the  chromosomes.  This  separation  of  the 
daughter  chromosomes,  and  their  movement  toward  the  daughter 
centrosomes,  is  called  metakinesis.  As  they  approach  their  des- 
tination, we  have  the  appearance  of  two  stars  in  the  nucleus — the 
daughter  stars,  or  diasters. 

3.  Anaphase. — The  daughter  stars  undergo,  in  reverse  order,  much 

the  same  changes  that  the  mother  star  passed  through.  The  chro- 
mosomes become  much  convoluted,  and  perhaps  united  to  one 
another,  the  lateral  twigs  appear,  and  the  chromatin  resumes  the 
appearance  of  the  resting  nucleus.  The  nuclear  spindle,  with 
most  of  the  polar  radiation,  disappears,  and  the  nucleoli  and  the 
nuclear  membrane  reappear,  thus  forming  two  complete  daughter 
nuclei.  Meanwhile  the  protoplasm  becomes  constricted  midway 
between  the  young  nuclei.  This  constriction  gradually  deepens 
until  the  original  cell  is  divided,  with  the  formation  of  two  complete 
cells. 


CHAPTER  IV. 

HISTOLOGY  OF  THE  EPITHELIAL  AND  CONNECTIVE 

TISSUES. 

i.  EPITHELIAL  TISSUE. 

The  epithelial  tissue  consists  of  one  or  more  layers  of  cells  resting 
on  a  homogeneous  membrane,  the  other  side  of  which  is  abundantly 
supplied  with  blood-vessels  and  nerves.  The  form  of  the  epithelial 
cell  varies  in  different  situations,  and  may  be  flattened,  cuboid,  sphe- 
roid, or  columnar.  (See  Figs.  4,  5,  and  6.)  The  form  of  the  cell  in 
all  instances  is  related  to  some  specific  function.  When  arranged  in 
layers  or  strata,  the  cells  are  cemented  together  by  an  intercellular 
substance. 


Fig.  4. — Epithelial  Cells  of  Rabbit,  Isolated.  X  560  1.  Squamous  cells 
(mucous  membrane  of  mouth).  2.  Columnar  cells  (corneal  epithelium).  3.  Colum- 
nar cells,  with  cuticular  border,  5  (intestinal  epithelium).  4.  Ciliated  cells;  h,  cilia 
(bronchial  epithelium). — (Stohr.) 


The  epithelial  tissue  forms  a  continuous  covering  for  the  surfaces  of 
the  body.  The  external  investment  (the  skin)  and  the  internal 
investment  (the  mucous  membrane,  which  lines  the  entire  alimentary 
canal  as  well  as  associated  body  cavities)  are  both  formed,  in  all 
situations,  by  the  homogeneous  basement  membrane,  covered  with 
one  or  more  layers  of  cells.  The  glands  of  the  skin,  the  lungs  and  the 
glands  in  connection  with  the  alimentary  canal  and  the  uro-genital 
apparatus  are  formed  of  the  same  elemental  structures.  All  materials, 
therefore,  whether  nutritive,  secretory,  or  excretory,  must  pass  through 
epithelial  cells  before  they  can  enter  into  the  formation  of  the  blood 
or  be  eliminated  from  it.  The  nutrition  of  the  epithelial  tissue  is 
maintained  by  the  nutritive  material  derived  from  the  blood  diffusing 
itself  into  and  through  the  basement  membrane.  Chemically,  the 
epithelial  cells  of  the  epidermis — hair,  nails,  etc. — are  composed  'of 

3  33 


34 


TEXT-BOOK  OF  PHYSIOLOGY. 


an  albuminoid  material  (keratin),  a  small  quantity  of  water,  and 
inorganic  salts.  In  other  situations,  especially  on  the  mucous  mem- 
branes, the  cells  consist  largely  of  mucin,  in  association  with  other 
proteins.  The  consistency  of  epithelium  varies  in  accordance  with 
external  influences,  such  as  the  presence  or  absence  of  moisture,  pres- 
sure, friction,  etc.  This  is  well  seen  in  the  skin  of  the  palms  of  the 
hands  and  the  soles  of  the  feet — situations  where  it  acquires  its  greatest 
density.  In  the  alimentary  canal,  in  the  lungs,  and  in  other  cavities, 
where  the  reverse  conditions  prevail,  the  epithelium  is  extremely  soft. 
Epithelial  tissues  also  possess  varying  degrees  of  cohesion  and  elasticity 
— physical  properties  which  enable  them  to  resist  considerable  pressure 
and  distention  without  having  their  physiologic  integrity  destroyed. 
Inasmuch  as  these  tissues  are  poor  conductors  of  heat,  they  assist  in 

preventing  too  rapid  radiation  of 
H  heat  from  the  body,  and  cooperate 

with  other  mechanisms  in  main- 
~  taining  the  normal  temperature. 


0    ®_; 


Fig.  5. —  Stratified  Squamous 
Epithelium  (Larynx  of  Man). 
X  240.  1.  Columnar  cells.  2.  Prickle- 
cells.     3.  Squamous  cells. — (Stohr.) 


Fig.  6.  —  Stratified  Ciliated 
Epithelium.  X  560.  From  the  res- 
piratory nasal  mucous  membrane 
of  man.  1.  Oval  cells.  2.  Spindle- 
shaped  cells.  3.  Columnar  cells. — 
(Stohr.) 


The  physiologic  activity  of  all  epithelial  tissue  depends  on  a  due  supply  of 
nutritive  material  derived  from  the  blood,  which  not  only  maintains 
its  nutrition,  but  affords  those  materials  out  of  which  are  formed  the 
secretions  of  the  glands,  whether  of  the  skin  or  mucous  membrane. 

The  cells  lining  the  blood-vessels,  the  lymph- vessels,  the  peritoneal, 
pleural,  pericardial,  and  other  closed  cavities  are  usually  termed 
endothelial  cells.  These  cells  are  flat,  irregular  in  shape,  with  borders 
more  or  less  wavy  or  sinuous  in  outline. 

Functions  of  Epithelial  Tissue. — In  succeeding  chapters  the 
form,  chemic  composition,  and  functions  of  epithelial  cells  will  be 
considered  in  connection  with  the  functions  of  the  organs  of  which 
they  constitute  a  part.  In  this  connection  it  may  be  stated  in  a  general 
way  that  the  functions  of  the  epithelial  tissues  are: 
1 .  To  serve  on  the  surface  of  the  body  as  a  protective  covering  to  the 


THE  CONNECTIVE  TISSUES.  35 

underlying  structures  which  collectively  form  the  true  skin,  thus 
protecting  them  from  the  injurious  influences  of  moisture,  air, 
dust,  microorganisms,  etc.,  which  would  otherwise  impair  their 
vitality.  Wherever  continuous  pressure  is  applied  to  the  skin, 
as  on  the  palms  of  the  hands  and  soles  of  the  feet,  the  epithelium 
increases  in  thickness  and  density,  and  thus  prevents  undue 
pressure  on  the  nerves  of  the  true  skin.  The  density  of  the 
epidermis  enables  it  to  resist,  within  limits,  the  injurious  influence 
of  acids,  alkalies,  and  poisons. 

2.  To  promote  absorption.     Inasmuch  as  the  skin  and  mucous  mem- 

branes cover  the  surfaces  of  the  body,  it  is  obvious  that  all  nutritive 
material  entering  the  body  must  first  traverse  the  epithelial 
tissue.  Owing  to  their  density,  however,  the  epithelial  cells 
covering  the  skin  play  but  a  feeble  role  as  absorbing  agents  in  man 
and  the  higher  animals.  The  epithelium  of  the  mucous  mem- 
brane of  the  alimentary  canal,  particularly  that  of  the  small 
intestine,  is  especially  adapted,  from  its  situation,  consistency, 
and  properties,  to  play  the  chief  role  in  the  absorption  of  new 
materials  from  the  canal.  The  epithelium  lining  the  air-vesicles 
of  the  lungs  is  engaged  in  promoting  the  absorption  of  oxygen  and 
the  exhalation  of  carbon  dioxid. 

3.  To    form   secretions  and  excretions.     Each  secretory  gland  con- 

nected with  the  surfaces  of  the  body  is  lined  by  epithelial  cells, 
which  are  actively  concerned  in  the  formation  of  the  secretion 
peculiar  to  the  gland.  Each  excretory  organ  is  similarly  provided 
with  epithelial  cells,  which  are  engaged  either  in  the  production 
of  the  constituents  of  the  excretion  or  in  their  removal  from  the 
blood. 

2.  THE  CONNECTIVE  TISSUES. 

The  connective  tissues,  in  their  collective  capacity,  constitute 
a  framework  which  pervades  the  body  in  all  directions,  and,  as  the 
name  implies,  serve  as  a  bond  of  connection  between  the  individual 
parts,  at  the  same  time  affording  a  basis  of  support  for  the  muscle, 
nerve,  and  gland  tissues.  The  connective-tissue  group  includes  a 
number  of  varieties,  among  which  may  be  mentioned  the  areolar, 
adipose,  retiform,  white  fibrous,  yellow  elastic,  cartilaginous  and 
osseous.  Notwithstanding  their  apparent  diversity,  they  possess 
many  points  of  similarity.  They  have  a  common  origin,  developing 
from  the  same  embryonic  material;  they  have  much  the  same  struc- 
ture, passing  imperceptibly  into  one  another,  and  perform  practically 
the  same  functions. 

Areolar  Tissue. — This  variety  is  found  widely  distributed  through- 
out the  body.  It  serves  to  unite  the  skin  and  mucous  membrane  to 
the  structures  on  which  they  rest;  to  form  sheaths  for  the  support 
of  blood-vessels,  nerves,  and  lymphatics;  to  unite  into  compact  masses 
the  muscular  tissue  of  the  body,  etc.     Examined  with  the  naked  eye, 


36  TEXT-BOOK  OF  PHYSIOLOGY. 

it  presents  the  appearance  of  being  composed  of  bundles  of  fine  fibers 
interlacing  in  every  direction.  In  the  embryonic  state  the  elements 
of  this  form  of  connective  tissue  are  united  by  a  ground  substance, 
gelatinous  in  character.  In  the  adult  state  this  substance  shrinks 
and  largely  disappears,  leaving  intercommunicating  spaces  of  varying 
size  and  shape,  from  which  the  tissue  takes  its  name.  When  subjected 
to  the  action  of  various  reagents,  and  examined  microscopically,  the 
bundles  can  be  shown  to  consist  of  extremely  delicate,  colorless, 
transparent,  wavy  fibers,  which  are  cemented  together  by  a  ground 
substance  composed  largely  of  mucin.  Other  fibers  are  also  observed, 
which  are  distinguished  by  a  straight  course,  a  sharp,  well-defined 
outline,  a  tendency  to  branch  and  unite  with  adjoining  fibers,  and  to 


super-  c\55@^^5S^T1^1  FlG"     8' — Fai'-cells      FROM     THE 

posed      — ^W^S&^/\}^yTir\  Axilla    of    Man.     i.  The   equator 

M^&vQ^^C/ik  of    the    cell    in    focus.     2.   The    ob- 

jective somewhat  elevated.  3,  4. 
Forms  changed  by  pressure,  p. 
Traces  of  protoplasm  in  the  vicinity 

Fig.  7. — Adipose  Tissue. — (Stdhr.)  of  the  flat  nucleus  k. — (Stdhr.) 

curl  up  at  their  extremities  when  torn.  From  their  color  and  elasticity 
they  are  known  as  yellow  elastic  fibers.  Distributed  throughout  the 
meshes  of  the  areolar  tissue  are  found  flattened,  irregularly  branched, 
or  stellate  corpuscles,  connective-tissue  corpuscles,  plasma  cells,  and 
granule  cells. 

Adipose  Tissue.- — This  tissue,  which  exists  very  generally  through- 
out the  body,  though  found  most  abundantly  beneath  the  skin,  around 
the  kidneys,  and  in  the  bones,  is  practically  but  a  modification  of 
areolar  tissue.  In  these  situations  it  presents  itself  in  small  masses 
or  lobules  of  varying  size  and  shape,  surrounded  and  penetrated  by  the 
fibers  of  connective  tissue.  (See  Fig.  7.)  Microscopic  examination 
shows  that  these  masses  consist  of  small  vesicles  or  cells,  round,  ellip- 
tical or  polyhedral  in  shape,  depending  somewhat  on  pressure.  (See  Fig. 
8.)  Each  vesicle  consists  of  a  thin,  colorless,  protoplasmic  membrane, 
thickened  at  one  point,  in  which  a  nucleus  can  usually  be  detected. 
This  membrane  incloses  a  globule  of  fat,  which  during  life  is  in  the 
liquid  state.  It  is  composed  of  olein,  stearin,  and  palmitin.  The 
origin  of  the  fat  is  to  be  referred  to  a  retrograde  change  in  the  proto- 


THE  CONNECTIVE  TISSUES. 


37 


plasmic  material  of  the  connective-tissue  cells.  When  this  protoplasm 
becomes  rich  in  carbon  and  hydrogen,  it  is  speedily  converted  into  fat, 
which  makes  its  appearance  in  the  form  of  minute  drops  in  different 
portions  of  the  cell.  As  the  drops  accumulate,  at  the  expense  of  the 
cell  protoplasm,  they  gradually  coalesce,  until  there  remains  but  a  thin 
stratum  of  the  protoplasm,  which  forms  the  wall  of  the  vesicle.  Adi- 
pose tissue  may,  therefore,  be  regarded  as  areolar  tissue,  in  which, 
and  at  the  expense  of  some  of  its  elements,  fat  is  stored  for  the  future 
needs  of  the  organism.  A  diminution  of  food,  especially  of  fat  and 
carbohydrates,  is  promptly  followed  by  an  absorption  of  fat  by  the 
blood-vessels  and  by  its  transference  to  the  tissues,  where  it  is  either 
utilized  for  tissue  construction  or  for  oxidation  purposes.  In  the 
situations  in  which  adipose  tissue  is  found  it  serves,  by  its  chemic 
and  physical  properties,  to  assist  in  the 
prevention  of  a  too  rapid  radiation  of 
heat  from  the  body,  to  give  form  and 
roundness,  and  to  diminish  angularities, 
etc. 

Retiform  and  adenoid  tissue  are 
also  modifications  of  areolar  tissue.  The 
meshes  of  the  former  contain  but  little 
ground  substance,  its  place  being  taken 
by  fluids;  the  meshes  of  the  latter  contain 
large  numbers  of  lymph  corpuscles. 

Fibrous  Tissue. — This  variety  of 
connective  tissue  is  widely  distributed 
throughout  the  body.  It  constitutes 
almost  entirely  the  ligaments  around  the 
joints,  the  tendons  of  the  muscles,  the 
membranes  covering  organs  such  as  the 
heart,   liver,  nerve  system,   bones,    etc. 

All  fibrous  tissue,  wherever  found,  can  be  resolved  into  elementary 
bundles,  which  on  microscopic  examination  are  seen  to  consist  of 
delicate,  wavy,  transparent,  homogeneous  fibers,  which  pursue  an 
independent  course,  neither  branching  nor  uniting  with  adjoining 
fibers.  (See  Fig.  9.)  A  small  amount  of  ground  substance  serves  to 
hold  them  together.  Fibrous  tissue  is  tough  and  inextensible,  and  in 
consequence  is  admirably  adapted  to  fulfil  various  mechanical  func- 
tions in  the  body.  It  is,  however,  quite  pliant,  bending  easily  in  all 
directions.  When  boiled,  fibrous  tissue  yields  gelatin,  a  derivative 
of  collagen. 

Elastic  Tissue. — The  fibers  of  elastic  tissue  are  usually  associated 
in  varying  proportions  with  the  white  fibrous  tissue;  but  in  some 
structures — as  the  ligamentum  nuchas,  the  ligamenta  subflava,  the 
middle  coat  of  the  larger  blood-vessels — the  elastic  fibers  are  almost 
the  only  elements  present,  and  give  to  these  structures  a  distinctly 
yellow    appearance.     The    fibers    throughout    their    course    give    off 


Fig.  9. — Connective-tissue 
Bundles  of  Various  Thick- 
nesses of  the  Intermuscular 
Connective  Tissue  of  Man. 
X  240. — (Stohr.) 


38 


TEXT-BOOK  OF  PHYSIOLOGY. 


many  branches,  which  unite  with  adjoining  branches  to  form  a  more 
or  less  close  network.  As  the  name  implies,  these  fibers  are  highly 
elastic,  and  are  capable  of  being  extended  as  much  as  60  per  cent, 
before  breaking.     (See  Fig.  10.) 

Cartilaginous  Tissue. — This  form  of  connective  tissue  differs 
from  the  preceding  varieties  chiefly  in  its  density.  As  a  rule,  it  is 
firm  in  consistency,  though  somewhat  elastic.  '  It  is  opaque,  bluish- 
white  in  color,  though  in  thin  sections  translucent.  All  cartilaginous 
tissues  consist  of  connective-tissue  cells  embedded  in  a  solid  ground 


Fig.  10. — Elastic  Fibers.  X  560.  A.  Fine  elastic  fibers,  /,  from  intermuscular 
connective  tissue  of  man;  b,  connective-tissue  bundles  swelled  by  treatment  with  acetic 
acid-  B.  Very  thick  elastic  fibers,  /,  from  ligamentum  nuchae  of  ox;  b,  connective-tissue 
bundles.  C.  From  a  cross-section  of  the  ligamentum  nuchae  of  ox;  /,  elastic  fibers; 
b,  connective-tissue  bundles. — (Stohr.) 


substance.  According  to  the  amount  and  texture  of  the  ground 
substance,  three  principal  varieties  may  be  distinguished: 
1.  Hyaline  cartilage,  in  which  the  cells,  relatively  few  in  number,  are 
embedded  in  an  abundant  quantity  of  ground  substance-  (Fig.  n). 
The  body  of  the  cells  is  in  many  instances  distinctly  marked  off 
from  the  surrounding  substance  by  concentric  fines  or  fibers, 
which  form  a  capsule  for  the  cell.  Repeated  division  of  the  cell 
substance  takes  place,  until  the  whole  capsule  is  completely 
occupied  by  daughter  cells.  The  ground  substance  is  pervaded 
by  minute  channels,  which  communicate  on  one  hand  with  the 
spaces  around  the  cells,  and  on  the  other  with  lymph-spaces  in 
the  connective  tissue  surrounding  the  cartilage.  By  means  of 
these  channels,  nutritive  fluid  can  permeate  the  entire  structure. 
Hyaline  cartilage  is  found  on  the  ends  of  the  long  bones,  where 
it  enters  into  the  formation  of  the  joints;  between  the  ribs  and 
sternum,  forming  the  costal  cartilage,  as  well  as  in  the  nose  and 
larynx. 


THE  CONNECTIVE  TISSUES.  39 

White  fibro-cartilage,  the  ground  substance  of  which  is  pervaded 
by  white  fibers,  arranged  in  bundles  or  layers,  between  which 
are  scattered  the  usual  encapsulated  cells.  (See  Fig.  12.)  White 
fibro-cartilage  is  tough,  resistant,  but  flexible,  and  is  found  in 
joints  where  strength  and  fixedness  are  required.  Hence  it  is 
present  between  the  vertebras,  forming  the  intervertebral  discs, 
between  the  condyle  of  the  lower  jaw  and  the  glenoid  fossa,  in 
the  knee-joint,  around  the  margins  of  the  joint  cavities,  etc.  In 
these  situations  it  assists  in  maintaining  the  apposition  of  the 


it 


s- 


Fig.  11. — Hyaline  Cartilage.  X  240.  A.  Surface  view  of  the  ensiform  process 
of  frog,  fresh;  p,  protoplasm  of  cartilage-cell,  which  entirely  fills  the  lacuna;  k,  nucleus; 
g,  hyaline  matrix.  B.  Portion  of  cross-section  of  human  rib-cartilage  several  days 
after' death;  examined  in  water:  the  protoplasm,  z,  of  the  cartilage-cells  has  withdrawn 
from  the  walls  of  the  lacuna?,  h;  the  nuclei  are  invisible.  1.  Two  cells  within  one  capsule, 
k;  x,  a  developing  partition.  2.  Five  cartilage-cells  within  one  capsule;  the  lowest  cell 
has  fallen  out,  and  here  only  the  empty  space  is  seen.  3.  Capsule  cut  obliquely,  and 
apparently  thicker  on  one  side.  4.  Capsule  not  cut,  but  showing  the  cell  within,  g. 
Hyaline  matrix  transformed  into  rigid  fibers,  /• — (Stohr.) 

bones,  in  giving  a  certain  degree  of  mobility  to  the  joints,  and  in 
diminishing  the  effects  of  shock  and  pressure  imparted  to  the 
bones. 

3.  Yellow  fibro-cartilage,  the  ground  substance  of  which  is  pervaded 
by  opaque,  yellow  elastic  fibers,  which  form,  by  the  interlacing 
of  their  branches,  a  complicated  network,  in  the  meshes  of  which 
are  to  be  found  the  usual  corpuscles.  (See  Fig.  13.)  As  these 
fibers  are  elastic,  they  impart  to  the  cartilage  a  very  considerable 
degree  of  elasticity.  Yellow  fibro-cartilage  is  well  adapted,  there- 
fore, for  entering  into  the  formation  of  the  external  ear,  epiglottis, 
Eustachian  tube,  etc. — structures  which  require  for  their  func- 
tional activity  a  certain  degree  of  flexibility  and  elasticity. 
Osseous  Tissue. — Osseous  tissue,   as  distinguished   from   bone, 

is  a  member  of  the  connective-tissue  group,  the  ground  substance  of 


40 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  is  permeated  with  insoluble  lime  salts,  of  which  the  phosphate 
and  carbonate  are  the  most  abundant.  Immersed  in  dilute  solutions 
of  hydrochloric  acid,  they  can  be  converted  into  soluble  salts  and  dis- 
solved out.  The  osseous  matrix  left 
behind  is  soft  and  pliable.  When 
boiled,  it  yields  gelatin. 

A  thin,  transverse  section  of  a 
decalcified  bone,  when  examined 
microscopically,  reveals  a  number  of 
small,  round,  or  oval  openings,  which 
represent  transverse  sections  of  canals 
which  run  through  the  bone,  for  the 
most  part  in  a  longitudinal  direction, 
though  frequently  anastomosing  with 
one  another.  These  so-called  Haver- 
sian canals  in  the  living  state  contain 
blood-vessels  and  lymphatics.  (See 
Fig.  14.) 

Around  each  Haversian  canal  is  a 
series  of  concentric  lamina;,  composed 
of  white  fibers.  Between  every  two 
laminae  are  found  small  cavities 
(lacunas),  from  which  radiate  in  all  directions  small  canals  (canaliculi), 
which  communicate  freely  with  one  another.  The  Haversian  canals, 
with  their  associated  lacunas  and  canaliculi,  form  a  system  of  inter- 
communicating   passages,    which    circulate   lymph    destined   for  the 


Fig.  12. — From  a  Horizontal 
Section  of  the  Intervertebral 
Disc  of  Man.  g.  Fibrillar  connec- 
tive tissue.  2.  Cartilage-cell  (nucleus 
invisible),  k.  Capsule  surrounded 
bv  calcareous  granules.  X  240. — 
(Stohr.) 


til^m-^''^*^1 


Fig.  13. — Elastic  Cartilage.  X  240.  1.  Portion  of  section  of  vocal  process  (ante- 
rior angle)  of  arytenoid  cartilage  of  a  woman  thirty  years  old;  the  elastic  substance  in  the 
form  of  granules.  2  and  3.  Portions  of  sections  of  epiglottis  of  a  woman  sixty  years  old; 
a  fine  network  of  elastic  fibers  in  2,  a  coarser  network  in  3.  z.  Cartilage-cell,  nucleus 
not  visible;  k,  capsule. — (Stohr.) 


nourishment  of  bone.  Each  lacuna  contains  the  bone  corpuscle,  which 
bears  a  close  resemblance  to  the  usual  branched  connective-tissue  cor- 
puscle, and  whose  function  appears  to  be  the  maintenance  of  the 
nutrition  of  the  bone. 


THE  CONNECTIVE  TISSUES. 


4i 


The  surface  of  every  bone  in  the  living  state  is  invested  with  a 
fibrous  membrane,  the  periosteum,  except  where  it  is  covered  with 
cartilage.  The  inner  surface  of  this  membrane  is  loose  in  texture, 
and  supports  a  fine  plexus  of  capillary  blood-vessels  and  numerous 
protoplasmic  cells — the  osteoblasts.  As  this  layer  is  directly  con- 
cerned in  the  formation  of  bone,  it  is  spoken  of  as  the  osteo genetic 
layer. 

A  section  of  a  bone  shows  that  it  is  composed  of  two  kinds  of 
tissue — compact  and  cancellated.  The  compact  is  dense,  resembling 
ivory,  and  is  found  on  the  outer  portion  of  the  bone;  the  cancellated 

Periosteum. 

Outer  ground  lamellae. 

Haversian  canals. 


Haversian  lamella?. 


Interstitial  lamella?. 
Inner  ground  lamella?. 


Marrow. 


Fig.  14. — From  a  Cross-section  of  a  Metacarp  of  Man.     X  5°.     The  Haver- 
sian canals  contain  a  little  marrow  (fat-cells).     Resorption  line  at  h. —  (Stohr.) 


is  spongy,  and  appears  to  be  made  up  of  thin,  bony  plates,  winch 
intersect  one  another  in  all  directions,  and  is  found  in  greatest  abun- 
dance in  the  interior  of  the  bones.  The  shaft  of  a  long  bone  is  hollow. 
This  central  cavity,  which  extends  from  one  end  of  the  bone  to  the 
other,  as  well  as  the  interstices  of  the  cancellated  tissue,  is  filled  in  the 
living  state  with  marrow.  The  marrow  or  medulla  is  composed  of  a 
connective-tissue  framework  supporting  blood-vessels.  In  its  meshes 
are  to  be  found  characteristic  bone  cells  or  osteoblasts,  the  function 
of  which  is  supposed  to  be  the  formation  of  bone.  In  the  long  bones 
the  marrow  is  yellow,  from  the  presence  in  the  connective-tissue 
corpuscle  of  fat  globules,  which  arise  through  the  transformation  of 
the  cell  protoplasm.  In  the  cancellated  tissue,  near  the  extremities 
of  the  long  bones,  this  fatty  transformation  does  not  take  place  to  the 
same  extent,  and  the  marrow  appears  red.  The  cells  of  the  red 
marrow  arc  believed  to  give  birth  indirectly  to  the  red  blood-cor- 
puscles. 

Physical  and  Physiologic  Properties  of  Connective  Tissues. — 
Among  the  physical  properties  may  be  mentioned  consistency,  cohesion, 
and  elasticity.  Their  consistency  varies  from  the  semiliquid  to  the 
solid  state,  and  depends  on  the  quantity  of  water  which  enters  into 


42  TEXT-BOOK  OF  PHYSIOLOGY. 

their  composition.  Their  cohesion,  except  in  the  softer  varieties,  is 
very  considerable,  and  offers  great  resistance  to  traction,  pressure, 
torsion,  etc.  In  all  the  movements  of  the  body,  in  the  contraction  of 
muscles,  in  the  performance  of  work,  the  consistence  and  cohesion  of 
these  tissues  play  most  important  roles.  Wherever  the  various  forms 
of  connective  tissue  are  found,  their  chemic  composition  and  structure 
are  in  relation  to  their  functions.  If  traction  be  the  preponderating 
force,  the  structure  becomes  fibrous,  as  in  ligaments  and  tendons,  and 
the  cohesion  greatest  in  the  longitudinal  direction.  If  pressure  be 
exerted  in  all  directions,  as  upon  membranes,  the  fibers  interlace  and 
offer  a  uniform  resistance.  When  pressure  is  exerted  in  a  definite 
direction,  as  on  the  extremities  of  the  long  bones,  the  tissue  becomes 
expanded  and  cancellated.  The  lamellae  of  the  cancellated  tissue 
arrange  themselves  in  curves  which  correspond  to  the  direction  of  the 
greatest  pressure  or  traction.  Extensibility  is  not  a  characteristic 
feature,  except  in  those  forms  containing  an  abundance  of  yellow 
elastic  fibers.  The  elasticity  is  an  essential  factor  in  many  physiologic 
actions.  It  not  only  opposes  and  limits  forces  of  traction,  pressure, 
torsion,  etc.,  but  on  their  cessation  returns  the  tissues  or  organs  to  their 
original  condition.  Elasticity  thus  assists  in  maintaining  the  natural 
form  and  position  of  the  organs  by  counterbalancing  and  opposing 
temporarily  acting  forces. 

The  Skeleton. — The  connective  tissues  in  their  entirety  con- 
stitute a  framework  which  presents  itself  under  two  aspects:  (i) 
As  a  solid,  bony  skeleton,  situated  in  the  trunk  and  limbs,  affording 
attachment  for  muscles  and  viscera;  (2)  as  a  fine,  fibrous  skeleton, 
found  everywhere  throughout  the  body,  connecting  the  various  viscera 
and  affording  support  for  the  epithelial,  muscle,  and  nerve  tissues. 


CHAPTER  V. 
THE  PHYSIOLOGY  OF  MOVEMENT. 

The  Animal  Body  as  a  Machine  for  Doing  Work. — Of  the  four 
phenomena  presented  by  an  animal,  that  which  more  immediately 
interests  the  physiologist  is  movement,  for  the  reason  that  it  is  not 
only  its  most  characteristic  form  of  activity,  and  that  which  serves 
to  distinguish  it  in  the  main  from  forms  of  vegetable  life,  but  its  solution 
affords  an  explanation  of  many  physiologic  processes  occurring  within 
the  human  body-  It  is  also  for  this  reason  that  movement  constitutes 
for  the  most  part  the  subject  matter  of  physiologic  experimentation. 

The  movements  may  be  general  as  when  the  animal  changes  its 
position  relatively  to  its  environment  as  in  the  various  acts  of  loco- 
motion, or  special  as  in  the  changes  of  relation  of  one  part  of  the  body 
with  reference  to  another. 

In  the  execution  of  these  movements  the  animal,  of  necessity,  meets 
with  various  forms  of  resistance,  viz. :  gravity,  cohesion,  friction,  etc. 
When  its  different  parts  are  applied  or  directed,  either  volitionally 
and  in  a  determinate  manner,  or  non-volitionally  and  in  an  indeter- 
minate or  reflex  manner  to  the  overcoming  of  these  opposing  forces 
in  the  environment,  the  animal  may  be  said  to  be  doing  work. 

In  addition  to  these  obvious  external  movements,  a  series  of  less 
obvious  though  no  less  characteristic  internal  movements  are  being 
exhibited  by  the  various  organs  of  the  animal,  e.  g.,  heart,  lungs,  stomach, 
intestines,  bladder,  etc.;  and  when  these  organs  are  applied  to  the 
overcoming  of  opposing  forces  or  resistances,  as  they  are  from  moment 
to  moment  in  the  performance  of  their  functions,  it  may  be  said  that 
they  also  are  doing  work.  The  cooperation  of  both  external  and 
internal  organs  is  necessary,  not  only  for  the  maintenance  of  the  life 
of  the  animal  but  also  for  the  accomplishment  of  work.  In  the  con- 
ception of  the  animal  body  as  a  machine  for  doing  work,  the  skeletal, 
the  muscle  and  nerve  tissues  constitute  a  primary  mechanism  in  which 
each  bears  a  certain  definite  relation  to  the  other.  The  internal  organs 
collectively  constitute  a  secondary  mechanism  in  which  each  bears 
not  only  a  definite  relation  to  the  other,  but  to  the  primary  mechanism 
as  well.  The  relation  of  skeletal,  muscle  and  nerve  tissue,  are  shown 
in  Fig.  15. 

General  Considerations. — The  skeleton  is  the  passive  framework 
of  the  body,  the  axial  portions  of  which  (the  vertebral  column,  ribs, 
sternum  and  skull)  impart  more  or  less  fixity  and  rigidity  to  the 
mechanism,  while  the  appendicular  portions  (the  bones  of  the  arms 
and  legs)  impart  extreme  mobility.     The  bones  of  the  arms  and  legs, 

43 


44 


TEXT-BOOK  OF  PHYSIOLOGY. 


c.s.c. 


Fig.  15. — Diagram  Showing  the  Relaton  of  Skeletal,  Muscle  and  Nerve 
Tissues.  (G.  Bachman.)  j.a.  Bones  of  the  forearm  representing  the  skeletal  tissue;  e.j. 
the  elbow  joint,  the  fulcrum  of  the  lever  formed  by  the  bones  of  the  forearm;  W.  a  weight 
acting  in  a  downward  direction  and  representing  the  passive  force  of  gravity;  sk.m.  a 
skeletal  muscle  acting  in  an  upward  direction  and  the  source  of  the  active  power  to  be  ap- 
plied to  the  lever;  sp.c.  transection  of  the  spinal  cord  showing  the  relation  of  the  white  and 
the  gray  matter:  m.c.  a  motor  cell  in  the  anterior  horn  of  the  gray  matter;  ej.n.  an  effer- 
ent nerve-fiber  connecting  the  motor  cell  from  which  it  arises  with  the  skeletal  muscle  and 
contained  in  the  ventral  roots  of  the  spinal  nerves;  af.n.  an  afferent  nerve-fiber  arising  from 
the  ganglion  cell  along  its  course  and  connecting  the  skin,  s.,  on  the  one  hand  with  the  spinal 
cord  on  the  other  hand  and  contained  in  the  dorsal  roots  of  the  spinal  nerves;  c.s.c. 
coronal  section  of  the  cerebrum  showing  the  relation  of  the  gray  to  the  white  matter;  v.c. 
a  volitional  or  motor  cell;  d.a.  a  descending  axon  or  nerve-fiber  connecting  the  volitional 
cell  from  which  it  arises  with  the  motor  cell  in  the  spinal  cord;  s.c.  a  sensor  cell;  a.a.  an 
ascending  axon  or  nerve-fiber  connecting  a  receptive  cell  from  which  it  arises  (not  shown  in 
the  diagram)  with  the  sensor  cell  in  the  gray  matter  of  the  cerebrum.  The  nerve-fibers 
which  pass  outward  from  the  spinal  cord  to  the  glands,  blood-vessels,  and  the  muscle 
walls  of  the  viscera,  have  for  the  sake  of  simplicity  been  omitted  from  the  diagram. 


THE  PHYSIOLOGY  OF  MOVEMENT.  45 

more  especially,  may  be  looked  upon  as  constituting  a  system  of 
levers,  the  fulcra  of  which,  the  points  around  which  they  move,  lie  in 
the  joints. 

That  a  lever  may  be  effective  as  an  instrument  for  the  accomplish- 
ment of  work,  it  must  not  only  be  capable  of  moving  around  its  ful- 
crum, but  it  must,  at  the  same  time,  be  acted  on  by  two  opposing 
forces,  one  passive,  the  other  active. 

In  the  movements  of  the  bony  levers  of  the  animal  body,  the 
passive  forces  to  be  overcome  are  largely  those  connected  with  the 
environment,  e.  g.,  gravity,  cohesion,  friction,  elasticity,  etc.  The 
active  forces  by  which  these  are  opposed  and  overcome  through  the 
mediation  of  the  bony  levers  are  found  in  the  muscles  attached  to 
them. 

The  muscle  tissue  may  therefore  be  regarded  as  the  seat  of 
those  energies  that  impart  movement  to  the  levers.  This  tissue 
is  arranged  in  masses  of  irregular  shape  and  size,  termed  muscles. 
The  majority  of  the  muscles  of  the  body  are  connected  with  the  bones 
of  the  body  in  such  a  manner  that  by  an  alteration  in  their  form,  they 
can  change  not  only  the  position  of  the  bones  with  reference  to  one 
another,  but  also  the  individual's  relation  to  surrounding  objects. 
When  the  lever  is  applied  to  the  overcoming  of  an  opposing  force,  the 
muscle  is  said  to  be  doing  work. 

Though  consisting  of  a  highly  active  tissue,  muscles  in  themselves 
do  not  possess  spontaneity  of  action,  but  require  for  the  manifestation 
of  their  energy  the  stimulating  influence  of  nerve  energy  or  nerve 
impulses  developed  in,  and  transmitted  by,  nerve  tissue  to  the  muscles. 

The  nerve  tissue  is  the  seat  of  origin  of  those  energies  or  impulses 
necessary  to  the  physiologic  excitation  of  the  muscles.  This  tissue  is 
partly  arranged  in  masses  contained  in  the  cavities  of  the  head  and 
spinal  column  (the  brain  and  spinal  cord),  forming  the  central  organs 
of  the  nerve  system,  and  partly  in  the  form  of  cords  or  nerves,  forming 
the  peripheral  organs  of  the  nerve  system;  the  latter  connects  the  for- 
mer, not  only  with  the  muscles,  but  with  glands,  blood-vessels,  skin, 
mucous  membranes,  etc.,  as  well. 

A  transection  of  the  spinal  cord  shows  that  it  is  composed  externally 
of  white  matter  and  internally  of  grey  matter.  The  grey  matter  is 
arranged  in  the  form  of  two  crescents  united  in  the  median  line  by  a 
transverse  band  or  commissure  forming  a  figure  resembling  the  letter 
H.  Though  varying  in  shape  in  different  regions  of  the  cord,  the  grey 
matter  in  all  situations  presents  on  either  side  an  anterior  or  ventral 
and  a  posterior  or  dorsal  horn. 

In  the  ventral  horns  of  the  grey  matter  are  located  nerve  cells 
which  not  only  generate,  but  under  appropriate  circumstances  discharge, 
a  form  of  energy  termed  a  nerve  impulse,  which  is  transmitted  by 
efferent  nerve  fibers  arising  from  the  cells,  and  by  way  of  the  ventral 
roots  of  the  spinal  nerves,  to  the  muscles,  glands,  blood-vessels,  and 
walls  of  the  viscera  with  which  they  are  directly  or  indirectly  connected. 


46  TEXT-BOOK  OF  PHYSIOLOGY. 

The  arrival  of  the  nerve  impulse  at  once  calls  forth  the  form  of  activity 
characteristic  of  the  structure  excited. 

Thus  the  muscle,  for  example,  passes  from  the  passive  to  the  active 
state,  that  is,  the  muscle  becomes  shorter  and  thicker,  and  the  bone 
to  which  it  is  attached  is  moved.  This  is  at  once  followed  by  a 
return  of  the  muscle  to  the  passive  state;  that  is,  it  lengthens,  be- 
comes narrower,  and  resumes  its  original  form;  the  bone  at  the  same 
time  returns  to  its  former  position. 

Coincident  with  this  change  of  shape  there  is  a  liberation  of  heat 
and  electricity.  The  nerve  impulse  which  occasions  this  transforma- 
tion of  potential  into  kinetic  energy  is  the  normal  or  the  physiologic 
stimulus.  The  muscle  responding  to  the  stimulus  is  said  to  be  irrit- 
able or  to  possess  irritability.  The  glands  in  response  to  the  nerve 
impulse  pour  out  a  secretion.  The  blood-vessels  and  viscera  change 
their  caliber.  These  tissues,  too,  responding  to  the  nerve  impulse,  are 
said  to  be  irritable.  The  nerve  cells  of  the  spinal  cord,  however,  do 
not  possess  spontaneity  of  action  but  require  for  their  excitation  the 
arrival  of  other  nerve  impulses.  These  may  come,  (i)  from  the  per- 
iphery through  afferent  nerve  fibers  by  way  of  the  dorsal  roots  of  the 
spinal  nerves;  and  (2)  from  the  brain  as  a  result  of  an  act  of  volition 
through  descending  axons  or  nerve  fibers.  In  the  first  instance,  the 
resulting  movement  taking  place  independently  of  volition,  and  in  re- 
sponse to  a  peripheral  or  surface  excitation,  is  termed  a  reflex  move- 
ment; in  the  second  instance,  the  resulting  movement  taking  place  in 
response  to  an  act  of  volition  is  termed  conventionally  a  volitional  or 
voluntary  movement. 

In  the  case  of  reflex  movements,  the  nerve  impulses  are  primarily 
developed  in  specialized  organs  located  in  the  skin  or  mucous  membranes 
and  as  a  result  of  the  impact  of  various  external  agents,  which  for 
this  reason  are  termed  stimuli.  The  nerve  impulses  thus  developed 
are  transmitted  by  the  afferent  nerves  to  the  nerve  cells  which  are  in 
turn  excited  to  activity. 

In  the  case  of  volitional  muscle  movements,  the  nerve  impulses 
which  cause  the  movement  are  discharged  from  certain  motor  or 
volitional  nerve  cells  in  the  grey  matter  of  the  cerebrum  and  trans- 
mitted by  descending  axons  or  nerve  fibers  direct  to  the  nerve  cells  in 
the  spinal  cord,  by  which  they  in  turn  are  excited  to  activity. 

The  volitional  movements  are  however  the  immediate  or  the  more 
or  kss  remote  effects  of  sensations  which  have  been  evoked  in  the 
sense  areas  of  the  brain,  by  the  arrival  of  nerve  impulses  coming  through 
ascending  axons  or  nerve  fibers  from  peripheral  sense  organs,  e.  g.,  skin, 
ar,  nose,  tongue,  and  which  have  been  developed  by  the  impact  of 
objects  in  the  external  world. 

The  nerve  cells  and  their  related  nerve  fibers,  responding  by  the 
development  and  conduction  of  nerve  impulses  are  also  said  to  be 
irritable.  The  transformation  of  energy,  however,  manifests  itself 
mainlv  as  electricitv  and  molecular  motion.     The  animal  bodv  in  its 


THE  PHYSIOLOGY  OF  MOVEMENT.  47 

entirety  may  therefore  be  regarded  as  a  machine  for  the  transformation 
of  potential  energy  into  kinetic  energy,  viz. :  heat  and  electricity,  move- 
ments of  muscles  and  bony  levers,  secretion,  sensation  and  other  forms 
of  nerve  activity.  When  muscles  and  bones  are  applied  to  the  over- 
coming of  opposing  forces,  mechanic  work  is  accomplished.  In  the 
following  chapters  some  of  the  problems  connected  with  the  activities 
of  the  skeletal,  muscle  and  nerve  tissues  will  be  considered. 


CHAPTER  VI. 
THE  PHYSIOLOGY  OF  THE  SKELETON. 

The  skeleton  in  its  entirety  determines  the  plan  of  organization 
of  the  animal  body.  Its  axial  portion  is  the  foundation  element 
and  the  center  around  which  the  appendicular  portions  are  developed 
and  arranged  with  a  certain  degree  of  conformity.  The  character  and 
the  arrangement  of  the  bones  of  the  axial  portion  endow  the  animal 
mechanism  with  a  certain  degree  of  fixity,  combined  with  slight  mo- 
bility, while  the  character  and  arrangement  of  the  bones  of  the  append- 
icular portions  endow  it  with  extreme  mobility.  The  bones  collectively 
constitute  a  system  of  levers,  the  fulcra  of  which  lie  in  the  points  of 
union  of  the  bones,  and  with  which  the  animal  is  enabled  to  execute  a 
variety  of  movements,  to  change  its  position  relatively  to  its  environ- 
ment and  overcome  opposing  forces.  The  structure  and  the  chemic 
composition  of  the  bones,  consisting  as  they  do  of  inorganic  matter 
67  per  cent,  and  of  organic  matter  33  per  cent,  endow  them  with  both 
rigidity  and  elasticity,  physical  properties  which  admirably  adapt  them 
to  the  character  of  the  work  necessitated  by  the  environment  and  the 
organization  of  the  animal.  The  rigidity  of  bone  is  considerable  as 
compared  with  other  hard  and  rigid  materials.  The  breaking  limit, 
in  terms  of  the  weight  in  kilos  required  to  tear  across  a  rod  one  square 
millimeter  in  cross-section  of  various  materials  is  as  follows :  Cast  iron 
13;  bone  12;  oak  6.5;  granite  1.9.  The  elasticity  is  about  one-sixth 
that  of  wrought  iron  and  twice  that  of  oak  parallel  to  the  grain 
(MacAlister).  In  youth  bones  are  quite  elastic;  in  old  age  they  are 
fragile  because  of  a  diminution  of  tissue  and  an  increased  porosity,  and, 
therefore,  at  both  periods  less  capable  of  functionating  as  effectively 
as  in  the  middle  period  of  life.  The  skeleton  also  serves  for  the 
attachment  of  muscles  and  affords  support  and  protection  to  viscera. 

For  the  manifestation  of  the  activities  of  the  animal  it  is  essential 
that  the  relation  of  the  various  portions  of  the  bony  skeleton  to  one 
another  shall  be  such  as  to  permit  of  movement  while  yet  retaining 
close  apposition.  This  is  accomplished  by  the  mechanical  conditions 
which  have  been  evolved  at  the  points  of  union  of  bones,  and  which 
are  technically  known  as  articulations  or  joints. 

A  consideration  of  the  body  movements  involves  an  account  of 
d)  the  static  conditions,  or  those  states  of  equilibrium  in  which  the 
body  is  at  rest — e.  g.,  standing,  sitting;  (2)  the  dynamic  conditions, 
or  those  states  of  activity  characterized  by  movement — e.  g.,  walking, 
running,  etc.  Jn  this  connection,  however,  only  those  physical  and 
physiologic  peculiarities  of  the  skeleton,  especially  in  its  relation  to 


THE  PHYSIOLOGY  OF  THE  SKELETON.  49 

joints,  will  be  referred  to,  which  underlie  and  determine  both  the 
static  and  dynamic  states  of  the  body. 

Structure  of  Joints. — The  structures  entering  into  the  formation 
of  joints  are: 

1.  Bones,  the   articulating  surfaces  of  which  are  often  more  or  less 

expanded,  especially  in  the  case  of  long  bones,  and  at  the  same 
time  variously  modified  and  adapted  to  one  another  in  accordance 
with  the  character  and  extent  of  the  movements  which  there 
take  place. 

2.  Hyaline  cartilage,  which  is  closely  applied  to  the  articulating  end 

of  each  bone.  The  smoothness  of  this  form  of  cartilage  facili- 
tates the  movements  of  the  opposing  surfaces,  while  its  elasticity 
diminishes  the  force  of  shocks  and  jars  imparted  to  the  bones 
during  various  muscular  acts.  In  a  number  of  joints,  plates 
or  discs  of  white  fibro-cartilage  are  inserted  between  the  surfaces 
of  the  bones. 

3.  A  synovial  membrane,  which  is  attached  to  the  edge  of  the  hyaline 

cartilage,  entirely  inclosing  the  cavity  of  the  joint.  This  mem- 
brane is  composed  largely  of  connective  tissue,  the  inner  surface 
of  which  is  lined  by  endothelial  cells,  which  secrete  a  clear,  color- 
less, viscid  fluid — the  synovia.  This  fluid  not  only  fills  up  the 
joint-cavity,  but,  flowing  over  the  articulating  surfaces,  diminishes 
or  prevents  friction. 

4.  Ligaments — tough,   inelastic    bands,    composed    of    white    fibrous 

tissue — which  pass  from  bone  to  bone  in  various  directions  on 
the  different  aspects  of  the  joint.  As  white  fibrous  tissue  is  in- 
extensible  but  pliant,  ligaments  assist  in  keeping  the  bones  in 
apposition,  and  prevent  displacement  while  yet  permitting  of 
free  and  easy  movements. 
Classification  of  Joints. — All  joints  may  be  divided,  according 

to  the  extent  and  kind  of  movements  permitted  by  them,  into   (1) 

diarthroses;  (2)  amphiarthroses;  (3)  synarthroses. 

1.  Diarthroses. — In  this  division  of  the  joints  are  included  all  those 
which  permit  of  free  movement.  In  the  majority  of  instances 
the  articulating  surfaces  are  mutually  adapted  to  each  other. 
If  the  articulating  surface  of  one  bone  is  convex,  the  opposing 
but  corresponding  surface  is  concave.  Each  surface,  therefore, 
represents  a  section  of  a  sphere  or  a  cylinder,  which  latter  arises 
by  rotation  of  a  line  around  an  axis  in  space.  According  to  the 
number  of  axes  around  which  the  movements  take  place  all 
diarthrodial  joints  may  be  divided  into : 

1.  Uniaxial  Joints. — In  tins  group  the  convex  articulating  surface  is 
a  segment  of  a  cylinder  or  cone,  to  which  the  opposing  surface 
more  or  less  completely  corresponds.  In  such  a  joint  the  single 
axis  of  rotation,  though  nearly,  is  not  exactly  at  right  angles 
to  the  long  axis  of  the  bone,  and  hence  the  movements — flexion 
and  extension — which  take  place  are  not  confined  to  one  plane. 


5o  TEXT-BOOK  OF  PHYSIOLOGY. 

Joints  of  this  character — e.  g.,  the  elbow,  knee,  ankle,  the  pha- 
■  langeal  joints  of  the  fingers  and  toes — are,  therefore,  termed 
ginglymi,  or  hinge-joints.  Owing  to  the  obliquity  of  their  ar- 
ticulating surfaces,  the  elbow  and  ankle  are  cochleoid  or  screw- 
ginglymi.  Inasmuch  as  the  axes  of  these  joints  on  the  opposite 
sides  of  the  body  are  not  coincident,  the  right  elbow  and  left 
ankle  are  right-handed  screws;  the  left  elbow  and  right  ankle, 
left-handed  screws.  In  the  knee-joint  the  form  and  arrangement 
of  the  articulating  surfaces  are  such  as  to  produce  that  modifica- 
tion of  a  simple  hinge  known  as  a  spiral  hinge,  or  helicoid.  As 
the  articulating  surfaces  of  the  condyles  of  the  femur  increase  in 
convexity  from  before  backward,  and  as  the  inner  condyle  is 
longer  than  the  outer,  and,  therefore,  represents  a  spiral  surface, 
the  line  of  translation  or  the  movement  of  the  leg  is  also  a  spiral 
movement.  During  flexion  of  the  leg  there  is  a  simultaneous 
inward  rotation  around  a  vertical  axis  passing  through  the  outer 
condyle  of  the  femur;  during  extension  a  reverse  movement  takes 
place.  Moreover,  the  slightly  concave  articulating  surfaces  of 
the  tibia  do  not  revolve  around  a  single  fixed  transverse  axis,  as 
in  the  elbow-joint,  for  during  flexion  they  slide  backward,  during 
extension  forward,  around  a  shifting  axis,  which  varies  in  posi- 
tion with  the  point  of  contact. 

In  some  few  instances  the  axis  of  rotation  of  the  articulating 
surface  is  parallel  with  rather  than  transverse  to  the  long  axis  of 
the  bone,  and  as  the  movement  then  takes  place  around  a  more 
or  less  conic  surface,  the  joint  is  termed  a  trochoid  or  pulley — 
e.  g.,  the  odonto-atlantal  and  the  radio-ulnar.  In  the  former  the 
collar  formed  by  the  atlas  and  its  transverse  ligament  rotates 
around  the  vertical  odontoid  process  of  the  axis.  In  the  latter  the 
head  of  the  radius  revolves  around  its  own  long  axis  upon  the  ulna, 
giving  rise  to  the  movements  of  pronation  and  supination  of  the 
hand.  The  axis  around  which  these  two  movements  take  place 
is  continued  through  the  head  of  the  radius  to  the  styloid  process 
of  the  ulna.  . 
2.  Biaxial  Joints. — In  this  group  the  articulating  surfaces  are  un- 
equally curved,  though  intersecting  each  other.  When  the  sur- 
faces lie  in  the  same  direction,  the  joint  is  termed  an  ovoid  joint 
— e.  g.,  the  radio-carpal  and  the  atlanto-occipital.  As  the  axes 
of  these  surfaces  are  vertical  to  each  other,  the  movements  per- 
mitted by  the  former  joint  are  flexion,  extension,  adduction,  and 
abduction,  combined  with  a  slight  amount  of  circumduction; 
the  latter  joint  permits  of  flexion  and  extension  of  the  head,  with 
inclination  to  either  side.'  When  the  surfaces  do  not  take  the 
same  direction,  the  joint,  from  its  resemblance  to  the  surfaces  of 
a  saddle,  is  termed  a  saddle-joint — e.  g.,  the  trapezio-metacarpal. 
The  movements  permitted  by  this  joint  are  also  flexion,  exten- 
sion, adduction,  abduction,  and  circumduction. 


THE  PHYSIOLOGY  OF  THE  SKELETON.  51 

3.  Polyaxial  Joints. — In  this  group  the  convex  articulating  surface 
is  a  segment  of  a  sphere,  which  is  received  by  a  socket  formed 
by  the  opposing  articulating  surface.  In  such  a  joint,  termed  an 
enarthrodial  or  ball-and-socket  joint — e.  g.,  the  shoulder-joint, 
hip-joint — the  distal  bone  revolves  around  an  indefinite  number 
of  axes,  all  of  which  intersect  one  another  at  the  center  of  rotation. 
For  simplicity,  however,  the  movement  may  be  described  as 
taking  place  around  axes  in  the  three  ordinal  planes— viz.,  a 
transverse,  a  sagittal,  and  a  vertical  axis.  The  movements  around 
the  transverse  axis  are  termed  flexion  and  extension;  around  the 
sagittal  axis,  adduction  and  abduction;  around  the  vertical 
axis,  rotation.  When  the  bone  revolves  around  the  surface  of  an 
imaginary  cone,  the  apex  of  which  is  the  center  of  rotation  and 
the  base  the  curve  described  by  the  hand,  the  movement  is 
termed  circumduction. 

2.  Amphiarthroses. — In    this    division    are  included   all  those  joints 

which  permit  of  but  slight  movement — e.  g.,  the  intervertebral, 
the  interpubic,  and  the  sacro-iliac  joints.  The  surfaces  of  the 
opposing  bones  are  united  and  held  in  position  largely  by  the 
intervention  of  a  firm,  elastic  disc  of  fibro-cartilage.  Each  joint 
is  also  strengthened  by  ligaments. 

3.  Synarthroses. — In    this  division   are  included  all  those    joints  in 

which  the  opposing  surfaces  of  the  bones  are  immovably  united, 
and  hence  do  not  permit  of  any  movement — e.  g.,  the  joints 
between  the  bones  of  the  skull. 
The  Vertebral  Column. — In  all  static  and  dynamic  states  of  the 
body  the  vertebral  column  plays  a  most  essential  role.  Situated  in 
the  middle  of  the  back  of  the  trunk,  it  forms  the  foundation  of  the 
entire  skeleton.  It  is  composed  of  a  series  of  superimposed  bones, 
termed  vertebrae,  which  increase  in  size  from  above  downward  as 
far  as  the  brim  of  the  pelvic  cavity.  Superiorly,  it  supports  the  skull ; 
laterally,  it  affords  attachment  for  the  ribs,  which  in  turn  support  the 
weight  of  the  upper  extremities;  below,  it  rests  upon  the  pelvic  bones, 
which  transmit  the  weight  of  the  body  to  the  inferior  extremities. 
The  bodies  of  the  vertebras  are  united  one  to  another  by  tough  elastic 
discs  of  fibro-cartilage,  which,  collectively,  constitute  about  one- 
quarter  of  the  length  of  the  vertebral  column.  The  vertebrae  are  held 
together  by  ligaments  situated  on  the  anterior  and  posterior  surfaces 
of  their  bodies,  and  by  short,  elastic  ligaments  between  the  neural 
arches  and  processes.  These  structures  combine  to  render  the  verte- 
bral column  elastic  and  flexible,  and  enable  it  to  resist  and  diminish 
the  force  of  shocks  communicated  to  it. 

The  amphiarthrodial  character  of  the  intervertebral  joints  endows 
the  entire  column  with  certain  forms  of  movement  which  are  neces- 
sary to  the  performance  of  many  body  activities.  While  the  range 
of  movement  between  any  two  vertebrae  is  slight,  the  sum  total  of  move- 
ment of  the  entire  series  of  vertebrae  is  considerable.     In  different 


52  TEXT-BOOK  OF  PHYSIOLOGY. 

regions  of  the  column  the  character,  as  well  as  the  range  of  move- 
ment, varies  in  accordance  with  the  form  of  the  vertebrae  and  the 
inclination  of  their  articular  processes.  In  the  cervical  and  lumbar 
regions  extension  and  flexion  are  freely  permitted,  though  the  former 
is  greater  in  the  cervical,  the  latter  in  the  lumbar  region,  especially 
between  the  fourth  and  fifth  vertebras.  Lateral  flexion  takes  place 
in  all  portions  of  the  column,  but  is  particularly  marked  in  the  cer- 
vical region.  A  rotatory  movement  of  the  column  as  a  whole  takes 
place  through  an  angle  of  about  twenty-eight  degrees.  This  is  most 
evident  in  the  lower  cervical  and  dorsal  regions. 


CHAPTER  VII. 
GENERAL  PHYSIOLOGY   OF  MUSCLE-TISSUE. 

The  muscle-tissue,  which  closely  invests  the  bones  of  the  body 
and  which  is  familiar  to  all  as  the  flesh  of  animals,  is  the  immediate 
cause  of  the  active  movements  of  the  body.  This  tissue  is  grouped 
in  masses  of  varying  size  and  shape,  which  are  technically  known  as 
muscles.  The  majority  of  the  muscles  of  the  body  are  connected 
with  the  bones  of  the  skeleton  in  such  a  manner  that,  by  an  alteration 
in  their  form,  they  can  change  not  only  the  position  of  the  bones  with 
reference  to  one  another,  but  can  also  change  the  individual's  relation 
to  surrounding  objects.  They  are,  therefore,  the  active  organs  of 
both  motion  and  locomotion,  in  contradistinction  to  the  bones  and 
joints,  which  are  but  passive  agents  in  the  performance  of  the  corre- 
sponding movements.  In  addition  to  the  muscle  masses  which  are 
attached  to  the  skeleton,  there  are  also  other  collections  of  muscle- 
tissue  surrounding  cavities  such  as  the  stomach,  intestine,  blood- 
vessels, etc.,  which  impart  to  their  walls  motility,  and  so  influence  the 
passage  of  material  through  them. 

Muscles  produce  movement  of  the  structures  to  which  they  are 
attached  by  the  property  with  which  they  are  endowed  of  changing 
their  shape,  shortening  or  contracting  under  the  influence  of  a  stimulus 
transmitted  to  them  from  the  nervous  system.  Muscles  are  divided 
into: 
i.  Voluntary   muscles,    comprising    those    the   activity   of    which    is 

called  forth  by  an  act  or  effort  of  volition. 
2.  Involuntary  muscles,   comprising   those  the  activity  of    which  is 
entirely  independent  of  the  volition. 

The  voluntary  muscles  are  also  known  from  their  attachment  to 
the  skeleton  as  skeletal,  and  from  their  microscopic  appearance  as 
striped  or  striated  muscles.  Though  for  the  most  part  these  muscles 
are  red,  there  are  certain  muscles  in  man  and  other  animals  winch 
are  pale  in  color  and  in  many  muscles  pale  fibers  are  extensively  dis- 
tributed among  the  red  fibers.  The  involuntary  muscles,  from  their 
relation  to  the  viscera  of  the  body,  are  known  also  as  visceral,  and  from 
their  microscopic  appearance  as  plain,  smooth,  or  non-striated  muscles. 

THE  VOLUNTARY  OR  SKELETAL  MUSCLE. 

x\ll  skeletal  muscles  consist  of  a  central  fleshy  portion,  the  bodv 
■or  belly,  provided  at  either  extremity  with  a  tendon  in  the  form  of  a 
cord  or  membrane.     The  body  is  the  active,  contractile  region,  the 

53 


54 


TEXT-BOOK  OF  PHYSIOLOGY. 


source  of  the  movement ;  the  tendon  is  the  inactive  region,  the  passive 
transmitter  of  the  movement  to  the  bones. 

A  skeletal  muscle  is  a  complex  organ  consisting  of  a  framework  of 
connective  tissue,  supporting  muscle-fibers,  blood-vessels,  nerves,  and 
lymphatics.  The  general  body  of  the  muscle  is  covered  by  a  dense 
layer  of  connective  tissue,  the  epi-mysium,  which  blends  with  and 
partly  forms  the  tendon.  From  the  under  surface  of  this  covering, 
septa  of  connective  tissue  pass  inward,  dividing  and  grouping  the 
fibers  into  larger  and  smaller  bundles,  termed  fasciculi.     The  fasciculi, 

invested  by  a  special  sheath,  the 
r^ll^sLsffla  peri-mysium,    are    prismatic   in 

shape  and  on  cross-section  pre- 
sent an  irregular  outline.  The 
muscle-fibers  composing  the  fas- 
ciculi are  separated  one  from 
another  and  supported  by  a  very 
delicate  connective  tissue,  the 
endo-mysium.  The  connective 
tissue  thus  surrounding  and 
penetrating  the  muscle  binds  the 
fibers  into  a  distinct  organ  and 
affords  support  to  all  remaining 
structures  (Fig.  16). 

Histology  of  the  Skeletal 
Muscle -fiber. — The  muscle- 
fiber  is  the  ultimate  anatomic 
units  of  the  muscle  system.  The 
fibers  for  the  most  part  are  ar- 
ranged parallel  one  to  another 
and  in  a  direction  corresponding 
to  the  long  axis  of  the  muscle. 
They  vary  in  length  from  30  to 
40  millimeters  and  in  breadth 
from  20  to  30  mi cromilli meters. 
There  are  exceptional  fibers,  however,  which  have  a  much  greater 
length.  As  the  fibers  have  but  a  limited  length  in  the  vast  majority 
of  muscles,  each  end,  more  or  less  pointed  or  beveled,  is  united  to 
adjoining  fibers  by  cement.  In  this  way  a  muscle  is  increased  in 
length. 

When  examined  with  the  microscope,  the  muscle-fiber  is  seen  to  be 
cylindric  or  prismatic  in  shape  and  to  consist  of  a  thin  transparent 
membrane,  the  sarcolemma,  in  which  is  contained  the  true  muscle 
or  sarcous  substance.  The  sarcolemma  is  elastic  and  adapts  itself 
to  all  changes  of  form  the  sarcous  substance  undergoes.  Beneath 
the  sarcolemma  there  are  several  nuclei  surrounded  by  granular 
material.  Each  fiber  also  presents  a  series  of  transverse  bands  alter- 
nately dim  and  bright  which  give  to  it  a  striated  appearance.     If  the 


t# 


Fig.  16. — From  a  Cross-section  of  the 
Adductor  Muscle  of  a  Rabbit.  P.  Peri- 
mysium, containing  two  blood-vessels,  at  g; 
m,  muscle-fibers;  many  are  shrunken  and  be- 
tween them  the  endomysium,  p,  can  be  seen; 
at  x  the  section  of  muscle-fiber  has  fallen  out. 
X  60.— {Stohr.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


55 


Fig.  17. — Muscle-fiber 
of  a  Rabbit.  a.  Dark 
band.  b.  Light  band.  c.  In- 
termediate line,  n.  Nucleus. 
— {Landois  and  Stirling. — 
Ranvier.) 


bright  bands  are  examined  with  high  magnifying  powers,  each  one  is 
seen  to  be  crossed  by  a  fine  dark  line  which  at  the  time  of  its  discovery 
was  regarded  as  the  optic  expression  of  a  mem- 
brane attached  laterally  to  the  sarcolemma. 
It  has  since  been  resolved  into  a  row  of  gran- 
ules (Fig.  17). 

The  muscle-fiber  also  presents  a  longit- 
udinal striation  which  indicates  that  it  is 
composed  of  finer  elements  placed  side  by  side, 
termed  fibrillae.  The  fibrillae  extend  through- 
out the  entire  length  of  the  fiber,  though  they 
are  not  of  uniform  thickness  (Fig.  18).  That 
portion  of  the  fibril  corresponding  in  position 
to  the  dim  band  is  thick,  prismatic,  or  rod-like 
in  shape,  and  termed  a  sarcostyle;  that  por- 
tion corresponding  in  position  to  the  bright 
band  is  extremely  thin  and  narrow  and  pre- 
sents at  its  middle  a  slight  enlargement  or 
granule.  The  fibrillae  are  embedded  in  a  clear 
transparent  fluid  which,  from  its  supposed  nutritive  character,  is  termed 
sarcoplasm.  The  diminution  in  caliber  of  the  fibrillae  at  different  levels 
permits  of  the  accumulation  and  storage  of  a 
larger  amount  of  nutritive  material  than  could 
otherwise  be  the  case.  It  is  for  this  reason 
that  the  fiber  at  these  points  presents  a 
brighter  appearance. 

When  the  muscle-fiber  is  examined  under 
crossed  Nichol  prisms,  the  dim  band  appears 
bright  and  the  bright  band  appears  dim  against 
a  dark  background,  indicating  that  the  former 
is  doubly  refracting  or  anisotropic,  the  latter 
singly  refracting  or  isotropic. 

The  pale  muscle  fiber  is  histologically 
similar  to  the  red.  It  is,  however,  somewhat 
larger,  paler  in  color  and  does  not  contain  so 
much  muscle  material.  The  transverse  striation 
is  closer  and  the  nuclei  are  not  so  abundant. 
The  Blood-supply. — Muscles  in  the 
physiologic  condition  require  for  the  main- 
tenance of  their  activity  a  large  amount  of 
nutritive  material.  Tins  is  obtained  di recti v 
from  the  lymph  and  indirectly  from  the  blood 
furnished  by  the  blood-vessels.  The  vascular 
supply  to  the  muscles  is  very  great  and  the 
disposition  of  the  capillary  vessels  with  refer- 
ence to  the  muscle-fiber  is  very  characteristic.  The  arterial  vessels, 
after  entering  the  muscle,  are  supported  by  the  peri-mysium;  in  this 


Fig.  18 — .4.  Diagram  of 
arrangement  of  the  contrac- 
tile substance  according  to 
the  view  of  Rollett;  the 
granular  figures  represent 
the  contractile  elements,  the 
intervening  light  areas  the 
sarcoplasm.  B.  Small 
muscle-fiber  of  man,  the 
corresponding  parts  in  the 
two  figures  are  indicated; 
t,  i,  I,  respectively  the  trans- 
verse, the  intermediate,  and 
lateral  discs,  n.  Muscle 
nu  clei . — (P  iersol. ) 


56 


TEXT-BOOK  OF  PHYSIOLOGY. 


LYMPH  SPACE 


MUSCLE 
FIBER 


-CAPILLARY  BLOOD 
VESSEL 


situation  they  give  off  short,  transverse  branches,  which  immediately 
break  up  into  a  capillary  network  of  rectangular  shape  within  which 
the  muscle-fibers  are  contained.  The  relation  of  the  capillary  vessel 
to  the  muscle-fiber  is  shown  in  Fig.  19. 

The  muscle-fiber,  in  intimate  relation  with  the  capillary,  is  bathed 
with  lymph  derived  from  it.  Its  contractile  substance,  however,  is 
separated  from  the  lymph  by  its  own  investing  membrane,  through 
which  all  interchange  of  nutritive  and  waste  material  must  take  place. 
The  nutritive  material  passes  through  the  capillary  wall  into  the 
lymph-space,  then  through  the  sarcolemma  into  the  interior  of  the 

fiber,  where  it  comes  into  re- 
lation with  the  living  muscle 
material.  The  waste  prod- 
ucts arising  in  the  muscle 
as  a  result  of  nutritive  changes 
pass  in  the  reverse  direction 
first  into  the  lymph  and  then 
into  the  blood,  by  which  they 
are  carried  away  to  eliminat- 
ing organs.  Lymphatics  are 
present  in  muscle,  but  confined 
to  the  connective  tissue,  in  the 
spaces  of  which  they  take  their 
origin. 

The  Nerve-supply. — The 
nerves  which  carry  the  stimuli 
to  a  muscle  enter  near  its 
geometric  center.  Many  of 
the  fibers  pass  directly  to  the 
muscle-fibers  with  which  they 
are  connected;  others  are  dis- 
tributed to  blood-vessels. 
Every  muscle-fiber  is  supplied  with  a  special  nerve-fiber  except  in 
those  instances  where  the  nerve-trunks  entering  a  muscle  do  not  con- 
tain as  many  fibers  as  the  muscle.  In  such  cases  the  nerve-fibers  divide- 
near  their  termination  until  the  number  of  branches  equals  the  number 
of  muscle-fibers.  The  individual  muscle-fiber  is  penetrated  near  its 
center  by  the  nerve  where  it  terminates;  the  ends  being  practically 
free  from  nerve  influence.  The  stimulus  that  comes  to  the  muscle- 
fiber  acts  primarily  upon  its  center,  the  effect  of  which  then  travels  in 
both  directions  to  the  ends.  The  manner  in  which  the  nerve-fibers 
terminate  in  muscle  will  be  more  fully  described  in  connection  with 
the  histology  of  the  nerve  tissue. 

CHEMIC  COMPOSITION  OF  MUSCLE. 

The  chemic  composition  of  living  muscle  is  but  imperfectly  under- 
stood owing  to  the  fact  that  shortly  after  death  some  of  its  constituents 


Fig.  19. — Relation  of  the  Blood-ves- 
sel to  the  Muscle-fiber. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  57 

undergo  a  spontaneous  coagulation  and  for  the  reason  that  the  methods 
employed  for  analysis  also  tend  to  alter  its  composition.  To  human 
muscle,  the  following  average  percentage  composition  has  been  given: 

Water, 73.5 

Proteins,    including    those    of    sarcolemma,    connective- 
tissue,  pigments, 18.02 

Gelatin, 1.99 

Fat, 2.27 

Extractives, 0.22 

Inorganic  salts, 3. 12.     (Halliburton.) 

(The  composition  of  muscles  of  different  animals,  consumed  as 
foods,  will  be  found  in  the  chapter  on  Foods.) 

When  fresh  muscle  is  freed  from  fat  and  connective  tissue,  frozen, 
rubbed  up  in  a  mortar,  and  expressed  through  linen,  a  slightly  yellow 
syrupy  alkaline  or  neutral  liquid  is  obtained  which  has  been  termed 
muscle-plasma.  This  fluid  at  normal  temperatures  coagulates 
spontaneously,  the  phenomena  resembling  in  many  respects  those 
observed  in  the  coagulation  of  blood-plasma.  The  coagulum  subse- 
quently contracts  and  squeezes  out  an  acid  muscle-serum.  The 
coagulated  protein  is  known  as  myosin  and  belongs  to  the  class  of 
globulins.  Inasmuch  as  it  is  not  present  in  living  muscle  and  only 
makes  its  appearance  under  conditions  not  strictly  physiologic,  it  is 
regarded  as  a  derivative  of  a  pre-existing  protein  which  has  been 
termed  myosinogen.  According  to  Halliburton,  the  proteins  of  living 
muscle  are  four  in  number,  distinguished  by  their  varying  solubilities 
in  different  salts  and  by  the  varying  temperatures  at  which  they 
coagulate.  From  muscle-plasma  may  then  be  obtained:  (1)  Para- 
myosinogen and  (2)  myosinogen,  the  former  coagulating  at  47  °  C,  the 
latter  at  56 °  C.  It  is  myosinogen  which  is  converted  into  myosin 
under  the  influence  of  some  special  ferment,  though  both  enter  into 
the  formation  of  the  muscle-clot.  From  the  muscle-serum  may  also 
be  obtained  at  68°  C.  a  globulin  body  termed  myoglobulin  and  a 
small  quantity  of  myoalbumin.  Among  the  proteids  may  be  men- 
tioned hemoglobin,  which  gives  the  color  to  the  muscles.  Spectro- 
scopic investigation  reveals  the  presence  of  a  special  pigment,  myo- 
hematin,  which  is  supposed  to  have  a  respiratory  function,  inasmuch 
as  its  absorption  bands  change  by  oxidation  and  reduction. 

Among  the  extractives  containing  nitrogen  may  be  mentioned 
creatin,  creatinin,  xanthin,  carnin,  urea,  uric  acid,  carnic  acid,  etc. 
Among  the  extractives  free  of  nitrogen,  glycogen,  dextrose,  inosite, 
lactic  acid,  fat,  are  the  most  important.  Inorganic  salts  are  relatively 
abundant,  of  which  potassium  is  the  most  abundant  among  the  bases, 
and  phosphoric  acid  among  the  acids. 

THE    PHYSICAL    AND    PHYSIOLOGIC    PROPERTIES    OF   MUSCLE- 
TISSUE. 

Consistency. — The  consistency  of  muscle-tissue  during  life 
varies  considerablv  in  accordance  with  different  states  of  the  muscle. 


58 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  20. — Extension   Curve  of 
Muscle. — (Gad.) 


In  a  state  of  tension  it  is  hard  and  resistant;  in  the  absence  of  tension 
it  is  soft  and  fluctuating  to  the  sense  of  touch.  Tension  alone  gives 
rise  to  hardness. 

Cohesion. — The  cohesion  of  a  muscle  is  largely  dependent  on  the 

quantity  of  connective  tissue  it  contains. 
A  band  of  fresh  human  muscle  one  square 
centimeter  in  cross-section  was  able  to 
resist  a  weight  of  14  kilograms  without 
rupture  (MacAlister).  Cohesion  resists 
the  forces  of  traction  and  pressure. 

Elasticity. — Muscle,  in  common  with 
many  other  organic  as  well  as  inorganic 
substances,  is  capable  of  being  extended 
beyond  the  normal  length  through  the 
action  of  external  forces  and  of  resuming 
the  normal  length  when  these  forces 
cease  to  act.  All  such  bodies  are  said  to 
be  elastic;  and  the  greater  the  variations 
between  the  natural  and  acquired  lengths, 
the  greater  is  their  elasticity  said  to  be. 
Muscle,  therefore,  possesses  extensibility 
and  elasticity.*  If  the  muscle  of  a  frog, 
preferably  the  sartorius,  the  fibers  of 
which  are  arranged  in  a  practically  parallel  manner,  be  fastened  at  one 
extremity  by  a  clamp,  and  then  extended  by  a  series  of  successive 
weights  which  differ  by  a  common  increment,  it  will  be  found  that  the 
extensibility  of  muscle  does  not  follow  the  law  of  elasticity  as  deter- 
mined for  inorganic  bodies;  i.  e.,  directly 
proportional  to  the  weight  and  to  the  length 
of  the  body  extended;  but  that  while  in- 
creasing in  length  with  each  successive 
weight,  the  increase  is  always  in  a  diminish- 
ing ratio.  Thus,  for  example,  as  shown  in 
Fig.  20:  The  extension  produced  by  5 
grams  is  5  millimeters,  that  produced  by  10 
grams  is  only  4  millimeters  more,  and  so  on 
with  additional  weights  until  the  increase  in 
passing  from  25  to  30  grams  is  only  1  milli- 
meter. The  extensibility  is  thus  shown  to 
be  proportionately  greater  with  small  than 

with  larger  weights.  It  is,  however,  actually  greater  with  the  larger 
weights.  The  extension  curve  A  B  formed  by  joining  the  ends  of  the 
muscle  approximates  that  of  a  parabola.  The  muscle  in  returning  to 
its  original  length  also  shows  a  variation  from  the  behavior  of  inorganic 
bodies.     With  the  successive  removal  of  the  weights,  the  elasticity  of  the 

*  By  this  latter  term  is  here  meant  the  power  by  virtue  of  which  the  muscle  returns 
to  its  original  length  and  is  used  synonymously  with  perfect  retractibility. 


Fig.  21. — Curve  of  Elas- 
ticity Produced  by  Continu- 
ous Extension  and  Recoil 
of  a  Frog's  Muscle.  0  x.  Ab- 
scissa before;  x',  after  exten- 
sion.— (Landois  and  Stirling.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  59 

muscle  asserts  itself  with  gradually  increasing  energy  until  its  normal 
length  is  nearly,  if  not  entirely,  regained  (Fig.  21).  Though  it  is 
usually  stated  that  the  elasticity  of  muscle  is  incomplete,  it  must  be 
borne  in  mind  that  the  experiments  have  usually  been  made  on  muscles 
removed  from  the  body,  deprived  of  blood  and  nerve  influences,  and 
hence  under  abnormal  conditions.  It  is  highly  probable  that  in  the 
living  body  muscles  possess  perfect  elasticity  which  enables  them  to 
completely  return  to  their  normal  length  after  extension.  The  extension 
and  elastic  recoil  of  muscle  depends  on  the  maintenance  of  physiologic 
conditions.  If  the  nutrition  is  impaired  by  fatigue,  deficient  blood- 
supply,  or  any  pathologic  condition,  the  elasticity  is  at  once  impaired. 

Tonicity. — This  is  a  property  possessed  by  all  muscles  in  the 
body  in  consequence  of  being  stretched  to  a  slight  extent  beyond  their 
normal  length.  This  may  be  due  to  the  action  of  antagonistic  muscles 
or  to  their  mode  of  growth,  the  muscles  growing  somewhat  more 
slowly  than  the  bones  to  which  they  are  attached.  That  muscles  are 
so  stretched  is  shown  by  the  shortening  which  at  once  takes  place 
when  their  tendons  are  divided.  This  muscle  tonus  or  tension  is 
closely  connected  with  the  elasticity  and  plays  an  important  role  in 
muscle  contraction;  being  always  on  the  stretch,  the  muscle  loses  no 
time  in  acquiring  that  degree  of  tension  necessary  to  immediate  action 
on  the  bone  to  which  it  is  attached.  The  working  power  of  a  muscle 
is  also  increased  by  the  presence,  within  limits,  of  some  resistance  to 
the  act  of  contraction.  According  to  Marey,  the  amount  of  work  is 
considerably  increased  when  the  muscle  energy  is  transmitted  by  an 
elastic  body  to  the  mass  to  be  moved,  while  at  the  same  time  the  shock 
of  the  contraction  is  lessened.  The  position  of  a  passive  limb  is  the 
resultant  also  of  the  elastic  tension  of  antagonistic  groups  of  muscles. 

Another  explanation  for  the  tonicity  of  muscle  is  found  in  the  fact 
that  the  skeletal  muscles  of  the  body  receive  continuously  nerve  im- 
pulses from  the  nerve  cells  of  the  spinal  cord  as  a  consequence  of  the 
arrival  of  nerve  impulses  reflected  through  afferent  nerves  from  the 
tendons  and  muscles  themselves.  The  stimulus  here  being  the  slight 
degree  of  extension  and  variations  in  extension  to  which  the  muscles 
are  being  subjected  from  moment  to  moment.  That  this  is  a  con- 
siderable factor  in  the  production  of  the  tonus  is  shown  by  the  effects 
which  follow  division  of  the  afferent  or  dorsal  roots  of  those  spinal 
nerves  coming  from  any  given  muscle  group.  With  the  division  of 
the  nerves  the  muscles  relax  and  lose  their  usual  tone.  As  a  result  of 
this  slight  but  constant  stimulation  from  the  spinal  cord,  the  metabolic 
changes  of  the  muscle  material  are  maintained  at  a  certain  level  with 
a  corresponding  liberation  of  heat.  The  chief  function  of  the  tonicity 
would  thus  be  the  production  of  heat,  other  functions  which  the  tone 
subserves  being  merely  secondary. 

Irritability,  Contractility. — These  are  terms  employed  to  denote 
that  property  of  muscle-tissue  by  virtue  of  which  it  responds  by  a 
change  of  form,  becoming  shorter  and  thicker  on  the  application  of 


60  TEXT-BOOK  OF  PHYSIOLOGY. 

a  stimulus.  On  the  withdrawal  of  the  stimulus  the  muscle  again 
undergoes  a  reverse  change  of  form,  becoming  longer  and  narrower, 
and  returning  to  its  original  condition.  All  muscles  which  possess  this 
capability  are  said  to  be  irritable  and  contractile;  and  all  agents  which 
call  forth  this  response  of  the  muscle  are  termed  stimuli.  The  rapid 
change  of  form  which  a  highly  irritable  muscle  undergoes  in  response 
to  the  action  of  a  stimulus  of  short  duration  is  usually  termed  a  twitch 
or  pulsation.  With  appropriate  apparatus  it  can  be  shown  that  the 
muscle  at  the  time  of  the  twitch  becomes  warmer  and  exhibits  electric 
phenomena.  The  muscle  is  therefore  an  apparatus  for  the  conversion 
of  potential  into  kinetic  energy:  viz.,  heat,  electricity,  and  mechanic 
motion. 

Though  usually  associated  with  the  activity  of  the  nerve  system, 
and  to  some  extent  dependent  on  it,  irritability  is  nevertheless  an 
independent  endowment  of  the  muscle  and  persists  for  a  longer  or 
shorter  period,  as  shown  by  many  experiments,  after  all  nerve  con- 
nections have  been  destroyed.  Among  the  proofs  which  may  be 
presented  in  support  of  this  view  are  the  following :  The  introduction 
of  the  drug,  curara,  into  the  body  of  an  animal  produces  in  a  short 
time  complete  paralysis.  Experiment  has  shown  that  curara  sus- 
pends the  conductivity  of  the  intramuscular  terminations  of  the  nerve- 
fiber  and  thus  separates  the  muscle  entirely  from  the  nerve.  Though 
the  animal  is  incapable  of  executing  a  single  movement,  its  muscles 
respond  promptly  on  the  application  of  a  stimulus.  Moreover, 
portions  of  muscles  exhibit  irritability  in  which  there  is  no  trace  of 
nerve  structure.  This  is  the  case  with  the  ends  of  the  sartorius  muscle 
of  the  frog  and  the  anterior  end  of  the  retractor  muscle  of  the  eyeball 
of  the  cat.  These  and  other  facts  demonstrate  the  independence 
of  muscle  irritability. 

In  the  living  body  irritability  and  nutritive  activity,  with  which 
it  is  closely  associated,  are  maintained  by  a  due  supply  of  oxygen, 
of  nutritive  material,  the  removal  of  waste  products,  and  a  normal 
temperature.  The  muscles  of  the  cold-blooded  animals,  and  especially 
the  frog,  retain  their  irritability  for  a  much  longer  period  after  death 
than  the  muscles  of  the  warm-blooded  animals.  This  is  the  case  also 
with  the  individual  muscles  after  removal  from  the  body  of  the  animal. 
The  reason  for  this  is  found  in  all  probability  in  the  difference  in  the 
rate  of  their  nutritive  activities  and  in  the  quantity  of  nutritive  material 
stored  up  in  their  cells.  The  duration  of  the  irritability  of  isolated 
muscles  can  be  considerably  prolonged  by  keeping  them  in  a  moist 
atmosphere. 

Muscle  Stimuli. — Though  consisting  of  a  highly  irritable  tissue, 
muscles  do  not  possess  spontaneity  of  action.  They  require  for  the 
manifestation  of  their  characteristic  activity  the  application  of  a 
stimulus.  In  the  living  body  all  contractions,  at  least  of  the  skeletal 
muscles,  occurring  under  normal  or  physiologic  conditions  are  caused 
by  the  action  of  "nerve  impulses"  transmitted  by  the  nerves  from  the 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  61 

central  nerve  system  to  the  muscles.  The  nerve  impulse  is  the  normal 
or  physiologic  stimulus.  After  removal  from  the  body  and  freed  from 
nerve  connections  muscles  can  be  excited  to  action  by  various  agents 
of  a  mechanic,  chemic,  thermic,  or  electric  nature.  These  are  artificial 
or  non-physiologic  stimuli. 

i.  Mechanic  Stimuli. — Cutting,  pinching,  sharply  tapping  the  muscle 
will  cause  it  to  contract,  providing  the  stimulus  has  sufficient 
intensity.  With  each  stimulation  a  short,  fleeting  contraction 
ensues.  If  repeated  with  sufficient  rapidity,  a  series  of  con- 
tinuous but  irregular  pulsations  are  produced. 

2.  Chemic    Stimuli. — Various    chemic    substances    in    solution    will 

excite  single  or  continuous  pulsations  if  the  strength  of  the  solu- 
tion is  not  such  as  to  destroy  at  once  the  irritability.  They  owe 
their  efficiency  as  stimuli  to  the  rapidity  with  which  they  alter 
the  composition  of  the  muscle-substance.  Among  these  may  be 
mentioned  solutions  of  potassium  and  sodium,  weak  solutions  of 
the  mineral  and  organic  acids,  ammonium  vapor,  distilled  water, 
glycerin,  and  sugar. 

3.  Thermic  Stimuli. — The  application  of    a  heated  object,  such  as  a 

hot  wire,  causes  the  muscle  to  rapidly  contract.' 

4.  Electric    Stimuli. — The  most  efficient  stimulus  and  the  one  least 

injurious  to  the  tissue  is  the  electric  current.  Either  the  con- 
stant or  the  induced  current  may  be  used.* 

The  Constant  Current. — If  the  ends  of  the  wires  in  connection  with 
an  electric  cell  be  provided  with  non-polarizable  electrodes  and  the 
latter  placed  on  opposite  ends  of  a  freshly  prepared  sartorius  muscle 
of  a  frog  which  has  been  previously  curarized,  it  will  be  found  on 
closing  or  making  the  circuit  that  the  muscle  will  exhibit  a  short  quick 
pulsation.  During  the  actual  passage  of  the  current,  especially  if 
it  is  weak,  there  may  be  no  apparent  change  in  the  muscle.  If  the 
current  is  strong,  the  muscle  may,  on  the  contrary,  remain  in  a  state 
of  continuous  contraction.  With  the  opening  or  breaking  of  the 
current  the  muscle  at  once  relaxes,  or  perhaps  again  contracts  and 
then  relaxes.  The  extent  of  the  contraction  depends  mainly  on  the 
strength  of  the  current,  being  greater  with  strong,  less  with  weak 
currents.  When  the  current  is  sufficiently  strong  to  elicit  both  making 
and  breaking  contractions,  it  is  found  that  the  contraction  occurring 
on  the  make  or  closure  of  the  circuit  is  always  greater  than  that  occur- 
ring on  the  break  or  opening  of  the  circuit.  Moreover,  it  has  been 
shown  in  many  ways  that  the  contraction  occurring  on  the  closure  of 
the  circuit  has  its  origin  at  the  point  where  the  current  is  leaving  the 
muscle — i.  e.,  in  the  immediate  neighborhood  of  the  negative  pole  or 

*  Since  the  study  of  the  physiologic  properties  of  both  muscle-tissue  and  nerve-tissue 
involves  the  employment  of  electricity  as  a  stimulus,  it  becomes  necessary  for  the  student 
to  familiarize  himself  with  certain  forms  of  apparatus  by  which  it  is  generated,  controlled, 
and  applied.  For  the  purpose  of  not  interrupting  the  continuity  of  the  text  this  inform- 
ation is  embodied  in  an  appendix.  The  facts  therein  contained  should  be  mastered  at 
this  time  by  the  student. 


62  TEXT-BOOK  OF  PHYSIOLOGY. 

cathode — and  propagates  itself  to  the  opposite  extremity;  while  the 
contraction  occurring  on  the  opening  of  the  circuit  has  its  origin  at  the 
point  where  the  current  is  entering  the  muscle,  i.  e.,  in  the  neighborhood 
of  the  positive  pole  or  anode. 

These  facts  can  be  readily  demonstrated  by  destroying  the  ir- 
ritability and  contractility  of  one  extremity  of  a  muscle  with  parallel 
fibers  such  as  the  sartorius.  On  applying  non-polarizable  electrodes 
to  the  muscle  as  in  Fig.  22,  A,  it  will  be  found  that  when  the  circuit 
is  made  a  contraction  occurs  which  must,  of  course,  have  developed  at 
the  irritable  cathodic  region,  for  on  the  break  of  the  circuit  the  muscle 
remains  at  rest.  When  the  electrodes  are  applied  as  in  Fig.  22,  B,  and 
the  circuit  made  the  muscle  remains  at  rest,  but  on  the  break  of  the 
circuit  a  contraction  occurs  which  must  have  developed  at  the  irritable 
anodic  region. 

The  Induced  Current. — If  the  primary  spiral  of  the  inductorium 
be  connected  with  an  electric  cell  and  the  secondary  spiral  be  con- 
nected with  a  muscle,  it  will  be  found  that  the  current  induced  in  the 


A.  B. 

Fig.  22. — Diagram  to  Show  the  Effect  of  Local  Injury  on  the  Irritability  of 
A  Muscle  (after  Starling).  C  Z  an  electric  cell  from  which  wires  pass  to  non-polarizable 
electrodes,  anode  and  kathode,  in  contact  with  a  muscle,  the  injured  end  of  which  is  more 
deeply  shaded.     The  arrows  indicate  the  direction  of  the  current. 

secondary  circuit,  both  on  the  make  and  break  of  the  primary,  will 
also  cause  the  muscle  to  sharply  and  rapidly  pulsate  if  the  two  spirals 
are  sufficiently  near  each  other.  Observation,  however,  makes  it 
evident  that  the  pulsation  occurring  with  the  break  of  the  primary 
circuit  is  more  energetic  than  that  occurring  with  the  make,  a  result 
the  opposite  of  that  obtained  with  the  constant  current.  This  is 
not  due  to  any  difference  in  the  electricity,  however,  but  to  peculiarities 
in  the  construction  of  the  inductorium.  When  the  primary  circuit 
is  interrupted  with  sufficient  frequency,  as  it  can  be  by  throwing  into 
the  circuit  Neef's  hammer  or  some  other  form  of  interrupter,  the  con- 
tractions excited  by  the  induced  currents  may  be  made  to  succeed  one 
another  so  rapidly  that  they  become  fused  together,  producing  a 
spasm  or  tetanus  of  the  muscle.  The  rapidity  with  which  the  induced 
current  appears  and  disappears,  its  brief  duration,  the  ease  with  which 
its  strength  can  be  regulated,  combine  to  render  it  a  most  efficient 
stimulus  for  either  muscle  or  nerve. 

Conductivity. — All    muscle    protoplasm    possesses    conductivity. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  63 

The  change  excited  in  a  muscle-fiber  by  the  arrival  of  a  nerve  impulse 
is  at  once  conducted  with  great  rapidity  in  opposite  directions  to 
the  end  of  the  fibers;  the  advance  of  the  excitation  process  is  im- 
mediately succeeded  by  the  contraction  process,  the  change  of  form 
which  constitutes  the  contraction.  With  the  disappearance  of  the 
former,  the  latter  also  disappears  and  the  muscle  resumes  its  pre- 
vious passive  condition.  There  is  no  evidence,  however,  that  the 
excitation  process  travels  transversely — that  is,  into  adjoining  fibers 
— being  prevented  from  doing  so  by  the  presence  of  the  limiting 
membranes,  the  sarcolemmata.  The  fact  that  each  muscle-fiber 
receives  its  own,  or  at  least  a  branch  of  a  nerve-fiber,  and  hence  its 
own  nerve  impulse  or  stimulus,  would  also  indicate  that  the  excitation 
process  can  not  be  conducted  longitudinally  into  adjoining  fibers, 
or  at  least  with  sufficient  rapidity  for  the  purposes  of  ordinary  muscle 
actions.  Nevertheless  if  a  long  muscle,  such  as  the  sartorius,  from  a 
curarized  frog  be  stimulated  at  one  end  with  an  induced  electric  cur- 
rent, the  excitation  and  the  contraction  processes  will  be  conducted 
with  extreme  rapidity  to  the  opposite  end  of  the  muscle.  The  rapidity 
of  conduction  in  human  muscles  has  been  estimated  at  from  10  to  13 
meters  per  second,  and  in  frog's  muscle  at  from  3  to  4.5  meters  per 
second.  The  contraction  process,  the  thickening  of  the  muscle,  is 
termed  the  contraction  wave.  As  it  is  the  result  of  the  excitation 
process  and  immediately  succeeds  it,  its  rate  of  conduction  must  be 
the  same  as  that  given  above.  With  appropriate  apparatus  the  du- 
ration of  the  wave  at  any  given  point  has  been  shown  to  be,  in  the 
frog's  muscle,  one-tenth  of  a  second  and  its  length  three-tenths  of  a 
meter. 


PHENOMENA  ATTENDING  A  MUSCLE  CONTRACTION. 

PHYSICAL  PHENOMENA. 

The  most  obvious  change  in  a  muscle  during  the  contraction  is 
that  relating  to  its  form.  The  muscle  not  only  becomes  shorter,  but 
at  the  same  time  thicker.  The  extent  to  which  it  may  shorten  when 
unopposed  may  amount  to  30  per  cent,  or  more  of  its  original  length. 
The  increase  in  thickness  practically  compensates  for  the  diminution 
in  length,  for  there  is  no  observable  diminution  in  volume.  The 
change  in  form  of  the  entire  muscle  results  from  a  corresponding 
change  of  form  of  its  individual  fibers  as  determined  by  microscopic 
examination,  each  of  which  becomes  shorter  and  thicker.  The 
successive  changes  in  both  the  muscle  and  the  individual  fibers  are 
represented  in  Fig.  23. 

When  the  contraction  begins,  the  dim  band  increases  and  the 
bright  band  diminishes  in  width.  This  Engelmann  attributes  to  the 
passage  of  fluid  material  from  the  bright  into  the  dim  band.  At  the 
time  of  relaxation  there  is  a  return  of  this  material  and  the  bands 
assume  their  original  shape  and  volume.     As  the  contraction  wave 


64 


TEXT-BOOK  OF  PHYSIOLOGY. 


reaches  its  maximum  the  optic  properties  of  the  bright  and  dim  bands 
change.  The  former  now  becomes  darker  and  less  transparent 
until  at  the  crest  of  the  wave  it  assumes  the  appearance  of  a  distinct 
dark  band;  the  latter  now  becomes  clear  and  bright  in  comparison. 
This  change  in  the  appearance  of  the  fiber  is  due  to  an  increase  in 
refrangibility  of  the  bright,  and  a  decrease  in  the  refrangibility  of  the 
dim  band,  coincident  with  the  passage  of  the  fluid  from  the  former 
into  the  latter.  There  is  at  the  height  of  the  contraction  a  complete 
reversal  in  the  positions  of  the  striations.  At  a  certain  stage  between 
the  beginning  and  the  crest  of  the  wave  the  striae  almost  entirely  dis- 
appear, giving  to  the  fiber  an  appearance  of  homogeneity.     There  is, 


Fig.  23. — Showing  the  Changes  in  a  Muscle  and  Muscle-fiber  during 

Contraction. 


however,  no  change  in  refractive  power  as  shown  by  the  polarizing 
apparatus.  When  the  contraction  wave  has  reached  the  stage  of 
greatest  intensity,  there  is  a  reversal  of  the  above  phenomena  as  the 
fiber  returns  to  its  former  condition,  that  of  relaxation. 

Elasticity. — During  the  contraction  of  a  muscle  there  is  a  greater 
or  less  alteration  in  its  elasticity,  as  shown  by  the  fact  that  it  is  ex- 
tended to  a  greater  degree  by  the  same  weight  in  the  active  than  in 
the  passive  condition.  The  degree  to  which  the  extensibility  is  in- 
creased and  the  elasticity  decreased  is  dependent  on  the  amount  of 
the  resisting  force.  These  facts,  as  determined  experimentally,  are 
represented  in  Fig.  24.  Let  A  B  and  A  b  represent  the  length  of  the 
normal  unweighted  muscle,  passive  and  active  states  respectively;  the 
line  B  B',  the  extension  curve  of  the  passive  muscle  produced  by 
successive  weights,  5,  10,  15,  20,  25,  30  grams,  differing  by  a  com- 
mon increment;  the  line  b  B',  the  extension  curve  of  the  active  con- 
tracted muscle  when  weighted  with  the  same  weights;  A'  B'  the  length 
of  the  muscle  when  the  weight  is  sufficiently  great  to  prevent  shorten- 
ing. It  will  be  observed  from  these  facts  that  while  the  muscle  is 
extended   in   both    the    passive   and    active   states   by   corresponding 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


65 


weights,  the  extension  during  the  latter  is  progressively  greater,  until 
with  a  given  weight  the  length  of  the  muscle  is  the  same.  Under 
such  circumstances,  there  being  no  shortening  of  the  muscle,  the 
energy  of  its  contraction  manifests  itself  physically  merely  as  tension. 
In  the  successive  actions  of  the  muscle  represented  in  the  same  figure 
there  is  to  be  observed  also  a  combination  of  a  change  of  length  and 
a  change  of  tension,  the  ratio  of  the  one  to  the  other  being  deter- 


Fig.  24. — Extension  Curves:     B  B',  of  the  resting;  b  B',  of  the  contracting  muscle. 

mined  by  the  amount  of  the  supported  weights.  When  the  weight 
is  slight  in  amount,  the  shortening  of  the  muscle  reaches  a  maximum 
and  the  tension  a  minimum;  when  the  weight  is  large  in  amount,  the 
reverse  conditions  obtain. 


THE   CONTRACTION   PROCESS.     METHODS    OF  INVESTIGATION. 

The  contraction  of  a  muscle  as  it  takes  place  in  the  living  body  and 
under  normal  physiologic  conditions  is  a  complex  act  persisting  for 
a  variable  length  of  time  in  accordance  with  the  number  of  stimuli 
transmitted  to  it  in  a  given  unit  of  time,  and  as  determined  experi- 
mentally is  the  resultant  of  the  fusion  of  a  greater  or  less  number  of 
separate  and  individual  contractions  or  pulsations.  To  this  enduring 
contraction  the  term  tetanus  has  been  given.  With  the  aid  of  ap- 
propriate apparatus  it  has  become  possible  to  obtain  and  record 
single  muscle  contractions,  to  analyze  and  decompose  them  into  their 
constituent  elements,  or  to  combine  them  in  such  a  manner  as  to  pro- 
duce practically  a  normal  physiologic  tetanus.  As  in  the  experi- 
mental study  of  the  phenomena  of  a  muscle  contraction  it  frequently 
becomes  necessary  to  remove  the  muscle  from  the  body  of  the  animal, 
the  muscles  of  warm-blooded  animals  are  not  well  adapted  for  this 
purpose,  owing  to  the  rapid  alteration  in  composition  they  undergo, 
with  a  consequent  loss  of  irritability,  when  deprived  of  their  normal 
blood-supply.  The  excised  muscles  of  cold-blooded  animals,  par- 
ticularly of  the  frog — in  which,  owing  to  the  relatively  slow  rate  of 


66 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  nutritive  activities,  the  irritability  and  contractility  endure  for  a 
relatively  long  period  of  time,  even  though  deprived  of  blood— are 
particularly  valuable  for  experimental  studies.  The  muscles  generally 
employed  are  the  gastrocnemius,  the  sartorius,  and  the  hyoglossus. 
If  kept  at  a  normal  temperature  and  moistened  with  0.6  per  cent. 
solution  of  sodium  chlorid,  such  a  muscle  will  contract  for  a  long  period 
of  time  on  the  application  of  any  form  of  stimulus,  but  especially  the 
electric. 

Graphic  Record  of  a  Muscle  Contraction. — Inasmuch  as  the 
changes  in  the  form  of  a  muscle  during  a  single  contraction  take  place 
with  extreme  rapidity,  their  succession,  peculiarities,  and  time  re- 
lations cannot  be  determined  with  any  degree  of  accuracy  by  the 
unaided  eye.  This  difficulty  can  largely  be  overcome  by  the  employ- 
ment of  the  graphic  method,  the  principle  of  which  consists  in  record- 


Fig.  25. — Myograph.     K.  Recording  cylinder.     M.  Moist  chamber.     L. 
Recording  lever.     W.  Weight.     I.  Induction  coil. 

ing  the  movements  by  means  of  a  pen  on  some  appropriate  moving 
and  receiving  surface.  To  accomplish  this  object  the  muscle  is  at- 
tached at  one  extremity  by  a  clamp  to  a  firm  support,  and  at  the  other 
extremity  to  a  weighted  lever,  which  is,  however,  sufficiently  light  to 
enable  it  to  take  up,  reproduce,  and  magnify  its  movements.  The 
end  of  the  lever  provided  with  a  pen  is  applied  to  a  smooth  surface, 
such  as  glazed  paper  on  a  cylinder  or  plate,  and  covered  with  lamp- 
black. If  the  surface  is  stationary,  the  contraction  is  recorded  as  a 
vertical  line;  if  i+  is  placed  in  movement  at  a  uniform  rate  by  clock- 
work, the  contraction  is  recorded  in  the  form  of  a  curve,  the  width  of 
the  arms  of  which  will  depend  on  the  rate  of  movement.  The  time 
relations  of  the  phases  of  the  contraction  can  be  obtained  by  placing 
beneath  the  lever  a  pen  attached  to  an  electro-magnet  thrown  into 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  67 

action  by  a  tuning-fork  vibrating  in  hundredths  of  a  second.  In 
order  to  determine  the  rapidity  with  which  the  contraction  follows 
the  stimulation,  it  is  essential  that  the  movement  of  the  latter  be  also 
recorded.  This  is  accomplished  by  an  automatic  key,  the  opening 
or  closing  of  which  develops  the  stimulus  which  excites  the  muscle. 
A  combination  of  these  different  appliances  constitutes  a  myograph 
and  the  curve  of  contraction  a  myogram.     (See  Fig.  25.) 

The  Isotonic  Myogram. — With  the  object  of  obtaining  a  curve 
of  successive  changes  in  the  length  of  a  muscle  during  a  single  con- 
traction and  at  the  same  time  avoiding  changes  in  tension,  the  weight 
attached  to  the  lever  should  be  applied  close  to  its  axis,  a  mechanic 


Fig.  26. — The  Isotonic  Myogram. 

■condition  which  practically  maintains  a  uniform  tension  throughout 
the  contraction.  To  this  method  the  term  "isotonic"  has  been  given 
and  the  curve  so  obtained  an  isotonic  myogram.* 

The  Character  of  an  Isotonic  Myogram. — With  the  muscle 
arranged  as  previously  described  and  stimulated  directly  with  a  single 
induction  shock,  the  contraction  will  be  recorded  in  the  form  of  a 
curve  similar  to  that  represented  in  Fig.  26,  in  which  the  horizontal  line 
represents  the  abscissa  of  time;  a,  the  moment  of  stimulation;  and  bed, 
the  degree  of  shortening  at  each  successive  moment.  The  undulating 
line  shows  the  time  relations,  the  distance  from  crest  to  crest  represent- 
ing hundredths  of  a  second.  The  curve  may  be  divided  into  three 
portions: 

1.  A  short  but  measurable  portion  between  the  point  of  stimulation 
and  the  first  evidence  of  the  shortening,  a  b,  known  as  the  "latent 
period."  The  duration  of  this  period-  for  the  skeletal  muscle 
of  the  frog  was  originally  determined  to  be  0.01  second,  but  with 
the  employment  of  more  accurate  apparatus  it  has  been  reduced 
to  0.0025  to  0.004  second.  During  this  period  it  is  supposed 
that  certain  chemic  changes  are  taking  place  preparatory  to  the 

*  In  the  ordinary  method  of  recording  a  muscular  movement,  i.  e.,  with  the  weight 
attached  to  the  lever  immediately  beneath  the  muscle  and  known  as  the  "loaded  method." 
a  certain  momentum  is  imparted  to  the  weight,  which  continues  after  the  muscle  has 
ceased  to  act,  both  when  shortening  and  relaxing,  and  so  imparts  to  the  recording  lever 
additional  movements  which  vitiate  the  true  character  of  the  curve 


68  TEXT-BOOK  OF  PHYSIOLOGY. 

exhibition  of  the  movement.  The  duration  of  the  latent  period 
is  influenced  by  a  variety  of  conditions,  e.  g.,  temperature,  fatigue, 
strength  of  stimulus,  etc. 

2.  An  ascending  portion,  b  c,  the  contraction  or  period  of  increasing 

energy.  The  contraction  as  shown  by  the  character  of  the  curve 
begins  slowly,  then  proceeds  rapidly,  and  again  slowly  as  the 
shortening  reaches  its  maximum.  The  contraction  may  be  said 
to  end  when  the  tangent  to  the  curve  becomes  parallel  with  the 
abscissa. 

3.  A  descending  portion,  c  d,  the  relaxation  or  period  of  decreasing 

energy.     The  relaxation  as  shown  by  the  character  of  the  curve 
begins  slowly,  then  proceeds  rapidly,  and  again  slowly  as  the 
muscle  attains  its  original  length.     The  termination  of  the  re- 
laxation is  at  the  point  where  the  curve  cuts  the  abscissa.     The 
curve  beyond  this  point  may  be  complicated  by  the  presence 
of  one  or  more  residual  or  after-vibrations,  which  are  probably 
due  to  the  inertia  of  the  lever  as  well  as  to  changes  in  the  muscle 
elasticity. 
The  duration  of  the  period  of  shortening  is  about  0.04  second,  and 
of  the  period  of  relaxation  0.05  second.     A  single  pulsation  of  the 
isolated  muscle  of  the  frog  therefore  occupies,  from  the  moment  of 
stimulation  to  termination,  the  tenth  of  a  second.     Muscles  of  many 
other  animals  have  a  contraction  period  the  duration  of  which  varies 
considerably  from  this.     Thus,  in  man  the  time  of  a  single  contrac- 
tion is  one-twentieth  of  a  second,  in  some  insects  one  three-hundredth 
of  a  second,  and  in  the  turtle  one  second.     Pale  muscles  have  a  shorter 
period  than  the  red. 

Influences  Modifying  the  Contraction  Process. — The  con- 
traction process  in  its  entirety  as  well  as  in  its  individual  parts  is 
considerably  modified  by  both  external  and  internal  conditions,  among 
which  may  be  mentioned  the  following: 

1.  Stimulus. — As  the  contraction  is  the  response  of  the  muscle  to 
a  stimulus,  the  vigor  of  the  former  is  proportional,  within  limits, 
to  the  strength  of  the  latter.  Thus  using  as  a  stimulus  the  single 
induced  current,  it  has  been  found  that  if  the  strength  of  the 
current  is  progressively  increased,  the  height  of  the  contraction 
will  correspondingly  increase  until  a  certain  maximum  height  is 
attained  (Fig.  27,  A);  then  notwithstanding  a  continued  increase 
in  the  strength  of  the  stimulus,  this  height  will  not  be  exceeded 
for  some  time.  But  if  the  strength  of  the  stimulus  be  yet  further 
increased,  there  comes  a  moment  when  the  contractions  again 
increase  in  vigor  and  a  second  maximum  height  is  attained  (Fig. 
27,  B).  Beyond  this  no  further  increase  in  height  is  observed. 
The  second  maximum  has  been  attributed  to  the  presence  in  the 
muscle  of  two  different  substances  differently  affected  by  changes 
in  temperature,  by  fatigue  and  by  various  drugs. 

The  rate  at  which  the  muscle  is  stimulated  with  a  given  stimulus 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  69 

of  uniform  strength  will  also  influence  the  character  of  the  contrac- 
tion process.  If  the  intervals  between  the  successive  stimulations 
be  such  as  permit  the  muscle  to  recover  from  the  effects  of  the 
contraction,  it  may  contract  as  many  as  a  thousand  times  without 
showing  any  particular  variation  from  the  normal  form;  but  if 
the  intervals  are  shorter  than  that  just  stated  it  is  found  that  from 
the  beginning  of  the  stimulation  each  succeeding  contraction 
slightly  exceeds  in  height  the  preceding  contraction,  until  a  cer- 
tain maximum  is  reached  and  maintained,  indicating  that  for 
some  reason  the  irritability  and  the  energy  of  the  contraction  have 
been  increased.  This  gradual  increase  in  the  height  of  the  con- 
traction has  been  termed  the  stair  case  effect,  or  the  treppe.     In 


A.  B. 

Fig.  27. — Tracing  Showing  the  Effects  of  a  Gradual  Increase  in  the  Strength 
of  the  Stimulus  on  the  Height  of  the  Contraction,  a.  Minimal  contraction;  ab. 
progressive  increase  in  the  height;  b  c.  first  maximum  (a  number  of  contractions  have  been 
omitted  for  economy  of  space);  de.  second  maximum. 

the  beginning  of  the  period  of  stimulation  there  is  sometimes  ob- 
served a  decrease  in  the  height  of  the  contraction  following 
several  stimulations  before  the  stair  case  effect  develops,  indi- 
cating a  temporary  decrease  in  the  irritability.  These  stair  case 
contractions  have  been  observed  in  the  muscle  of  both  warm- 
blooded and  cold-blooded  animals.  The  cause  for  this  increase 
in  irritability  upon  which  the  effect  depends  is  attributed  to  the 
presence  of  certain  chemic  substances  in  the  muscle  arising  as  a 
result  of  its  katabolism,  such  as  carbon  dioxid,  mono-potassium 
phosphate,  and  paralactic  acid.  These  compounds  when  pres- 
ent in  small  amounts  or  in  larger  amounts  for  a  short  time,  aug- 
ment the  action  of  the  muscle  and  give  rise  to  the  treppe  effect. 
(Lee.)  In  time,  however,  if  the  stimulation  be  continued,  the 
irritability  declines,  the  height  of  the  contraction  diminishes  and 
finally  the  muscle  ceases  to  respond  to  any  stimulus. 
2.  Temperature. — The  temperature  at  winch  all  phases  of  the  con- 
traction process,  as  represented  by  the  myogram,  attain  their 
physiologic   maximum  value   is   about  30 °  C.     If  the  tempera- 


yo  TEXT-BOOK  OF  PHYSIOLOGY. 

ture  of  the  muscle  falls  to  20 °  C.  there  is  a  corresponding  decline 
in  activity,  as  shown  by  an  increase  of  the  latent  period,  a  de- 
crease in  the  height  of  curve — i.  e.,  in  the  shortening  of  the  mus- 
cle— an  increase  both  in  the  contraction  and  relaxation  periods. 
As  the  temperature  approaches  o°  C.  the  height  of  the  curve 
again  suddenly  increases,  indicating,  for  some  unknown  reason, 
an  increase  in  the  irritability.     This  is,  however,  scarcely  a  physio- 


Fig.  28. — Single  Contractions  of  the  Gastrocnemius  Muscle  at  Different 
Temperatures.     Time  tracing  200  per  second. — (Brodie.) 

logic  condition.  At  a  temperature  of  40  °  C.  to  50  °  C.  the  muscle 
suddenly  contracts  and  passes  into  the  condition  of  heat  rigor. 
The  proteid  constituents  of  the  muscle  are  coagulated  and  the 
irritability  destroyed.  (Fig.  28.) 
The  Load.- — The  extent  to  which  a  muscle  is  loaded  or  weighted 
will  not  only  determine  the  height  of  the  contraction,  but  also  the 
time  relations  of  all  its  phases.     This  is  apparent  from  an  ex- 


Fig.  29.— Contractions  of   a   Gastrocnemius  Muscle 
with  Different  Loads. — (Brodie.) 

amination  of  Fig.  29,  in  which  it  is  shown  that  with  an  increase 
in  load  there  is  a  decrease  in  the  height  of  the  contraction,  an 
increase  in  the  latent  period,  and  a  general  increase  in  the  dura- 
tion of  both  the  periods  of  rising  and  falling  energy. 
Continuous  Stimulation. — Prolonged  or  excessive  activity  of  our  own 
muscles  is  accompanied  by  a  feeling  of  stiffness  or  soreness  and 
lassitude.     There  is  at  the  same  time  a  diminution  in  the  speed 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  71 

and  vigor  of  the  contractions  and  the  power  of  doing  work.  To 
this  condition  the  term  fatigue  has  been  given.  The  cause  of  the 
fatigue  is  attributed  to  a  diminution  in  the  amount  of  the  energy- 
vielding  compounds  as  well  as  to  the  production  and  accumulation 
of  waste  products  resulting  from  katabolic  activity.  Among  the 
waste  products  mono-potassium  phosphate,  paralactic  acid,  and 
carbon  dioxid  are  the  most  important.  These  substances,  when 
present  in  small  amounts  or  in  larger  amounts  for  a  short  time, 
increase  the  irritability  of  'the  muscle,  but  when  they  accumulate 
more  rapidly  than  they  are  removed,  as  is  the  case  during  excessive 
activity,  they  exert  a  depressive  influence  on  the  irritability  of 
the  muscle  and  thus  diminish  its  contractile  power  and  its  capacity 
for  doing  work.  The  more  rapidly  they  are  removed,  the  sooner 
is  a  fatigued  muscle  restored  to  its  normal  condition.  The  condi- 
tion of  fatigue  with  its  attendant  phenomena  is  shown  by  stimulat- 


Fig.  30. — Fatigue  Curves.     Every  Twentieth  Contraction  Recorded. 

ing  through  its  nerve  an  excised  frog  muscle  with  induced  electric 
currents  at  intervals  of  one  second.  In  a  variable  period  of  time 
the  muscle  shows  an  increase  in  the  duration  of  the  latent  period,  a 
diminution  of  the  height  of  the  contraction,  in  the  power  of  doing 
work,  and  an  increase  in  the  time  required  for  relaxation.  (Fig.  30.) 
If  the  stimulation  is  continued  the  contractions  gradually  decline  as 
the  muscle  becomes  exhausted.  When  a  muscle  will  no  longer  re- 
spond to  stimulation  through  its  related  nerve,  it  can  be  made  to 
respond  to  direct  stimulation  with  the  electric  current.  This 
taken  in  connection  with  the  fact  that  stimulation  of  a  nerve-trunk 
even  for  several  hours  does  not  fatigue  it,  leads  to  the  inference 
that  the  cause  of  the  cessation  of  contraction  does  not  wholly  lie  in 
the  muscle  but  partly  in  the  nerve  endings  in  the  muscle.  These 
structures  it  is  believed  fatigue  more  readily  than  the  muscle 
structures,  and  hence  fail  to  conduct  the  nerve  impulse  to  the 
muscle.  By  this  means  it  is  protected  from  absolute  exhaustion. 
Nutrition. — The  irritability  of  a  muscle  which  conditions  the  con- 
traction process  is  dependent  on  the  maintenance  of  its  nutrition ; 
hence  a  continuous  and  sufficient  supply  of  nutritive  material 
and  a  rapid  removal  of  waste  products  are  essential  conditions 
for  the  exhibition  of  normal  contractions.  A  diminution  of  blood 
supply  or  an  accumulation  of  waste  products  sooner  or  later  im- 
pairs the  irritability  and  diminishes  the  vigor  and  extent  of  the 


72  TEXT-BOOK  OF  PHYSIOLOGY. 

contraction.  Various  drugs — e.  g.,  veratrin,  barium,  etc. — in- 
troduced into  the  circulation  and  finding  their  way  into  the  muscle 
modify  the  contraction  process,  as  shown  by  a  very  great  increase 
in  the  duration  of  the  relaxation  period. 

The  Isometric  Myogram. — With  the  object  of  obtaining  a  curve 
of  the  increase  and  decrease  in  the  tension  of  a  muscle  during  a  single 
contraction,  with  the  exclusion  as  far  as  possible  of  a  change  in  length, 
the  muscle  may  be  made  to  contract  against  a  strong  spring  or  similar 
resistance  sufficient  to  practically  though  not  absolutely  prevent  short- 
ening. To  this  method  the  term  isometric  has  been  given,  and  the 
curve  so  obtained  an  isometric  myogram  or  a  tonogram.  The  record- 
ing Jportion  of  the  lever  is  prolonged  some  distance  so  that  the  very 
slight  upward  movement  at  its  axis,  close  to  which  the  muscle  is  at- 
tached, will  be  considerably  magnified.     That  the  ordinate  value  of 

an  isometric  curve  may  be  known, 
the  apparatus  must  be  graduated 
by  subjecting  the  spring  to  a  series 
of  weights  playing  over  a  pulley 
supported  by  the  muscle  clamp. 
The   curve   of  the  variation 
in  tension  obtained  by  the  iso- 
metric method  is  shown  in  Fig. 
31,  b,  in  which  the  two  curves 
are   contrasted.       The  form  of 
Fig.  31.— a.  Diagram   of  Isotonic;   b,     the  curve  indicates  that  the  mus- 
of   Isometric  Muscle  Curves.— (Landois    c\e  attains  its  maximum  of  ten- 
and  Stirling.)  sion  m0Te  rapidly  than  its  max- 

imum of  shortening;  that  the 
tension  endures  for  a  certain  period  of  time  unchanged ;  that  the  fall  in 
tension  takes  place  more  rapidly  than  the  muscle  relaxes. 

The  Myogram  Due  to  the  Make  and  the  Break  of  a  Galvanic 
Current. — The  contraction  of  the  muscle  which  has  heretofore  been 
recorded  has  been  caused  by  the  momentary  action  of  an  induced 
current.  The  contraction  of  the  muscle  which  is  caused  by  the  action 
of  a  constant  or  galvanic  current  presents  features  which  are  some- 
what different  and  as  it  serves  to  illustrate  the  difference  in  the  effects 
of  a  constant  or  galvanic  and  an  induced  or  interrupted  current,  a 
myogram  of  a  contraction  due  to  the  make  and  break  of  a  galvanic 
current  is  introduced  at  this  place.  The  effects  which  are  observed  in 
a  muscle  during  the  passage  of  both  feeble  and  strong  currents  have  been 
alluded  to  in  a  previous  section.  (See  page  61.)  In  Figure  32  these 
effects  are  graphically  represented.  It  will  be  observed  that  on  the 
closure  of  the  circuit  at  c  the  muscle  at  once  contracted  and  so  long  as 
the  current  was  flowing,  the  muscle  remained  in  a  more  or  less  con- 
tracted state  known  as  galvanotonus;  on  opening  the  circuit  at  o  the 
muscle  again  contracted,  after  which  it  gradually  relaxed  and  returned 
to   its   original  condition.     The  record  shows  also  that  during  the 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


73 


actual  passage  of  the  current  the  muscle  substance  was  being  stimu- 
lated by  it. 

The  Work  Accomplished  by  a  Muscle  during  the  Time  of  a 
Single  Contraction. — By  work  is  meant  the  overcoming  of  opposing 
forces.  In  the  physiologic  activities  of  the  body  the  muscles  at  each 
contraction  not  only  overcome  the  resistances  of  antagonistic  muscles, 


Fig.  32. — A  Myogram  Due  to  the  Action  of  a  Galvanic  Current,  Applied  Di- 
rectly to  a  Muscle,  when  the  Circuit  was  Closed  (c)  and  when  it  was  Opened  (0). 

the  weight  of  the  limbs,  the  friction  of  joints,  etc.,  but  in  addition  over- 
come various  external  resistances  connected  with  the  environment — 
e.  g.,  gravity,  cohesion,  friction,  elasticity,  etc.  The  muscles  may 
therefore  be  regarded  as  machines  for  the  accomplishment  of  work. 
Experimentally  the  work  done  by  an  isolated  muscle  may  be  calcu- 
lated when  first  the  height 
of  the  contraction  is  ob- 
tained and  then  multiply- 
ing it  by  the  weight  raised. 
The  influence  of  the 
weight  on  the  height  of 
the  contraction  is  shown 
in  Fig.  33.  From  this 
tracing  it  will  be  observed 
that  the  extent  to  which 
a  muscle  will  shorten  in 
response  to  a  maximal 
stimulus  is  greatest  when 
it  is  unweighted;  but  as 
weights  differing  by  a 
common  increment  are 
added,  the  height  of  the 
contraction  diminishes  until  with  a  given  weight  it  is  nil. 

A  careful  study  of  the  results  of  this  experiment  will  show  that  the 
work  done  gradually  increased  as  the  load  was  increased  from  o  to  70 
grams,  when  it  amounted  to  210  milligrammeters;  but  that  after  this, 
even  though  the  weight  lifted  was  greater,  the  height  to  which  it  was 
lifted  was  less,  and  hence  the  work  done  gradually  decreased,  until 
it  amounted  to  nothing. 


Fig.  33. — Tracing  Showing  the  Gradual 
Diminution  in  the  Height  of  the  Contrac- 
tion as  the  Weight  was  Increased  by  a  Com- 
mon Increment  of  10  Grams  from  o  to  180 
Grams.     Magnification  of  the  Lever,  4. 


74  TEXT-BOOK  OF  PHYSIOLOGY. 

The  following  table  will  also  show  the  work  done  by  a  frog's  muscle 
according  to  Rosenthal. 


Weight. 

Height. 

Work 

Done. 

o  grams 

14  mm. 

0 

grarn- 

millimeters 

5°       " 

9     " 

45° 

" 

IOO         " 

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From  the  preceding  figures  it  is  evident  that  the  mechanical  work  of  a 
muscle  increases  with  increasing  weights  up  to  a  certain  maximum, 
and  then  declines  to  zero.  Equally  when  the  muscle  contracts  to  its 
maximum  without  being  weighted,  and  when  it  does  not  contract  at 
all  from  being  overweighted,  no  work  is  done.  Between  these  two 
extremes  the  muscle  performs  varying  amounts  of  work. 

The  maximum  amount  of  force  which  a  muscle  puts  forth  during  a 
contraction  is  naturally  measured  by  the  amount  of  work  done;  but 
as  this  varies  with  the  degree  to  which  the  muscle  is  weighted,  another 
measure  has  been  adopted,  to  which  the  term  absolute  muscle  force 
or  static  force  has  been  given.  The  absolute  force  is  measured  by 
the  weight  which  is  sufficient  to  prevent  the  muscle  from  shortening. 
This  is  best  determined  by  the  method  of  after-loading  in  which  the 
muscle  is  not  extended  by  the  weight  previous  to  the  contraction. 
It  has  been  found  that  the  absolute  force  of  a  muscle  is  directly  de- 
pendent on  the  number  and  not  the  length  of  the  fibers  it  contains 
and  proportional  to  the  physiologic  transverse  section  of  the  muscle. 
The  transverse  section  of  a  muscle  is  obtained  by  dividing  its  volume 
(obtained  by  dividing  its  actual  weight  by  the  specific  weight  of  mus- 
cle-tissue, 1 .058)  by  the  average  length  of  the  fibers.  Assuming  that  the 
muscle  weighs  622  grams,  its  volume  would  be  576  c.c;  and  if  it  be 
further  assumed  that  the  fibers  have  an  average  length  of  4  centimeters 
the  transverse  section  would  contain  144  sq.  centimeters  each  of  which 
would  have  a  length  of  4  centimeters. 

For  purposes  of  comparison  it  is  customary  to  refer  the  absolute 
force  to  these  units  of  diameter — viz.,  one  square  centimeter.  Rosen- 
thal estimates  the  force  for  the  square  centimeter  of  the  muscle  of  the 
frog  at  from  2  to  8  kilograms ;  for  the  muscles  of  man  at  6  to  8  kilograms; 
Koster  at  about  10  kilograms  for  the  muscles  of  the  leg  and  7  or  8 
kilograms  for  the  muscles  of  the  arm. 

Summation  Effects. — If  a  series  of  successive  stimuli  be  applied 
to  a  muscle,  the  effect  will  vary  according  to  the  rapidity  with  which 
they  follow  one  another.  As  previously  stated,  if  the  interval  preced- 
ing each  stimulus  be  sufficiently  long  to  enable  the  muscle  to  recover 
from  the  effects  of  the  previous  contraction,  there  will  be  no  change 
in  the  form  or  the  character  of  the  contraction  for  a  long  time  except  a 
slight  increase,  in  the  early  period,  of  the  irritability  as  shown  by  the  in- 
creased height  of  the  curve  or  shortening  of  the  muscle.  If,  however,  a 
second  stimulus  be  applied  to  a  muscle  during  the  period  of  relaxation, 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


75 


a  second  contraction  immediately  follows  which  is  added  to  or  super- 
posed on  the  first;  the  effect  produced  will  be  greater  than  that  pro- 
duced by  either  stimulus  separately.      See  Fig.  34. 

A  third  stimulus  applied  during  the  relaxation  of  the  second  con- 
traction produces  a  third  contraction  which  adds  itself  to  the  second, 
and  so  on.  The  increment  of  increase  in  the  extent  of  the  success- 
ive contractions  gradually  di- 
minishes, however,  until  the 
muscle  reaches  a  maximum  of 
contraction.  The  superposition 
of  the  second  contraction  on  the 
first,  the  third  on  the  second,  and 
so  on,  is  termed  summation  0} 
contractions  or  effects.  Experi- 
ment has  shown  that  the  greatest 
effect  of  a  second  stimulus — that 
is,  the  highest  contraction— is 
produced  when  the  stimulus  is 
applied  during  the  last  third  of 
the  period  of  rising  energy,  when 
the  sum  of  the  two  contractions 
is  almost  twice  as  great  as  the 
first  contraction  (Fig.  34).  The 
effects  following  both  maximal 
and  submaximal  stimuli  indicate 
that  the  muscle  cannot  attain  its 
maximum  of  shortening  except 
through  a  summation  of  several 
stimuli.  If  a  second  maximal 
stimulus  enter  a  muscle  during 
the  latent  period  following  the 
first,  the  effect  produced  will  be 
no  greater  than  that  produced  by 
a  single  stimulus.  The  muscle 
during  this  period  is  said  to  be 
refractory  or  non-responsive  to  a 
second  stimulus.  If,  however, 
the  stimuli  are  submaximal  they  read  from  below  uPward- 
add    themselves    together,    and 

though  the  effect  is  but  a  single  contraction,  it  is  larger  than  either  would 
have  produced  separately.      This  is  termed  the  summation  of  stimuli. 

Still  further,  if  a  series  of  subminimal  stimuli,  each  of  which  is 
alone  insufficient  to  produce  a  contraction  of  the  muscle,  be  applied 
in  rapid  succession,  a  contraction  frequently  results.  This  is  termed 
the  summation  0}  subminimal  stimuli. 

Tetanus. — Tetanus  may  be  defined  as  a  more  or  less  continuous 
contraction  of  a  muscle  which  arises  when  the  time  intervals  between 


Fig.  34. — Tracing  Showing  the  Ef- 
fects of  Two  Successive  Stimuli,  a.  a' 
with  Gradually  Diminishing  Inter- 
val on  a  Muscle  Contraction.     To  be 


76  TEXT-BOOK  OF  PHYSIOLOGY. 

the  stimuli  are  shorter  than  the  time  of  the  contraction  process. 
Tetanus  will  be  incomplete  or  complete  according  to  the  number  of 
stimuli  that  fall  into  the  muscle  in  a  second  of  time.  When  a  muscle 
is  stimulated  directly  or,  better,  indirectly  through  its  related  nerve  by 
a  series  of  induced  currents  at  the  rate  of  four  or  six  per  second,  it 
undergoes  a  corresponding  number  of  contractions  of  about  equal  ex- 
tent. If  the  rate  of  stimulation  is  increased  up  to  the  point  when  the 
interval  between  each  stimulus  is  less  than  the  duration  of  the  entire 
contraction  process,  the  muscle  does  not  have  time  to  completely  relax 
before  the  arrival  of  the  succeeding  stimulus,  and  hence  remains  in  a 
more  or  less  contracted  state,  during  which  it  exhibits  a  series  of 
alternate  partial  contractions  and  relaxations.  To  this  condition  of 
muscle  activity  the  term  incomplete  tetanus  or  clonus  is  applied.  A 
graphic  record  of  an  incomplete  tetanus  is  given  in  Fig.  35. 


Fig.  35. — Curves  Showing  the  Analysis  of  Tetanus  of  a  Frog's  Muscle 
(Gastrocnemius).  The  numbers  under  the  curves  indicate  the  number  of  shocks  per 
second  applied  to  the  muscle.  There  is  almost  complete  tetanus  with  twenty-five  per 
second,  and  it  is  a  little  lower  than  the  previous  one  because  the  muscle  was  slightly 
fatigued. — (Stirling.) 

In  such  a  tracing  it  is  observed  that  the  second  stimulation,  occurring 
before  the  muscle  had  time  to  relax,  gave  rise  to  a  second  contraction, 
which  was  superposed  on  the  first;  the  same  result  followed  the  third 
stimulus,  the  fourth,  the  fifth,  and  so  on.  Owing  largely  to  this  sum- 
mation of  the  contractions  there  is  a  gradual  rise  in  the  height  of  the  con- 
traction curve.  This  condition  of  the  muscle,  viz.,  continued  contrac- 
tion, combined  with  diminished  power  of  relaxation,  is  termed  con- 
tracture. The  tracing  also  shows  that  as  the  stimulus  continues,  the 
base  line,  that  connecting  the  lowest  points  of  the  contractions,  grad- 
ually rises  and  takes  the  form  of  a  curve  which  increases  in  height  with 
the  stimulation.  The  apex  line,  that  connecting  the  highest  points 
of  the  contractions,  also  rises  at  the  same  time,  indicating  a  continuous 
increase  in  the  height  of  the  contractions.  The  length  of  time  a  muscle 
will  exhibit  incomplete  tetanus  depends  on  a  variety  of  circumstances, 
e.  g.,  character  of  muscle,  rate  and  strength  of  stimulation,  etc.,  but 
mainly  on  the  rapidity  with  which  the  muscle  becomes  fatigued.  With 
the  oncoming  of  fatigue  the  muscle  begins  to  relax,  and  ultimately 
returns  to  its  normal  condition,  notwithstanding  the  continued  stimu- 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  77 

lation.  If  the  stimulation  be  withdrawn,  the  muscle  does  not  at  once 
return  to  its  original  length  but  remains  more  or  less  contracted  for  a 
variable  time.  This  contraction  after  stimulation  is  known  as  the  con- 
traction-remainder. 

If  the  stimulation  be  still  further  increased  in  frequency,  the  in- 
dividual contractions  become  fused  together  and  the  curve  described 
by  the  lever  becomes  a  continuous  line.  (See  Fig.  35.)  Notwith- 
standing the  fact  that  the  individual  contractions  are  no  longer  visible, 
it  can  be  shown  by  other  methods  that  the  muscle  is  undergoing  a 
series  of  slight  alternate  contractions  and  relaxations  or  vibrations 
at  least.  After  a  varying  length  of  time  the  muscle  becomes  fatigued, 
relaxes,  and  returns  to  its  natural  condition  even  though  the  stimu- 
lation be^continued.     The  number  of  stimuli  per  second  necessary  to 


Fig.  36. — Development  of  Fatigue  and  Contraction.     Muscle  stimulated  once  a 
second  by  a  strong  induced  current. 


develop  complete  tetanus  will  depend  under  normal  circumstances  on 
the  period  of  duration  of  the  individual  contractions.  The  longer  this 
period,  the  less  the  number  of  stimuli  required,  and  the  reverse.  Hence 
the  number  of  stimuli  will  vary  for  different  classes  of  animals  and  for 
different  muscles  in  the  same  animal,  e.  g.,  2  or  3  for  the  muscles  of 
the  tortoise,  10  for  the  muscles  of  the  rabbit,  15  to  20  for  the  frog,  70 
to  80  for  birds,  330  to  340  for  insects. 

An  effect  which  follows  frequent  stimulation  of  a  muscle,  e.  g.,  50  to 
60  times  per  minute,  and  especially  when  the  muscle  is  somewhat 
fatigued  or  cold  is  shown  in  Fig.  36.  It  is  evidently  a  combination  of 
contracture  and  fatigue.  It  will  be  observed  that  at  the  beginning 
of  the  stimulation  there  is  a  stair  case  effect,  a-b,  combined  with  dim- 
inished relaxation.  This  in  turn  is  followed  by  a  decline  in  the  height 
of  the  contractions,  b-c,  and  a  fall  of  the  base  line  which  may  be  attributed 
to  fatigue  conditions.  After  a  short  time  there  is  a  second  rise  of  the 
base  line,  d,  and  a  rapid  development  of  contracture.  The  muscle  at 
this  period  is  in  a  condition  of  incomplete  tetanus  which  gradually 
passes  into  complete  tetanus  attended  by  fatigue. 


78  TEXT-BOOK  OF  PHYSIOLOGY. 

The  tetani  of  muscles  may  be  classified  in  accordance  with  their 
causes  as  .follows : — 

-r,,      .  ,     .     !  Volitional, 
i.  Phvsiologic 


Reflex. 

2.  Experimental. 

3.  Pharmacologic. 

t,  ,ii     •       Bacterial. 

4.  Pathologic 

[  Reflex. 

1.  Physiologic  Tetanus. — 1.  Volitional.- — Because  of  the  fact  that 
during  the  continuance  of  a  volitional  movement  the  muscle  is  in  a 
state  of  continuous  contraction,  it  may  be  accepted  that  volitional  con- 
tractions are  states  of  tetanus,  more  or  less  complete;  for  the  shortest 
possible  volitional  contraction,  however  quickly  it  takes  place,  has  al- 
ways a  longer  duration  than  a  single  contraction  caused  by  an  induced 
electric  current.  As  the  volitional  contraction  is  similar  to  that  observed 
when  a  muscle  or  its  related  nerve  is  stimulated  by  rapidly  repeated 
induced  currents,  it  is  assumed  that  the  nerve-cells  in  the  spinal  cord 
are  discharging  in  a  rhythmic  manner  a  certain  number  of  nerve  im- 
pulses per  second  in  consequence  of  the  arrival  of  nerve  impulses  com- 
ing from  the  cerebral  cortex,  the  result  of  volitional  acts.  In  other 
words  the  volitional  tetanus  is  the  result  of  a  discontinuous  stimulation. 
The  number  of  stimuli  transmitted  to  a  muscle  during  a  volitional 
tetanus  has  been  estimated  by  the  employment  of  the  graphic  method 
at  from  8  to  13  per  second,  10  being  about  the  average.  When  a  voli- 
tional contraction  is  recorded  the  myogram  not  infrequently  exhibits  a 
series  of  small  wave-like  elevations  which  indicate  that  the  muscle  is 
not  in  a  state  of  complete  tetanus  but  is  undergoing  slight  alternate 
contractions  and  relaxations.  Unless  the  contraction  process  in  human 
muscle  differs  from  that  of  frogs  it  is  difficult  to  see  how  10  or  even 
20  stimuli  per  second  can  give  rise  to  even  an  incomplete  tetanus 
when  the  single  contraction  is  -^  of  a  second  in  duration. 

2.  Reflex. — A  tetanus  of  muscle,  physiologic  in  character,  arises 
during  the  performance  of  many  muscle  movements  in  consequence 
of  peripherally  acting  causes  and  may  therefore  be  termed  a  reflex 
tetanus.  The  duration  of  a  tetanus  thus  induced,  like  the  duration  of 
a  volitional  tetanus,  will  vary  with  the  duration  of  the  exciting  cause. 
Reflex  tetani  are  presented  by  the  muscles  of  the  lower  jaw  during 
mastication,  by  the  intercostal  muscles  during  breathing,  by  the 
muscles  of  the  limbs  during  walking,  etc.  In  these  and  other  in- 
stances there  are  reasons  for  believing  that  for  a  variable  period  of 
time  the  muscles  are  in  a  state  of  continuous  contraction  from  the  dis- 
charge of  nerve  impulses  from  the  nerve  cells  in  the  spinal  cord  as  the 
result  of  the  arrival  of  nerve  impulses  coming  from  a  peripheral  sur- 
face. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  79 

2.  Experimental  Tetanus. — The  tetanus  of  muscle  developed 
in  accordance  with  the  method  described  in  foregoing  paragraphs,  i.  e., 
by  the  employment  of  instrumental  procedures,  may  be  termed  ex- 
perimental tetanus.  Its  mode  of  development  serves  to  illustrate  and 
explain  the  method  by  which  individual  contractions  are  summated 
and  continuous  contractions  made  possible  for  the  performance  of 
volitional  acts. 

3.  Pharmacologic  Tetanus. — The  administration  of  certain 
drugs,  e.  g.,  strychnin,  in  sufficient  amounts,  is  followed  in  a  short  time 
by  a  series  of  intermittent  spasms  in  which  all  the  muscles  of  the  body 
are  involved.  At  the  beginning  of  the  spasms  the  muscles  are  thrown 
into  tonic  or  complete  tetanus,  during  the  continuance  of  which  the 
muscles  are  hard  and  firm.  In  a  short  time  this  tonic  state  begins 
to  subside,  giving  way  to  tremors  or  a  series  of  irregular  contractions 
resembling  incomplete  tetanus  or  clonus.  A  tetanus  of  this  char- 
acter may  be  termed  pharmacologic.  Though  the  onset  of  the 
tetanus  is  occasioned  largely  by  peripheral  stimulation,  the  seat  of 
action  of  strychnin  is  central  and  for  the  most  part  focalized  in  the 
spinal  cord.  The  exact  seat  of  its  action  is  not  definitely  determined 
but  there  are  reasons  for  believing  that  it  is  on  the  end-tufts  of  afferent 
nerves  in  the  spinal  cord  or  on  the  intercalated  neuron  between  them 
and  the  nerve-cells  in  the  anterior  horns  of  the  gray  matter,  the  irrita- 
bility of  which  is  raised  and  the  resistance  to  the  transmission  of  nerve 
impulses  coming  from  the  periphery  diminished.  As  a  result  the  nerve 
impulses  are  transmitted  to  the  nerve-cells  more  readily,  not  only  in  a 
horizontal  but  also  in  a  longitudinal  direction,  and  the  effects  they  pro- 
duce enormously  increased. 

4.  Pathologic  Tetanus.— 1.  Bacterial. — The  introduction  of  a 
specific  bacillus  into  a  wound  in  any  region  of  the  body  is  followed 
after  a  period  of  incubation  of  from  three  or  four  days  to  a  week  by  a 
tetanus  in  which  nearly  all  the  muscles  of  the  body  are  involved,  char- 
acterized by  a  tonic  contraction  and  clonic  exacerbations.  A  tetanus 
of  this  character  may  be  termed  pathologic.  The  persistent  tonic 
contraction  is  the  result  of  a  more  or  less  continuous  discharge  of  nerve  ■ 
impulses  from  the  nerve- cells  of  the  spinal  cord  which  have  been 
rendered  abnormally  irritable  by  the  action  of  a  toxin,  produced  by  the 
bacilli,  and  which  has  a  selective  action  on  these  structures.  The 
clonic  exacerbations  are  evoked  from  time  to  time  by  various  forms  of 
peripheral  stimulation. 

2.  Keflex — A  tetanus  of  individual  muscles  more  or  less  continu- 
ous in  character  is  occasionally  the  result  of  peripheral  irritations  of 
a  pathologic  character.  A  tonic  contraction  of  the  masseter  muscles, 
for  example,  firmly  closing  the  jaws  for  weeks  and  months  at  a  time 
is  caused  in  some  instances  by  an  impacted  wisdom  tooth  or  an  ulcera- 
tive condition  of  the  mouth.  Since  the  removal  of  the  cause  is  followed 
by  a  relaxation  of  the  muscle,  this  form  of  tetanus,  known  as  trismus, 
may  be  regarded  as  pathologic  in  character  and  reflex  in  origin. 


8o  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Muscle  Sound. — If  a  stethoscope  or  a  myophone  with  tele- 
phone connections  be  placed  on  a  muscle  while  in  a  condition  of  voli- 
tional tetanus  and  at  the  same  time  kept  in  a  certain  degree  of  tension, 
there  will  be  developed  in  the  observer  a  sensation  of  sound  or  tone 
which  is  spoken  of  as  a  muscle  sound  or  tone.  It  is  also  readily  heard 
in  the  masseter  muscle  when  the  side  of  the  face  is  placed  on  a  receiv- 
ing body  such  as  a  pillow,  and  the  masseter  muscles  made  to  con- 
tract volitionally.  This  tone  is  attributed  to  a  vibration  or  an  alternate 
contraction  or  relaxation  of  the  muscle  or  to  an  intermittent  rhythmic 
variation  in  tension,  the  result  of  the  rate  of  stimulation.  This  tone 
corresponds  to  a  vibration  frequency  of  from  1 8  to  20  per  second  and  is 
accepted  as  one  of  the  proofs  that  the  physiologic  volitional  tetanus  is 
not  continuous  but  discontinuous  in  character.  If  a  muscle  is  tetanized 
with  induced  currents,  the  tone  increases  in  pitch  for  a  limited  time 
as  the  frequency  of  the  current  per  second  increases  up  to  a  certain 
maximum,  which  for  frogs  is  about  200  and  for  mammals  about  1000. 

CHEMIC  PHENOMENA. 

The  chemic  changes  which  underlie  the  transformation  of  energy 
in  the  living  muscle  even  when  in  a  state  of  rest  are  active  and  com- 
plex, though  but  little  is  known  as  to  their  exact  character.  As  shown 
by  an  analysis  of  the  blood  flowing  to  and  from  the  resting  muscle, 
it  has,  while  flowing  through  the  capillaries,  lost  oxygen  and  gained 
carbon  dioxid.  The  amount  of  oxygen  absorbed  by  the  muscle  (9 
per  cent.)  is  greater  than  the  amount  of  carbon  dioxid  (6.7  per  cent.) 
given  off.  Notwithstanding  the  relation  of  the  oxygen  absorbed  to 
the  carbon  dioxid  produced,  there  is  no  parallelism  between  these  two 
processes,  as  the  carbon  dioxid  will  be  given  off  in  the  absence  of  free 
oxygen  or  in  an  atmosphere  of  nitrogen. 

In  the  active  or  contracting  muscle  all  the  chemic  changes  are 
increased,  as  shown  both  by  an  increased  absorption  of  oxygen  and 
an  increased  production  of  carbon  dioxid,  though  the  ratio  existing 
between  them  differs  considerably  from  that  of  the  resting  muscle. 
Thus,  according  to  Ludwig,  an  active  muscle  absorbs  12.26  per  cent. 
of  oxygen  and  gives  off  10.8  per  cent,  carbon  dioxid.  During  the 
activity  of  a  muscle  its  tissue  changes  from  a  neutral  to  an  acid  reac- 
tion, from  the  development  of  sarcolactic  acid  and  possibly  phos- 
phoric acid.  The  degree  of  the  acidity  depends  to  some  extent  on  the 
duration  of  the  contraction  periods.  Chemic  analysis  of  a  tetanized 
muscle  shows  that  it  contains  less  glycogen  than  a  resting  muscle,  and 
that  it  contains  a  larger  amount  of  water.  Coincident  with  muscular 
contraction,  the  blood-vessels  become  widely  dilated,  leading  to  a 
large  increase  in  the  blood-supply  and  a  rapid  removal  of  the  products 
of  decomposition. 

Rigor  Mortis.— A  short  time  after  death  the  muscles  pass  into  a 
condition  of  extreme  rigidity  or  contraction  known  as  death  stiffening 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  81 

or  rigor  mortis,  which  lasts  from  one  to  five  days.  In  this  state  they 
offer  great  resistance  to  extension.  At  the  same  time  their  tonicity  dis- 
appears, their  cohesion  diminishes,  and  their  irritability  ceases.  The 
time  of  the  appearance  of  this  post-mortem  rigidity  varies  from  a  quarter 
of  an  hour  to  seven  hours.  Its  onset  and  duration  are  influenced  by  the 
condition  of  the  muscle  irritability  at  the  time  of  death.  When  the  ir- 
ritability is  impaired  from  any  cause,  such  as. chronic  disease  or  defec- 
tive blood-supply,  the  rigidity  appears  promptly  but  is  of  short  duration. 
After  death  from  acute  diseases  it  is  apt  to  be  delayed,  but  will  continue 
for  a  longer  period.  The  rigidity  first  appears  in  the  muscles  of  the  lower 
jaw  and  neck;  next  in  the  muscles  of  the  abdomen  and  upper  extrem- 
ities; finally  in  the  trunk  and  lower  extremities.  It  disappears  in  prac- 
tically the  same  order.  Chemic  changes  of  a  marked  character  ac- 
company this  process.  The  muscle  becomes  acid  in  reaction  from 
the  development  of  sarcolactic  acid  and  there  is  a  large  increase  in 
the  amount  of  carbon  dioxid  given  off.  The  immediate  cause  of  the 
rigidity  appears  to  be  coagulation  of  the  myosinogen  within  the  sarco- 
lemma  with  the  formation  of  an  insoluble  protein,  myosin.  In  the 
early  stages  of  the  coagulation  restitution  is  possible  by  the  circula- 
tion of  arterial  blood  through  the  vessels.  The  final  disappearance 
of  this  post-mortem  rigidity  is  due  probably  to  the  action  of  acids  which 
render  the  myosin  soluble,  and  possibly  to  the  action  of  various  micro- 
organisms which  give  rise  to  putrefactive  changes. 

Source  of  the  Muscle  Energy.— Notwithstanding  many  in- 
vestigations, the  nature  of  the  materials  which  are  the  immediate  source 
of  the  muscle  energy  is  not  known.  The  absence  of  any  noticeable 
increase  in  the  quantity  of  urea  or  other  nitrogen-holding  compounds 
excreted  renders  it  probable  that  the  energy  does  not  come  from  the 
metabolism  of  proteid  materials.  The  marked  production  of  carbon 
dioxid  and  sarcolactic  acid  points  to  the  decomposition  of  some  un- 
stable compound,  of  a  carbohydrate  character,  rich  in  carbon  and 
oxygen.  It  has  been  suggested  that  glycogen  furnishes  the  energy, 
inasmuch  as  this  substance,  generally  present  in  muscle,  disappears 
during  activity.  A  muscle  which  has  been  tetanized  contains  less 
glycogen  than  the  corresponding  muscle  at  rest.  A  muscle  which 
has  been  separated  from  the  nervous  system  by  division  of  its  nerves 
and  thus  prevented  from  contracting  accumulates  glycogen.  Bunge 
is  of  the  opinion  that  though  the  carbohydrates  are  the  main,  they  are 
not  the  only  sources  of  muscle  energy.  If  there  is  a  deficiency  or  ab- 
sence of  carbohydrate  food,  the  muscle  will  utilize  fat  and  proteid, 
for  experiment  has  shown  that  the  available  glycogen  is  entirely  con- 
sumed the  second  or  third  day.  The  mechanism  by  which  the  energy 
is  liberated,  whether  by  decomposition  or  direct  oxidation,  is  unknown. 
The  fact  that  muscle  will  contract  in  an  atmosphere  free  of  oxygen,  that 
no  free  oxygen  can  be  obtained  from  muscle,  would  support  the  idea 
that  the  mechanism  is  one  of  decomposition.  Hermann  suggests  that 
the  energy  of  a  contraction  is  liberated  by  the  splitting  and  subsequent 
6 


82  TEXT-BOOK  OF  PHYSIOLOGY. 

re-formation  of  a  complex  body  belonging  neither  to  the  carbohydrates 
nor  fats,  but  to  the  proteids — to  this  hypothetic  body  the  term  inogen  is 
given.  This  complex  molecule,  the  product  of  the  nutritive  activity  of 
the  muscle-cell  in  undergoing  decomposition,  would  yield  carbon  dioxid, 
sarcolactic  acid,  and  a  proteid  residue  resembling  myosin.  On  the 
cessation  of  the  contraction  the  muscle-cell  recombines  the  proteid 
residue  with  oxygen,  carbohydrates,  and  fats,  and  again  forms  the 
energy-holding  compound,  inogen.  The  phenomena  of  rigor  mortis 
support  this  view.  At  the  moment  of  this  contraction  the  muscle 
gives  off  C02  in  large  amount,  develops  sarcolactic  acid  and  myosin. 
There  is  thus  a  close  analogy  between  the  two  processes;  in  other  words, 
a  contraction  is  a  partial  death  of  the  muscle.  If  this  view  is  correct, 
then  the  oxygen  is  required  mainly  for  heat  production  through  oxida- 
tion processes. 

THERMIC  PHENOMENA. 

The  potential  energy  liberated  during  a  contraction  is  transformed 
into  kinetic  energy — viz.,  heat  and  mechanic  motion.  Though  heat 
production  is  taking  place  even  during  the  passive  condition,  prob- 
ably through  oxidation  processes,  it  is  largely  increased  by  muscle 
activity.  The  skeletal  muscle  of  the  frog,  the  gastrocnemius,  shows 
after  tetanization  an  increase  in  temperature  from  0.140  C.  to  0.180 
C,  and  after  a  single  contraction  from  0.0010  C.  to  0.005  °  C.  The 
amount  of  heat  thus  produced  will  vary  with  a  variety  of  conditions, 
as  strength  of  stimulus,  tension,  work  done,  etc. 

Stimulus. — It  has  been  experimentally  determined  that  an  in- 
crease in  the  strength  of  the  stimulus  from  a  minimal  to  a  maximal 
value  increases  the  amount  of  heat  liberated.  This  is  the  direct  result 
of  increased  chemic  change  naturally  following  increased  stimulation. 

Tension. — The  greater  the  tension  of  a  muscle,  the  greater,  other 
conditions  being  the  same,  is  the  amount  of  heat  liberated.  If  the 
muscle  is  securely  fastened  at  both  extremities  so  that  shortening  is 
practically  impossible  during  the  stimulation,  the  maximum  of  heat 
production  is  reached.  In  the  tetanic  state  the  great  increase  in  tem- 
perature is  due  to  the  tension  of  antagonistic  and  strongly  contracted 
muscles.  In  both  instances,  mechanic  motion  being  prevented,  the 
liberated  energy  is  transformed  into  heat. 

Mechanic  Work. — If  the  muscle  is  permitted  to  shorten  and 
raise  a  weight,  some  of  the  energy  liberated  takes  the  form  of  mechanic 
motion.  If  the  weight  is  removed  at  the  height  of  the  contraction, 
external  work  is  accomplished.  The  greater  the  weight  raised,  within 
limits,  the  greater  is  the  percentage  of  energy  which  takes  the  direction 
of  mechanic  motion.  The  percentage  of  the  total  energy  liberated 
which  is  thus  utilized,  has  been  estimated  at  from  25  to  40  per  cent.  In 
accordance  with  the  law  of  the  conservation  of  energy,  the  heat  produced, 
stated  in  calories,  plus  the  energy  required  in  the  raising  of  the  weight, 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  83 

expressed  in  kilogrammeters  of  work,  must  equal  the  potential  energy 
transformed. 

A  muscle  during  a  tetanic  contraction  of  short  duration  accom- 
plishes more  work  than  during  a  single  contraction;  the  weight  in  each 
case  being  the  same.  In  the  former  condition  the  height  of  contrac- 
tion through  summation,  and  hence  the  work  done,  is  greater  than  in 
the  latter.  The  work  done  by  a  short  tetanic  contraction  may  be  two 
or  three  times  that  of  a  single  contraction,  but  after  the  muscle  reaches 
its  maximum  degree  of  shortening  and  then  continues  in  a  state  of  tet- 
anus, no  further  work  is  done.  Internal  work  is  done,  however,  as 
shown  by  an  increase  in  the  temperature. 

When  a  weight  which  is  lifted  by  a  muscle  during  a  single  con- 
traction is  allowed  to  act  on  the  muscle  during  the  relaxation,  no  ex- 
ternal work  is  accomplished.  All  the  energy  set  free  manifests  itself 
as  heat.  Internal  work  is  done,  as  shown  by  the  fact  that  the  muscle 
becomes  fatigued. 

Work  Done  Daily. — The  muscle  system  in  its  entirety  is  to  be 
regarded  as  a  machine  for  the  transformation  of  potential  into  kinetic 
energy,  and  in  so  doing  accomplishes  work.  Through  the  interme- 
diations of  the  bones  of  the  sketelon  which  play  the  part  of  levers  the 
individual  not  only  changes  his  position  in  space,  but  overcomes  to 
some  extent  the  resistances  offered  by  the  environment.  The  em- 
ployment of  artificial  levers,  tools,  as  distinguished  from  natural  levers, 
bones,  materially  adds  to  the  effectiveness  of  the  muscle  machine. 
The  amount  of  work  which  a  man  of  average  physical  development 
weighing  72  kilos  can  perform  in  eight  hours  has  been  variously  esti- 
mated. It  will  naturally  vary  according  to  the  character  of  the  occupa- 
tion. If  the  work  done  be  calculated  from  the  number  of  kilograms 
raised  one  meter,  the  average  laboring-man  performs  about  300,000 
kilogrammeters. 

ELECTRIC  PHENOMENA. 

Electric  Currents  from  Injured  Muscles. — The  energy  liber- 
ated during  a  muscle  contraction  is  not  only  transformed  into  heat 
and  mechanic  motion,  but  to  some  extent  also  into  electric  energy. 
The  presence  of  points  of  different  potential  on  the  surface  of  the 
muscle,,  the  necessary  condition  for  the  development  of  electric  currents, 
is  tested  by  means  of  non-polarizable  electrodes  connected  by  wires 
with  a  sensitive  galvanometer  or  capillary  electrometer.  When  such 
electrodes  are  brought  in  contact  with  a  muscle  properly  prepared, 
there  is  at  once  developed  and  conducted  to  the  galvanometer  an  electric 
current  the  intensity  and  direction  of  which  are  indicated  by  the  de- 
flection of  the  galvanometer  needle.  The  existence  of  this  current  is 
most  conveniently  demonstrated  with  single  muscles  the  fibers  of  which 
are  parallel — e.  g.,  the  sartorius,  or  the  semimembranosus  of  the  frog. 
If  the  tendinous  ends  of  either  of  these  muscles  be  removed  bv  a  section 


84 


TEXT-BOOK  OF  PHYSIOLOGY. 


made  at  right  angles  to  the  long  axis,  a  muscle  prism  is  obtained  which 
presents  a  natural  longitudinal  surface  and  two  artificial  transverse 
surfaces.  A  line  drawn  around  the  surface  of  such  a  muscle  prism  at  a 
point  midway  between  the  two  transverse  sections  constitutes  the  equator. 
"When  the  natural  longitudinal  and  artificial  transverse  surfaces  are 
connected  with  the  wires  of  a  galvanometer  the  terminals  of  which  are 
provided  with  non-polarizable  electrodes,  an  electric  current  is  at  once 
developed.     In  all  instances  the  current,  as  shown  by  the  deflection 

of  the  needle,  originates  at  the  transverse 
surface,  passes  through  the  muscle  to  the 
longitudinal  surface,  thence  through  the 
galvanometer  to  the  transverse  surface. 
The  longitudinal  surface  is,  therefore, 
electropositive,  the  transverse  surface 
electronegative.  The  two  points  exhibit- 
ing the  greatest  difference  of  potential, 
and  hence  the  most  powerful  current,  lie 
in  the  equator  and  in  the  center  of  the 
transverse  surface.  Currents  of  gradually 
diminishing  intensity  are  obtained  when 
the  electrode  placed  on  the  longitudinal 
surface  is  removed  toward  either  end. 
Feeble  currents  are  developed  when  two 
points  situated  at  unequal  distances, 
either  on  corresponding  or  opposite  sides 
of  the  equator,  are  connected;  in  either 
case  the  current  flows  from  the  point 
lying  nearest  the  equator  to  the  point 
farthest  from  it.  Similar  currents  are 
obtained  when  two  points  on  the  cross- 
section  situated  at  unequal  distances  from 
the  central  axis  are  connected,  in  which 
case  the  direction  of  the  currents  will  be 
from  the  point  lying  nearest  the  periphery 
toward  the  center.  On  the  contrary,  no 
current  is  developed  when  two  points  on  the  longitudinal  surface 
equally  distant  from  the  equator,  or  two  points  on  the  transverse  sur- 
face equally  distant  from  the  central  axis,  are  connected.  Such  points 
are  said  to  be  isoelectric.  These  facts  are  shown  in  Fig.  37.  The 
natural  ends  of  the  muscle,  enclosed  by  sarcolemma  and  tendon,  do  not 
exhibit,  if  carefully  preserved  from  injury,  the  negativity  characteristic 
of  the  artificial  transverse  ends. 

Similar  electric  conditions  are  exhibited  by  the  muscles  of  man 
and  other  mammals,  by  the  muscles  of  birds,  reptiles,  amphibia,  etc. 
The  currents  developed  by  connecting  the  equator  on  the  longitu- 
dinal surface  with  the  axis  of  the  transverse  surface  have  an  electromo- 
tive force  in  the  frog  muscle  of  from  0.037  to  °-°75  °f  a  Daniell  cell. 


Fig.  37. — Diagram  to  Illus- 
trate the  Current  in  Muscle. 
The  arrowheads  indicate  the  direc- 
tion; the  thickness  of  the  lines  in- 
dicates the  strength  of  the  currents. 
— (Landois  and  Stirling.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


85 


The  electric  currents  in  the  muscle  are  intimately  associated  with 
the  chemic  changes  underlying  its  nutrition,  and  hence  their  intensity 
rises  and  falls  with  all  the  conditions  which  maintain  or  impair  mus- 
cle nutrition  and  irritability.  The  currents  observed  in  the  injured 
muscle  during  the  inactive  state  have  been  termed  currents  0}  rest. 
du  Bois-Reymond  regarded  them  as  pre-existent,  intimately  connected 
with  the  living  condition  of  the  muscle,  and  essential  to  the  performance 
of  its  functions,  and  to  be  explained  by  the  view  that  the  entire  muscle 
is  composed  of  molecules  each  of  which  exhibits  the  same  difference 
of  potential  on  its  longitudinal  and  transverse  surfaces  as  the  muscle 
prism  itself.  Hermann  denies  the  existence  of  currents  in  normal 
resting  muscle  and  attributes  them  to  injuries  of  the  surface,  due  to 
methods  of  preparation,  in  consequence  of  which  the  tissue  dies  and 
becomes  electronegative  to  the  uninjured  area,  which  remains  electro- 
positive.    These  currents  Hermann  terms  "demarcation  currents." 

Negative  Variation  of  the  Muscle  Current. — If  a  muscle  ex- 
hibiting a  current  of  injury 
be  excited  to  activity  by 
tetanizing  induced  currents 
applied  to  the  opposite  end 
of  the  muscle,  it  will  be  ob- 
served that  as  the  contrac- 
tion wave  passes  over  the 
muscle  there  is  a  movement 
of  the  galvanometer  needle 
toward  the  zero  point,  in- 
dicating a  diminution  of  the 
potential  on  the  longitudinal 
surface.  To  this  diminu- 
tion in  the  strength  of  the 
current  the  term   negative 

variation  was  given.  On  the  withdrawal  of  the  stimulus  the  needle 
again  returns  in  a  short  time  to  its  former  position.  The  diminution 
of  potential  on  the  longitudinal  surface  of  the  muscle  is  now  attributed 
to  the  passage  of  the  excitation  and  contraction  processes,  to  a  tem- 
porary disintegration  of  the  muscle  substance  (Fig.  38).  With  their 
disappearance  and  the  subsequent  restoration  of  the  nutrition  of  the 
muscle,  the  former  electric  condition  returns. 

The  primary  deflection  of  the  galvanometer  needle  is  due  to  the 
demarcation  current  which  arises  as  a  result  of  the  difference  in 
electric  potential  produced  by  the  destructive  chemic  changes  taking 
place  at  the  cut  end  of  the  muscle.  The  negative  variation  is  caused 
by  the  fact  that  the  activity  of  the  muscle,  with  its  attendant  chemic 
changes,  will  always  be  greater  in  the  uninjured  equatorial  region, 
and  hence  will  always  tend  to  counterbalance  the  original  source  of 
difference  in  electric  potential. 

Electric   Currents   from  Non-injured  Muscles. — Though  per- 


Fig.  38. — The  Negative  Variation  of  the 
Demarcation  Current.  A.  The  contraction 
wave,  which  as  it  passes  beneath  the  electrode  at  B 
causes  a  diminution  of  potential- 


86 


TEXT-BOOK  OF  PHYSIOLOGY. 


fectly  normal  resting  muscle,  according  to  Hermann,  is  isoelectric, 
nevertheless  electric  currents  are  developed  during  activity  to  which 
he  has  given  the  term  action  currents,  and  which  are  attributed  to 
the  propagation  of  the  contraction  wave. 

Action  Currents. — When  two  isoelectric  points  on  the  longitu- 
dinal surface  of  a  muscle  are  connected  with  a  galvanometer  and  a 
single  stimulus  applied  directly  to  one  extremity,  it  can  be  shown  that 
as  the  contraction  wave  passes  beneath  A,  Fig.  39,  the  muscle-tissue 
at  that  point  becomes  electronegative  toward  B  and  a  current  at  once 
passes  through  the  galvanometer  from  B  to  A,  as  shown  by  the  deflec- 
tion of  the  needle  toward  A.  As  the  contraction  wave  passes  beneath 
B  it  in  turn  becomes  electronegative,  and  a  temporary  condition  of 


Fig. 


39. — The  Condition  Leading  to  the  Development  of  the  First  Action 

Current. 


equal  potential  is  established  when  the  needle  returns  to  the  zero  point. 
In  a  very  short  time  the  nutrition  of  A  is  restored  and  becomes  elec- 
tropositive toward  B,  when  a  current  will  pass  through  the  galvan- 
ometer in  the  opposite  direction  from  A  to  B,  as  shown  by  the  movement 
of  the  needle  toward  B,  Fig.  40.  As  the  contraction  wave  passes  beyond 
B  its  nutrition  is  restored  and  becomes  of  equal  potential  with  A.  The 
term  phasic  is  applied  to  these  currents.  The  first  current  flows  in 
the  muscle  in  the  direction  of  progress  of  the  contraction  wave — first 
phase;  the  second  current  flows  in  the  reverse  direction — second  phase; 
the  current  is  therefore  diphasic.  When  a  muscle  is  tetanized,  there 
is  but  a  single  current  observed,  which,  however,  endures  so  long  as 
the  tetanic  contraction  is  maintained.  .  To  this  current  the  term  de- 
cremential  is  given.  When  a  muscle  is  excited  to  action  by  the  nerve 
impulse  which  enters  at  its  center,  two  contraction  waves  are  developed, 
one  in  each  half  of  the  muscle,  and  hence  there  are  two  sets  of  diphasic 
action  currents. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


87 


The  presence  of  action  currents  in  the  muscle  of  the  living  body 
during  a  single  contraction  was  demonstrated  by  Hermann  in  the  mus- 
cles of  the  forearm.  The  arrangement  of  the  experiment  was,  briefly, 
as  follows :  The  forearm  was  surrounded  by  two  twine  electrodes  sat- 
urated with  zinc  solution,  one  being  placed  at  the  physiologic  middle 
— the  nervous  equator — the  other  at  the  wrist.  Both  electrodes  were 
then  connected  with  the  galvanometer.  When  the  brachial  plexus  was 
stimulated  in  the  axillary  space,  the  deflections  of  the  galvanometer 
needle,  when  analyzed  with  the  repeating  rheotome,  indicated  phasic 
currents  with  a  single  contraction.     In  the  first  phase — atterminal — '■ 


Fig.  40. — The  Condition  Leading  to  the  Development  of  the  Second  Action 

Current. 

the  wrist  became  positive  and  the  current  passed  in  the  muscle  toward 
its  termination;  and  in  the  second — abterminal — it  became  negative 
and  the  current  now  passed  in  the  reverse  direction.  The  action  cur- 
rents which  are  observed  in  the  frog's  muscle  were  thus  shown  to  be 
present  in  the  living  human  muscle,  with  this  difference,  however: 
that  the  second  phase — abterminal — instead  of  being  weaker  in  man, 
is  equally  strong  with  the  atterminal.  This  experiment  also  revealed 
the  fact  that  the  rapidity  of  propagation  of  the  excitation  wave  was 
much  greater  in  man,  amounting  to  about  twelve  meters  per  second. 
Hermann  therefore  denies  the  pre-existence  of  electric  currents  and 
regards  them  as  due  to  localized  temporary  disintegration  of  the  mus- 
cle in  consequence  of  activity,  as  they  disappear  on  the  restoration  of 
the  muscle  to  its  normal  condition. 


SPECIAL  ACTION  OF  MUSCLE  GROUPS. 

The  individual  muscles  of  the  axial  and  appendicular  portions  of 
the  body  are  named  with  reference  to  their  shape,  action,  structure, 
etc.;  e.  g.,  deltoid,   flexor,  penniform,  etc.     In  different  localities  a 


88  TEXT-BOOK  OF  PHYSIOLOGY. 

group  of  muscles  having  a  common  function  is  named  in  accordance 
with  the  kind  of  motion  it  produces  or  to  which  it  gives  rise:  e.  g., 
groups  of  muscles  which  alternately  diminish  or  increase  the  angular 
distance  between  two  bones  are  known  respectively  as  flexors  and 
extensors;  such  muscle  groups  are  usually  found  in  association  with 
ginglymus  joints.  Muscles  which  rotate  the  bone  to  which  they  are 
attached  around  its  own  axis  without  producing  any  great  change  of 
position  are  known  as  rotators,  and  are  found  in  association  with  en- 
arthrodial  or  ball-and-socket  joints.  Muscles  which  impart  an  angu- 
lar movement  to  the  extremities  to  and  from 
|  the  median  line  of  the  body  are  termed  ad- 

r^ — j5-(i)       ductors  and  abductors  respectively. 

a  In  addition  to   the  actions  of  individual 

F ® £ .  .       groups  of  muscles  in  producing  special  move- 

A  W      a      P  ments,  in  some  regions  of  the  body,  several 

•  |      f  groups  of  muscles  are  coordinated  for  the  ac- 

W  p     a  complishment   of   certain   definite    functions; 

Pig.  41.— The  Theee'oe-     e-  g->  tne  functions  of  respiration,  mastication, 
ders  of  Levers.  etc.     The  coordination  of  axial  and  appen- 

dicular muscles  enables  the  individual  to  as- 
sume certain  postures,  such  as  standing,  sitting,  and  lying;  to  engage 
in  various  acts  of  locomotion,  as  walking,  running,  dancing,  swimming. 
Levers. — The  function  or  special  mode  of  action  of  individual 
muscles  can  be  understood  only  when  the  bones  with  which  they  are 
connected  are  regarded  as  levers  whose  fulcra  or  fixed  points  lie  in 
the  joints  where  the  movement  takes  place,  and  the  muscles  as  sources 
of  power  for  imparting  movement  to  the  levers  with  the  object  of  over- 
coming resistance. 

In  mechanics  levers  of  three  kinds  or  orders  are  recognized  ac- 
cording to  the  relative  positions  of  the  fulcrum  or  axis  of  motion, 
the  applied  power,  and  the  weight  to  be  moved.     (See  Fig.  41.) 

In  levers  of  the  first  order  the  fulcrum,  F,  lies  between  the  weight 
or  resistance,  W,  and  the  power  or  moving  force,  P.  The  distance 
P  F  is  known  as  the  power  arm  and  the  distance  W  F  as  the  weight 
arm.  As  examples  of  this  form  of  lever  found  in  the  human  body  may 
be  mentioned : 

1.  The  elevation  of   the  trunk  from  the  flexed  position.     The  axis 

of  movement,  the  fulcrum,  lies  in  the  hip-joint;  the  weight,  that 
of  the  trunk,  acting  as  if  concentrated  at  the  center  of  gravity, 
which  lies  close  to  the  tenth  dorsal  vertebra;  the  power,  the  mus- 
cles attached  to  the  tuberosity  of  the  ischium.  The  opposite 
movement  is  equally  one  of  the  first  order,  but  the  relative  posi- 
tions of  P  and  W  are  reversed. 

2.  The  head  in  its  movement  backward  and  forward  on  the  atlas. 

In  levers  of  the  second  order  the  weight  lies  between  the  power 
and  the  fulcrum.  As  illustration  of  this  form  of  lever  may  be  men- 
tioned: 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  89 

1.  The  depression  of  the  lower  jaw,  in  which  movement  the  fulcrum  is 

the  temporomaxillary  articulation;  the  resistance,  the  tension 
of  the  elevator  muscles;  the  power,  the  contraction  of  the  de- 
pressor muscles. 

2.  The  raising  of  the  body  on  the  toes,  in  which  movement  the  ful- 

crum is  the  toes,  the  weight  that  of  the  body  acting  through  the 
ankle,  the  power  the  gastrocnemius  muscle  applied  to  the  heel 
bone. 
In  levers  of  the  third  order  the  power  is  applied  at  a  point  lying 

between  the  fulcrum  and  the  weight.     As  examples  of  this  form  of 

lever  may  be  mentioned : 

1.  The  flexion  of   the  forearm,  in  which  the  fulcrum  is  the  elbow- 

joint,  the  power  the  biceps  and  brachialis  anticus  muscles  ap- 
plied at  their  points  of  insertion,  the  weight  that  of  the  forearm 
and  hand. 

2.  The  extension  of  the  leg  on  the  thigh. 

When  levers  are  employed  in  mechanic  operations,  the  object 
aimed  at  is  the  overcoming  of  a  great  resistance  by  the  application  of 
a  small  force  acting  through  a  great  distance,  so  as  to  obtain  mechanic 
advantage.  In  the  mechanism  of  the  human  body  the  reverse  gener- 
ally obtains,  viz,,  the  overcoming  of  a  small  resistance  by  the  applic- 
ation of  a  large  force  acting  through  a  short  distance.  As  a  result 
there  is  a  gain  in  the  extent  and  rapidity  of  the  movement  of  the 
lever.  The  power,  however,  owing  to  its  point  of  application,  acts 
at  a  great  mechanic  disadvantage  in  many  instances,  especially  in 
levers  of  the  third  order. 

Postures. — Owing  to  its  system  of  joints,  levers,  and  muscles  the 
human  body  can  assume  a  series  of  positions  of  equilibrium,  such  as 
standing  and  sitting,  to  which  the  term  posture  has  been  given.  In 
order  that  the  body  may.  remain  in  a  state  of  stable  equilibrium  in 
any  posture,  it  is  essential  that  the  vertical  line  passing  through  its 
center  of  gravity  shall  fall  within  the  base  of  support. 

Standing  is  that  position  of  equilibrium  in  which  a  line  drawn 
through  the  center  of  gravity  of  the  entire  body  falls  within  the  base 
of  support.  This  position  is  maintained  largely  by  the  mechanical 
conditions  of  the  joints,  apparently  for  the  purpose  of  reducing  to  a 
minimum  muscular  action,  so  that  it  can  be  prolonged  for  some  time 
without  giving  rise  to  fatigue.  In  the  military  position,  which  may 
be  assumed  as  the  normal  position,  all  the  joints  must  be  in  such  a 
condition  of  extension  and  fixation  that  the  body  will  represent  a 
rigid  column  resting  on  the  astragalus  and  supported  by  the  arch  of 
the  foot.     This  is  accomplished: 

1.  By  balancing  the  head  on  the  apex  of  the  vertebral  column.  This 
is  done  by  the  action  of  the  muscles  on  the  back  of  the  neck. 
The  muscular  effort  is,  however,  very  slight,  as  the  center  of 
gravity  of  the  head  lies  but  a  short  distance  in  front  of  the  ar- 
ticulation. 


go  TEXT-BOOK  OF  PHYSIOLOGY. 

2.  By  making  the  vertebral  column  erect  and  rigid.     This  is  brought 
about  by  the  action  of  the  common  extensor  muscles  of  the  trunk. 
In  this  condition  the  center  of  gravity  lies  just  in  front  of  the 
tenth  dorsal  vertebra.     The  head,  trunk,  and  upper  extremities 
are  now  supported  by  the  hip-joints;  and  in  order  that  this  sup- 
port may  give  to  the  body  a  certain  degree  of  stable  equilibrium, 
independent  of  muscular  action,  the  line  of  gravity  falls  behind  the 
line  uniting  the  center  of  rotation  of  the  two  joints.     In  conse- 
quence the  body  would  fall  backward  were  it  not  prevented  by 
the  tension  of  the  iliofemoral  ligament  and  the  fascia  lata. 
The  line  of  gravity,  continued  downward,  passes  through  the  knee- 
joint  posterior  to  the  axis  of  rotation,  and  hence  the  body  would  now 
fall  backward  were  it  not  prevented  by  the  tension  of  the  lateral 
ligaments   and   the   contraction   of   the   quadriceps   femoris   muscle. 
Though  the  body  is  supported  by  the  astragalus,  the  line  of  grav- 
ity does  not  pass  through  the  line  uniting  the  two  joints,  for  in  so 
doing  constant  muscular  effort  would  be  required  to  maintain  stable 
equilibrium;  passing  a  short  distance  in  advance  of  this  line,  there 
would  be  a  tendency  of  the  body  to  fall  forward,  which  is  prevented 
by  the  extensor  muscles  of  the  foot.     When  the  body  is  in  the  erect 
or  military  position,  the  center  of  gravity  lies  between  the  sacrum 
and  last  lumbar  vertebra.     Standing  is  thus  an  act  of  balancing,  and 
requires  not  only  the  static  conditions  of  joints,  but  the  dynamic 
conditions  of  various  groups  of  muscles,  and  hence  is  not  a  position 
of  absolute  ease  and  cannot  be  maintained  for  any  length  of  time 
without    experiencing   discomfort    and    fatigue.     Sitting   erect   is   an 
attitude  of  equilibrium  in  which  the  body  is  balanced  on  the  tubera 
ischii,  when  the  head  and  trunk  together  form  a  rigid  column. 

Locomotion  is  the  act  of  transferring  the  body  as  a  whole  through 
space,  and  is  accomplished  by  the  combined  action  of  its  own  muscles. 
The  acts  involved  consist  of  walking,  running,  jumping,  etc. 

Walking  is  a  complicated  act  involving  almost  all  the  voluntary 
muscles  of  the  body  either  for  purposes  of  progression  or  for  bal- 
ancing the  head  and  trunk,  and  may  be  defined  as  a  progression  in 
a  forward  horizontal  direction  due  to  the  alternate  action  of  both 
legs.  In  walking  one  leg  becomes  for  the  time  being  the  active  or 
supporting  leg,  carrying  the  trunk  and  head;  the  other  the  passive 
but  progressing  leg,  to  become  in  turn  the  active  leg  when  the  foot 
touches  the  ground.  Each  leg  is  therefore  alternately  in  an  active 
and  passive  state. 

Running  is  distinguished  from  walking  by  the  fact  that  at  a  given 
moment  both  feet  are  off  the  ground  and  the  body  is  raised  in  the  air. 

THE  VISCERAL  MUSCLE. 

The  visceral  muscle,  as  the  name  implies,  is  found  in  the  walls 
of  hollow  viscera,  where  it  is  arranged  in  the  form  of  a  membrane 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  91 

or  sheet.  It  is  present  in  the  walls  of  the  alimentary  canal,  blood- 
vessels, respiratory  tract,  ureter,  bladder,  vas  deferens,  uterus,  fallopian 
tubes,  iris,  etc.  In  some  situations  it  is  especially  thick  and  well 
developed — e.  g.,  uterus  and  pyloric  end  of  the  stomach;  in  others  it  is 
thin  and  slightly  developed. 

The  Histology  of  the  Visceral  Muscle-fiber. — When  examined 
with  the  microscope,  the  muscle  sheet  is  seen  to  be  composed  of 
fibers,  narrow,   elongated,  and  fusiform  in  shape.     As  a  rule,  they 


Fig.  42. — Two  Smooth  Muscle-fibers  from  Small  Intestine  of  Frog.  X  240. 
Isolated  with  35  per  cent,  potash-lye.  The  nuclei  have  lost  their  characteristic  form 
through  the  action  of  the  lye. — (Stohr.) 

are  extremely  small,  measuring  only  from  40  to  250  micromillimeters 
in  length  and  from  4  to  8  micromilHmeters  in  breadth.  The  center 
of  each  fiber  presents  a  narrow,  elongated  nucleus.  The  muscle- 
protoplasm  which  makes  up  the  body  of  the  fiber  appears  to  be  en- 
closed by  a  delicate  elastic  membrane  resembling  in  some  respects 
the  sarcolemma  of  the  skeletal  muscle.  In  some  animals  the  visceral 
fiber  presents  a  longitudinal  striation  suggesting  the  existence  of 
fibrillse  surrounded  by  sarcoplasm  (Fig.  42).  The  fibers  are  united 
longitudinally  and  transversely  by  a  cement  material.  The  muscle 
is  increased  in  thickness  by  the 
superposition  of  successive  layers. 
At  varying  intervals  the  fibers  are 

I     ■     .        1  ii  r  t  Connective-tissue. 

grouped  into   bundles  or  fasciculi  septum. 

by  septa  of  connective  tissue  (Fig. 

43.)     Blood-vessels  ramify  in  the 

connective  tissue  and  furnish  the    „ 

,  .j.  ,     .  1  Smooth  muscie-fiber- 

necessary  nutritive  material.  in  transverse  section. 

The    visceral    muscle    receives 
stimuli    from    the   sninal   rord    not  Fig.  43-— Section  of  the  Circular 

stimuli   irom  me  spinal  cora,  not      layer  of  the  Muscular  Coat  of  the 

directly,  however,  as  in  the  case  of      Human  Intestine.— (Stohr.) 
the  skeletal  muscle,  but  indirectly 

through  the  intermediation  of  ganglion  cells,  which  may  be  located 
at  some  distance  from  the  muscle  or  near  the  walls  of  the  viscera. 
Non-medullated  fibers  from  the  ganglion  pass  directly  into  the  muscle, 
where  they  frequently  unite  to  form  a  general  plexus.  From  this 
plexus  fine  branches  take  their  origin  and  ultimately  become  phys- 
iologically associated  with  the  muscle-fiber. 

Physiologic  Properties. — The  visceral  muscles  which  have 
been  subjected  to  experiment  are  mainly  those  of  the  stomach,  in- 
testine, bladder,  ureter,  and  iris.  From  the  results  of  the  experiments 
which  have  been  published,  it  is  evident  that  all  visceral  muscles  pos- 
sess elasticity,  tonicity,  irritability,  and  conductivity. 


Nucleus. 


92  TEXT-BOOK  OF  PHYSIOLOGY. 

The  elasticity  of  the  bladder  muscle  of  the  cat  was  strikingly 
shown  in  the  experiments  published  by  Dr.  Colin  C.  Stewart.  When 
this  muscle  was  weighted  with  weights  differing  by  a  common  incre- 
ment, it  was  extended  on  the  addition  of  each  weight,  though  to  a 
progressively  less  extent.  On  the  removal  of  the  weights  the  muscle 
eventually  returned  to  its  former  length.  The  records  of  the  extension 
were  similar  to,  if  not  identical  with,  those  of  the  skeletal  muscle. 

Tonicity  is  a  property  common  to  all  visceral  muscles.  Each 
muscle  is  continuously  in  a  state  of  contraction  intermediate  between 
that  of  complete  contraction  and  that  of  relaxation.  In  how  far  this 
is  due  to  local  and  inherent  causes  or  to  stimuli  reflected  from  the 
nervous  system  as  a  result  of  peripherally  acting  causes  is  not  in 
individual  instances  readily  determinable.  From  time  to  time  the 
tonicity  varies,  increasing  and  decreasing  in  response  to  these  various 
stimuli  and  in  accordance  with  the  functional  activities  of  the  organs 
in  which  the  muscle  is  found. 

The  irritability  manifests  itself  by  a  change  of  form,  and  doubt- 
less by  the  liberation  of  heat  on  the  application  of  any  form  of  stimulus 
— mechanic,  chemic,  thermic,  electric. 

The  conductivity  is  less  marked  in  the  visceral  than  in  the  skeletal 
muscle,  and,  contrary  to  what  is  observed  in  the  latter,  the  conduction 
extends  laterally  as  well  as  longitudinally  from  fiber  to  fiber.  This 
is  shown  by  stimulation  of  the  exposed  intestine.  Shortly  after  the 
stimulus  is  applied  the  muscle  contracts  longitudinally — i.  e.,  in  a  di- 
rection at  right  angles  to  the  long  axis  of  the  intestine,  partially 
obliterating  its  lumen.  From  this  point  the  conduction  process  indi- 
cated by  the  contraction  wave  passes  in  opposite  directions  for  some 
distance  along  the  canal.  As  to  whether  this  is  accomplished  by 
protoplasmic  processes  extending  from  fiber  to  fiber,  or  whether  the 
uniting  membrane,  differs  in  conducting  power  from  the  sarcolemma, 
is  as  yet  a  matter  of  doubt.  From  the  fact  that  the  upper  two-thirds 
of  the  ureter,  though  free  of  nerve-cells,  exhibits  lateral  conduction, 
it  is  evident  that  it  may  take  place  independent  of  the  nervous 
system. 

The  Contraction  of  the  Visceral  Muscle. — The  general  character 
of  the  contraction  may  be  witnessed  on  opening  the  abdomen  of  a 
recently  killed  animal,  especially  the  rabbit.  Shortly  after  exposure 
to  the  air  the  walls  of  the  intestine  begin  to  contract  in  a  most  vig- 
orous manner.  The  contraction  wave  beginning  at  various  points 
is  propagated  in  both  directions,  running  along  the  intestinal  wall 
for  a  variable  distance.  A  succession  of  similar  waves  may  be  ob- 
served for  some  minutes.  To  the  alternate  contraction  and  relaxa- 
tion of  the  muscle-fibers,  which  are  circularly  arranged,  the  term 
peristalsis  is  usually  given.  The  excised  stomach  of  a  dog  kept 
under  suitable  conditions  will  exhibit  similar  movements.  The 
same  holds  true  of  the  bladder  muscle  of  the  cat,  the  muscle  of  the 
ureter,  etc.     Careful  observation  shows  a  certain  periodicity  in  the 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


93 


movements.     Inasmuch  as  the  cause  is  not  apparent,  these  contrac- 
tions are  termed  spontaneous  or  automatic. 

Graphic  Record  of  the  Contraction. — For  experimental  pur- 
poses narrow  transverse  sections  of  the  stomach  of  the  frog  or  the 
entire  bladder  muscle  of  the  cat,  excised  or  in  situ,  according  to  the 
method  of  Prof.  Colin  C.  Stewart,  may  be  employed.  If  kept  moist, 
they  will  retain  their  irritability  for  some  hours.  The  changes  of 
form  may  be  recorded  with  the  usual  muscle  lever.  When  thus  pre- 
pared, the  muscle  may  exhibit  for  several  hours  a  series  of  pulsa- 
tions, rhythmic  in  character.  With  spontaneously  acting  mammalian 
muscle  the  contraction  and  relaxation  periods  are  of  equal  duration. 
With  the  amphibian  muscle  they  are  of  unequal 
duration,  as  a  rule.  In  both  classes  of  animals 
the  character  of  the  record,  a  succession  of  large 
and  small  contractions,  would  indicate  that  the 
general  rhythmic  movement  is  compounded  of 
two  or  three  secondary  rhythms  which  differ  in 
rate  and  character.  A  single  pulsation  may  be 
recorded  by  stimulating  the  bladder  muscle  with 
the  induced  or  the  make  and  break  of  the  con- 
stant current.  A  curve  of  such  a  contraction  is 
shown  in  Fig.  44.  The  contraction  takes  place 
more  rapidly  than  the  relaxation;  the  two  phases 
occupying  five  and  thirty-five  seconds  respec- 
tively. The  latent  period  covered  0.25  second. 
With  other  muscles  the  time  relations  are  slightly 
different.  Tetanization  of  the  bladder  muscle  of 
the  cat  occurred  when  the  stimuli  succeeded  each 
other  with  a  certain  rapidity;  the  interval  between 
stimuli  approximating  a  period  somewhat  less 
than    two    seconds.      This  muscle  responds   to 

variations  in  temperature,  to  strength  of  stimulus,  to  the  load,  in  a 
manner  similar  to,  if  not  identical  with,  the  skeletal  muscle. 

The  Function  of  the  Visceral  Muscle. — In  a  general  way  it 
may  be  said  that  the  visceral  muscle  determines  and  regulates  the  pas- 
sage through  the  viscus  or  organ  of  the  material  contained  within  it. 
The  food  in  the  stomach  and  intestines  is  subjected  to  a  churning  proc- 
ess by  the  muscles,  in  consequence  of  which  the  digestive  fluids 
are  more  thoroughly  incorporated  and  their  characteristic  action  in- 
creased. At  the  same  time  the  food  is  carried  through  the  canal,  the 
absorption  of  the  nutritive  material  promoted,  and  the  indigestible 
residue  removed  from  the  body.  The  blood  is  delivered  in  larger  or 
smaller  volumes  according  to  the  needs  of  the  tissues  through  a  re- 
laxation or  contraction  of  the  muscle-fibers  of  the  blood-vessels. 
The  urine  is  forced  through  the  ureter  and  from  the  bladder  by  the 
contraction  of  their  respective  muscles.  The  mode  of  action  of  the 
individual  muscles  will  be  described  in  successive  chapters. 


itimmmiiiiimwimmimmiiiili 

Fig.  44. — The  Curve 
of  Contraction  of  the 
Bladder  Muscle  at 
Body-temperature  in 
Response  to  a  Single 
Induction  Current. 
The  time  'is  indicated 
in  seconds. — (Stewart.) 


94  TEXT-BOOK  OF  PHYSIOLOGY. 

Ciliary  Movement. — The  free  surface  of  the  epithelium  cover- 
ing the  mucous  membrane  in  certain  regions  of  the  body  is  charac- 
terized by  the  presence  of  delicate  filamentous  processes  termed 
cilia.  (See  Fig.  45.)  Ciliated  epithelium  is  found  in  man  and  mam- 
mals generally,  in  the  nose,  Eustachian  tube,  larynx,  with  the  ex- 
ception of  the  vocal  membranes,  trachea  and  bronchial  tubes  as  far 
as  the  pulmonary  lobules,  Fallopian  tubes,  uterus,  and  epididymis. 
The  lumen  of  the  central  canal  of  the  spinal  cord  and  the  cavities  of 
the  brain  are  lined,  especially  in  childhood,  by  cells  provided  with 
similar  cilia.  Ciliated  epithelium  is  also  found  in  all  classes  of  ani- 
mals, and  especially  in  the  invertebrates. 

The  cilia  found  in  the  human  body  vary  in  length  from  0.003  mm- 
to  0.005  mm.     They  are  apparently  structureless  and  colorless,  and 
appear  to  have  their  origin  in  and  to  be  a  pro- 
longation of  a  transparent  material  on  the  outer 
surface  of  the  cell  material.     The  number  of  cilia 
present    on   the   surface   of  any  individual  cell 
varies  approximately    from   five  to  twenty-five. 
When  ciliated  epithelial  cells,  freshly  removed 
from  the  mucous  membrane  and  moistened  with 
normal  saline,  are  examined  with  the  microscope, 
it  will  be  found  that  the  cilia  are  in  continuous 
Fig  4 s-— Ciliated  Epi-    anc'   rapid  vibratile  movement,  so  much  so  that 
thelium.  the  individual  cilium  can  not  be  distinguished. 

In  time,  however,  their  vitality  declines  and  the 
rapidity  of  movement  diminishes.  When  the  movement  of  the  in- 
dividual cilium  falls  to  about  eight  or  ten  per  second,  its  character  can 
be  readily  determined.  It  will  then  be  seen  that  the  movement  is,  as  a 
rule,  alternately  a  backward  and  a  forward  one,  the  cilium  lowering 
and  then  raising  itself,  the  latter  taking  place  more  quickly  and  ener- 
getically than  the  former.  As  the  cilium  raises  itself  it  becomes  some- 
what flexed  in  a  direction  corresponding  to  that  of  the  general  move- 
ment. The  movement,  however,  varies  in  character  in  different 
situations  and  in  different  animals.  The  cause  of  the  movements  and 
the  mechanism  of  their  coordination  are  unknown.  They  are,  as  far 
as  known,  independent  of  the  nerve  system.  The  force  of  ciliary 
motion  is  very  great.  A  load  of  twenty  grams  can  be  supported  and 
carried  forward  by  the  cilia  on  the  mucous  membrane  of  the  mouth 
and  esophagus  of  the  frog.  The  activity  of  the  cilia  is  associated 
with  the  nutrition  of  the  cell  of  which  they  are  a  part  and  rises  and 
falls  with  it.  Experimentally  it  has  been  found  that  the  rate  and 
energy  of  the  movement  are  greatest  at  a  temperature  of  about  35  °  to 
40 °  C.,  especially  if  they  arc  bathed  with  normal  saline,  rendered 
slightly  alkaline.  Low  temperatures,  acids,  alkalies,  carbon  dioxid, 
etc.,  retard  the  movement. 

The  function  of  the  cilia,  though  not  always  apparent,  is  asso- 
ciated with  the  function  of  the  passages  in  which  they  are  found.     As 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  95 

the  surfaces  of  these  passages  are  swept  by  a  current  of  considerable 
power,  it  is  probable  that  they  assist  in  the  passage  of  the  materials 
which  ordinarily  traverse  them.  Mucus  and  particles  of  dust  are 
carried  upward  through  the  air-passages;  the  ovarian  cell  is  carried 
from  the  ovary  toward  the  uterus;  the  spermatozoa,  as  well  as  the 
fluid  in  which  they  are  contained,  are  carried  forward  through  the 
epididymis  ducts. 


CHAPTER  VIII. 

THE  GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 

The  nerve-tissue,  which  unites  and  coordinates  the  various 
organs  and  tissues  of  the  body  and  brings  the  individual  into  relation- 
ship with  the  external  world,  is  arranged  in  two  systems,  termed 
the  encephalo  spinal  or  cerebrospinal  and  the  sympathetic. 

The  encephalospinal  system  consists  of: 
i.  The  brain   and  spinal  cord,  contained  within  the  cavities  of  the 

cranium  and  the  spinal  column  respectively,  and 
2.  The  cranial  and  spinal  nerves. 

The  sympathetic  system  consists  of: 
i.  A  double  chain  of  ganglia  situated  on  each  side  of  the  spinal  column 
and  extending  from  the  base  of  the  skull  to  the  tip  of  the  coccyx. 
2.  Various  collections  of  ganglia  situated  in  the  head,  face,  thorax, 
abdomen,  and  pelvis.  All  these  ganglia  are  united  by  an  elab- 
orate system  of  intercommunicating  nerves,  many  of  which  are 
connected  with  the  cerebrospinal  system. 

HISTOLOGY  OF  NERVE-TISSUE. 

The  Neuron. — The  nerve-tissue  has  been  resolved  by  the  in- 
vestigations of  modern  histologists  into  a  single  morphologic  unit,  to 
which  the  term  neuron  has  been  applied.  The  entire  nerve  system 
has  been  shown  to  be  but  an  aggregate  of  an  infinite  number  of  neurons, 
each  of  which  is  histologically  distinct  and  independent.  Though 
having  a  common  origin,  as  shown  by  embryologic  investigations, 
they  have  acquired  a  variety  of  forms  in  different  parts  of  the  nervous 
system  in  the  course  of  development.  The  old  conception  that  the 
nerve  system  consisted  of  two  distinct  histologic  elements,  nerve- 
cells  and  nerve-fibers,  which  differed  not  only  in  their  mode  of  origin, 
but  also  in  their  properties,  their  relation  to  each  other,  and  their 
functions,  has  been  entirely  disproved. 

The  neuron,  or  neurologic  unit,  is  histologically  a  nerve-cell,  the 
surface  of  which  presents  a  greater  or  less  number  of  processes  in 
varying  degrees  of  differentiation.  As  represented  in  Figure  46,  A,  the 
neuron  may  be  said  to  consist  of:  (1)  The  nerve-cell,  neurocyte,  or 
corpus;  (2)  the  axon,  or  nerve  process;  (3)  the  end-tufts,  or  terminal 
branches.  Though  these  three  main  histologic  features  are  every- 
where recognizable,  they  exhibit  a  variety  of  secondary  features  in 
different    situations    in    accordance    with    peculiarities    of    function. 

96 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


97 


Though  the  nerve-cell  and  the  nerve-fiber  are  but  part  of  the  same 
neuron,  it  is  convenient  at  present  to  describe  them  separately. 

The  Nerve-cell. — The  nerve-cell,  or  body  of  the  neuron,  presents 
a  variety  of  shapes  and  sizes  in  different  portions  of  the  nervous 
system.  Originally  ovoid  in  shape,  it  has  acquired,  in  course  of  de- 
velopment, peculiarities  of  form  which  are  described  as  pyramidal, 
stellate,  pear-shaped,  spindle-shaped,  etc.  The  size  of  the  cell  varies 
considerably,  the  smallest  having  a  diameter  of  not  more  than  3  0\  0  of  an 
inch,  the  largest  not  more  than  T^  of  an  inch.  Each  cell  consists  of 
granular,  striated  protoplasm,  containing  a  distinct  vesicular  nucleus 


Dendrites 


Nerve-cell. 


Nerve  process 
or  axon. 


Neurilemma. 


Medulla 


Neurilemma. 


.Nerve-cell. 


Fig.  46. — A.  Efferent  neuron.     B.  Afferent  neuron. 


and  a  well-defined  nucleolus.  A  cell  membrane  has  not  been  observed. 
From  the  surface  of  the  adult  cell  portions  of  the  protoplasm  are  pro- 
jected in  various  directions,  which  portions,  rapidly  dividing  and  sub- 
dividing, form  a  series  of  branches,  termed  dendrites  or  dendrons. 
In  some  situations  the  ultimate  branches  of  the  dendrites  present  short 
lateral  processes,  known  as  lateral  buds,  or  gemmules,  which  impart  to 
the  branches  a  feathery  appearance.  This  characteristic  is  common 
to  the  cells  of  the  cortex  of  the  cerebrum  and  of  the  cerebellum.  The 
ultimate  branches  of  the  dendrites,  though  forming  an  intricate  felt- 
work,  never  anastomose  with  one  another  nor  unite  with  dendrites 
of  adjoining  cells.  According  to  the  number  of  axons,  nerve-cells  are 
classified  as  monaxonic,  diaxonic,  polyaxonic.  Most  of  the  cells  of 
7 


9S  TEXT-BOOK  OF  PHYSIOLOGY. 

the  nervous  system  of  the  higher  vertebrates  are  monaxonic.  In  the 
ganglia  of  the  posterior  or  dorsal  roots  of  the  spinal  and  cranial  nerves, 
however,  they  are  diaxonic.  In  this  situation  the  axons,  emerging 
from  opposite  poles  of  the  cell,  either  remain  separate  and  pursue  oppo- 
site directions,  or  unite  to  form  a  common  stem,  which  subsequently 
divides  into  two  branches,  which  then  pursue  opposite  directions. 
(See  Fig.  46,  B.)  The  nerve-cell  maintains  its  own  nutrition,  and 
presides  over  that  of  the  dendrites  and  the  axon  as  well.  If  the  latter 
be  separated  in  any  part  of  its  course  from  the  cell,  it  speedily  degener- 
ates and  dies. 

The  axon,  or  nerve  process,  arises  from  a  cone-shaped  projection 
from  the  surface  of  the  cell,  and  is  the  first  outgrowth  from  its  pro- 
toplasm. At  a  short  distance  from  its  origin  it  becomes  markedly 
differentiated  from  the  dendrites  which  subsequently  develop.  It 
is  characterized  by  a  sharp,  regular  outline,  a  uniform  diameter,  and 
a  hyaline  appearance.  In  structure,  the  axon  appears  to  consist  of 
fine  fibrillae  embedded  in  a  clear,  protoplasmic  substance.  Shafer 
advocates  the  view  that  the  fibrillae  are  exceedingly  fine  tubes  filled 
with  fluid.  The  axon  varies  in  length  from  a  few  millimeters  to  one 
meter.  In  the  former  instance  the  axon,  at  a  short  distance  from  its 
origin,  divides  into  a  number  of  branches,  which  form  an  intricate 
feltwork  in  the  neighborhood  of  the  cell.  In  the  latter  instance 
the  axon  continues  for  an  indefinite  distance  as  an  individual  struc- 
ture. In  its  course,  however,  especially  in  the  brain  and  spinal  cord, 
it  gives  off  a  number  of  collateral  branches,  which  possess  all  its  his- 
tologic features.  The  long  axons  serve  to  bring  the  body  of  the  cell 
into  direct  relation  with  peripheral  organs,  or  with  more  or  less  re- 
mote portions  of  the  nerve  system,  thus  constituting  association  or 
commissural  fibers. 

The  more  or  less  elongated  axon  becomes  invested,  as  a  rule,  at  a 
short  distance  from  the  cell  with  nucleated  oblong  cells,  which  subse- 
quently become  modified  and  constitute  the  medullary  or  myelin 
sheath.  This  is  invested  by  a  thin,  cellular  membrane — the  neu- 
rilemma. These  three  structures  thus  constitute  what  is  known  as  a 
medullated  nerve-fiber.  In  the  brain  and  spinal  cord  the  outer  sheath, 
however,  is  frequently  absent.  In  the  sympathetic  system  the  myelin 
is  also  frequently  absent,  though  the  axon  is  inclosed  by  the  neurilemma, 
thus  constituting  a  non-medullaled  nerve-fiber. 

The  end-tujls  or  terminal  organs  are  formed  by  the  splitting  of  the 
axon  into  a  number  of  filaments,  which  remain  independent  of  one 
another  and  are  free  from  the  medullary  investment.  The  histologic 
peculiarities  of  the  terminal  organs  vary  in  different  situations,  and  in 
many  instances  are  quite  complex  and  characteristic.  In  peripheral 
organs,  as  muscles,  glands,  blood-vessels,  skin,  mucous  membrane, 
the  tufts  are  in  direct  histologic  and  physiologic  connection  with  their 
cellular  elements.  In  the  brain  and  spinal  cord  the  tufts  are  in  more 
or  less  intimate  relation  with  the  dendrites  of  adjacent  neurons. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  99 

The  neurons  in  their  totality  constitute  the  neuron  or  nerve  tissue. 
From  the  fact  that  they  are  arranged  both  serially  and  collaterally  into 
a  regular  and  connected  whole,  they  collectively  constitute  a  system 
known  as  the  neuron  or  nerve  system. 

Neurons,  moreover,  are  grouped  into  more  or  less  completely 
organized  masses,  termed  organs,  which  in  accordance  with  their 
locations  may  for  convenience  be  divided  into  central  and  peripheral 
organs.  The  central  organs  of  the  nerve  system  consist  of  the  en- 
cephalon  or  brain  and  the  spinal  cord ;  the  peripheral  organs  consist  of 
the  cranial  nerves,  the  spinal  nerves,  the  sympathetic  ganglia  and  their 
branches. 

Nerve-fibers. — The  nerve-fibers  which  constitute  by  far  the 
larger  part  of  both  the  peripheral  and  central  organs  of  the  nerve 
system,  are  simply  the  axonic  processes  of  neurons  with  their  secon- 
dary investments,  the  myelin  and  neurilemma;  according  as  they 
possess  or  do  not  possess  the  medullary  sheath,  they  may  be  divided 
into  two  groups — viz.,  mcdullated  and  non-medullated  fibers. 

Medullated  Nerve-fibers. — These  consist  for  the  most  part  of 
three  distinct  structures: 

1 .  An  external  investing  sheath,  tubular  in  shape,  termed  the  neuril- 

emma. 

2.  An  intermediate  semifluid  substance — the  medulla  or  myelin. 

3.  An  internal  dark  thread — the  axis- cylinder. 

The  neurilemma  is  a  thin,  transparent,  homogeneous  membrane 
closely  adherent  to  the  medulla.  Owing  to  its  colorless  appearance, 
it  can  be  seen  only  with  difficulty  in  fresh  tissue.  When  treated 
with  various  reagents,  it  becomes  distinct.  Physically,  it  is  quite 
resistant  and  elastic.  Its  function  is  doubtless  that  of  a  protective 
agent  to  the  structures  within. 

The  medulla,  myelin,  or  white  substance  0}  Schwann  completely 
fills  the  neurilemma  and  closely  invests  the  axis-cylinder  or  axon.  In 
fresh  tissue  the  medulla  is  clear,  homogeneous,  semifluid,  and  highly 
refracting.  When  the  nerve  is  treated  with  various  reagents  which 
alter  its  composition,  the  medulla  becomes  opaque  and  imparts  a 
white,  glistening  appearance.  The  function  of  the  medulla  is  quite 
unknown. 

At  intervals  of  about  seventy-five  times  its  diameter  the  medul- 
lated nerve-fiber  undergoes  a  remarkable  diminution  in  size,  due  to 
an  interruption  of  the  medullary  substance,  so  that  the  neurilemma 
lies  directly  on  the  axis-cylinder.  These  constrictions,  or  nodes  of 
Ranvier,  taking  their  name  from  their  discoverer,  occur  at  regular 
intervals  along  the  course  of  the  nerve,  separating  it  into  a  series  of 
segments.  The  portion  between  the  nodes  is  termed  the  internodal 
segment.  It  has  been  suggested  that  in  consequence  of  the  absence 
of  the  myelin  at  these  nodes,  a  free  exchange  of  nutritive  material 
and  decomposition  products  can  take  place  between  tbe  axis-cylinder 
and  the  surrounding  plasma.     Beneath  the  neurilemma  in  each  inter- 


ioo  TEXT-BOOK  OF  PHYSIOLOGY. 

nodal  segment  there  is  a  large  nucleus  surrounded  by  a  small  amount 
of  granular  protoplasm. 

The  axis-cylinder,  or  axon,  the  direct  outgrowth  of  the  nerve-cell, 
is  the  most  essential  element  of  the  nerve-fiber,  as  it  alone  is  uni- 
formly continuous  throughout.  In  the  natural  condition  it  is  trans- 
parent and  invisible;  but  when  treated  with  proper  reagents,  it  presents 
itself  as  a  pale,  granular,  flattened  band,  more  or  less  solid  and  some- 
what elastic.  It  is  albuminous  in  composition.  With  high  magnifi- 
cation the  axis  presents  a  longitudinal  striation,  indicating  a  fibrillar 
structure.  The  fibrillar  appear  to  be  embedded  in  an  intervening 
semifluid  substance,  the  neuroplasm. 

Non-Medullated  Nerve-fibers. — These  consist,  for  the  most 
part,  only  of  the  axis-cylinder,  though  in  some  portions  of  the  nerve 
system  a  neurilemma  is  also  present.  Though  much  less  abundant 
than  the  former  variety,  they  are  distributed  largely  throughout  the 
nerve  system,  but  are  particularly  abundant  in  the  sympathetic. 
Owing  to  the  absence  of  a  medulla,  they  present  a  rather  pale  or 
grayish  appearance. 

Sympathetic  Ganglia. — A  sympathetic  ganglion  consists  essen- 
tially of  a  connective-tissue  capsule  with  an  interior  framework. 
The  meshes  of  this  framework  contain  nerve-cells  provided  with 
dendrites  and  branching  axons.  The  majority  of  the  axons  are  non- 
medullated.  In  all  instances,  with  the  exception  of  the  ganglion  cells 
of  the  heart,  the  axons  are  distributed  to  non-striated  muscle  tissue 
and  to  the  epithelium  of  glands. 

The  nerve-cells  of  the  ganglia  are  also  in  histologic  connection 
with  the  terminal  branches  of  fine  medullated  nerve-fibers  which 
leave  the  spinal  cord  by  way  of  the  anterior  roots  of  the  spinal  nerves. 
These  nerve-fibers  are  designated  autonomic  or  pre- ganglionic  fibers, 
while  those  emerging  from  the  cells  are  designated  p ost- ganglionic 
fibers.     (See  Sympathetic  System.) 

The  Peripheral  Organs  of  the  Nerve  System. — These  consist 
of  the  cranial  and  spinal  nerves  and  the  sympathetic  ganglia.  Each 
nerve  consists  of  a  variable  number  of  nerve-fibers  united  into  firm 
bundles  by  connective  tissue  which  supports  blood-vessels  and  lym- 
phatics.    The  bundles  are  technically  known  as  nerve-trunks  or  nerves. 

The  nerve-trunks  connect  the  brain  and  cord  with  all  the  re- 
maining structures  of  the  body.  Each  nerve  is  invested  by  a  thick 
layer  of  lamellated  connective  tissue,  known  as  the  epineurium. 
A  transverse  section  of  a  nerve  shows  (see  Fig.  47)  that  it  is  made 
up  of  a  number  of  small  bundles  of  libers,  each  of  which  possesses 
a  separate  investment  of  connective  tissue — the  perineurium.  With- 
in this  membrane  the  nerve-fibers  are  supported  by  a  fine  stroma— 
the  endoneurium.  After  pursuing  a  longer  or  shorter  course,  the 
nerve-trunk  gives  off  branches,  which  interlace  very  freely  with  neigh- 
boring branches,  forming  plexuses,  the  fibers  of  which  are  distributed 
to  associated  organs  and  regions  of  the  body.     From  their  origin  to 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  101 

their  termination,  however,  nerve-fibers  retain  their  individuality,  and 
never  become  blended  with  adjoining  fibers. 

As  nerves  pass  from  their  origin  to  their  peripheral  terminations, 
they  give  off  a  number  of  branches,  each  of  which  becomes  invested 
with  a  lamellated  sheath— an  offshoot  from  that  investing  the  parent 
trunk.  This  division  of  nerve-bundles  and  sheath  continues  through- 
out all  the  branchings  down  to  the  ultimate  nerve-fibers,  each  of 
which  is  surrounded  by  a  sheath  of  its  own,  consisting  of  a  single 
layer  of  endothelial  cells.  This  delicate  transparent  membrane,  the 
sheath  of  Henle,  is  separated  from  the  nerve-fiber  by  a  considerable 


^%ii^5> 


Fig.  47. — Transverse    Section    of  a  Nerve  (Median),     ep.   Epineurium. 
pe.   Perineurium,     ed.  Endoneurium. — (Landois  and  Stirling.) 


space,  in  which  is  contained  lymph  destined  for  the  nutrition  of  the 
fiber.  Near  their  ultimate  terminations  the  nerve-fibers  themselves 
undergo  division,  so  that  a  single  fiber  may  give  origin  to  a  number 
of  branches,  each  of  which  contains  a  portion  of  the  parent  axis- 
cylinder  and  myelin. 

Blood-supply. — Nerves  being  parts  of  living  cells  require  for 
the  maintenance  of  their  nutrition  a  certain  amount  of  blood.  This 
is  furnished  by  the  blood-vessels  ramifying  in  and  supported  by  the 
connective-tissue  framework.  Here  as  elsewhere  there  is  a  constant 
exchange,  through  the  capillary  wall  and  the  neurilemma,  of  nutritive 
material  to  the  nerve  proper  and  of  waste  materials  to  the  blood. 

The  Chemic  Composition  and  Metabolism. — Chemic  analysis 
of  nerve-tissue  has  shown  the.  presence  of  water,  proteins  (two  glob- 
ulins and  a  nucleo-protein),  neurokeratin  and  nuclein,  two  phos- 
phorized  bodies   (protagon  and  lecithin),  several  cerebrosides  (nitro- 


102  TEXT-BOOK  OF  PHYSIOLOGY. 

gen-holding  bodies  of  a  glucoside  character,  as  shown  by  their  yielding 
the  reducing  carbohydrate  galactose),  inorganic  salts,  and  a  series  of 
nitrogen-holding  bodies  such  as  creatin,  xanthin,  urea,  leucin,  etc.  As 
to  the  metabolism  that  is  taking  place  in  nerve-cells  and  fibers,  practic- 
ally nothing  definite  is  known.  That  such  changes,  however,  are 
taking  place  would  be  indicated  first  by  the  blood-supply,  and  second 
by  the  fact  that  withdrawal  of  the  blood-supply  is  followed  by  a  loss 
of  irritability.  The  metabolism  of  the  central  organs  of  the  nerve  sys- 
tem is  more  active  and  extensive.  In  this  situation  any  withdrawal 
of  blood  from  compression  or  occlusion  of  blood-vessels  is  followed 
by  impairment  of  nutrition  and  loss  of  function. 

THE  RELATION  OF  THE  PERIPHERAL  ORGANS  OF  THE  NERVE 
SYSTEM  TO  THE  CENTRAL  ORGANS. 

Spinal  Nerves. — The  nerves  in  connection  with  the  spinal  cord 
are  thirty-one  in  number  on  each  side.  If  traced  toward  the  spinal 
column,  it  will  be  found  that  the  nerve-trunk  passes  through  an  in- 
tervertebral foramen.  Near  the  outer  limits  of  the  foramina  each 
nerve-trunk  divides  into  two  branches,  generally  termed  roots,  one 
of  which,  curving  slightly  forward  and  upward,  enters  the  spinal 
cord  on  its  anterior  or  ventral  surface,  while  the  other,  curving  back- 
ward and  upward,  enters  the  spinal  cord  on  its  posterior  or  dorsal 
surface.  The  former  is  termed  the  anterior  or  ventral  root ;  the  latter, 
the  posterior  or  dorsal  root.  Each  dorsal  root  presents  near  its  union 
with  the  ventral  root  a  small  ovoid  grayish  enlargement  known  as 
a  ganglion.  Both  roots  previous  to  entering  the  cord  subdivide  into 
from  four  to  six  fasciculi. 

A  microscopic  examination  of  a  cross-section  of  the  spinal  cord 
shows  that  the  fibers  of  the  ventral  roots  can  be  traced  directly  into 
the  body  of  the  nerve-cells  in  the  ventral  horns  of  the  gray  matter. 
The  fibers  of  the  dorsal  roots  are  not  so  easily  traced,  for  they  diverge 
in  several  directions  shortly  after  entering  the  cord.  In  their  course 
they  give  off  collateral  branches  which,  in  common  with  the  main 
fiber,  end  in  tufts  which  become  associated  with  nerve-cells  in  both 
the  ventral  and  dorsal  horns  of  the  gray  matter. 

Cranial  Nerves. — The  nerves  in  connection  with  the  base  of  the 
brain  are  known  as  cranial  nerves;  some  of  these  nerves  present  a 
similar  ganglionic  enlargement,  and  therefore  may  be  regarded  as 
dorsal  nerves,  while  others  may  be  regarded  as  ventral  nerves.  Their 
relations  within  the  medulla  oblongata  are  similar  to  those  within 
the  spinal  cord. 

Efferent  and  Afferent  Nerves. — Nerves  are  channels  of  com- 
munication between  the  brain  and  spinal  cord,  on  the  one  hand,  and 
the  muscles,  glands,  blood-vessels,  skin,  mucous  membrane,  viscera, 
etc.,  on  the  other.  Some  of  the  nerve-fibers  serve  for  the  transmission 
of  nerve  energy  from  the  brain  and  spinal  cord  to  certain  peripheral 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


103 


organs,  and  so  increase  or  retard  their  activities;  others  serve  for  the 
transmission  of  nerve  energy  from  certain  peripheral  organs  to  the 
brain  and  spinal  cord  which  gives  rise  to  sensation  or  other  modes  of 
nerve  activity.  The  former  are  termed  efferent  or  centrifugal,  the 
latter  afferent  or  centripetal  nerves.  Experimentally  it  has  been  de- 
termined that  the  anterior  or  ventral  roots  contain  all  the  efferent 
fibers,  the  posterior  or  dorsal  roots  all  the  afferent  fibers. 

THE  PERIPHERAL  ENDINGS  OF  NERVES. 

The  efferent  nerves  as  they  approach  their  ultimate  terminations 
lose  both  the  neurilemma  and  myelin  sheath.  The  axon  or  axis- 
cylinder  then  divides  into  a  number  of  branches  which  become  directly 


Motor  plate. 


Fig.  48. — Motor  Nerve-endings  of  Intercostal  Muscle-fibers  of  a  Rabbit. 

X  150. — (Stohr.) 


and  intimately  associated  with  tissue-cells.  The  particular  mode  of 
termination  varies  in  different  situations.  These  terminations  are 
generally  spoken  of  as  end-organs,  terminal  organs,  or  end-tufts. 

In  the  skeletal  muscle  the  nerve-fiber  loses  both  neurilemma  and 
myelin  sheath  at  the  point  where  it  comes  in  contact  with  the  muscle- 
fiber.  After  penetrating  the  sarcolemma,  the  axon  or  axis-cylinder 
divides  into  a  number  of  small  branches  which  appear  to  be  embedded 
in  a  relatively  large  mass  of  sarcoplasm  and  nuclei,  the  whole  form- 
ing the  so-called  "motor  plate."  Each  muscle-fiber  possesses  one 
such  plate  or  end-organ  in  mammalia,  several  in  the  frog.    (Fig.  48.) 

In  the  visceral  muscle  the  terminal  nerve-fibers  derived  from 
sympathetic  or  peripheral  neurons  are  primarily  non-medullated. 
The  axons  divide  and  subdivide  and  form  plexuses  which  surround 
the  muscle-cell  bundles.     Fine  fibers  from  the  plexuses  are  given  off 


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TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  49. — Terminations  of  Nerve- 
fibers  in  the  Gland-cells.  A.  Cell 
of  the  parotid  gland  of  a  rabbit.  B. 
Cells  of  the  mammary  gland  of  a  cat  in 
gestation. — (Doyon  and  Morat.) 


which  ultimately  come  into  relation  with  each  individual  cell,  on  the 
surface  of  which  they  terminate  in  the  form  of  one  or  more  granular 
masses. 

In  the  glands,  taking  as  an  illustration  the  parotid  and  mammary 
glands,  the  nerve-fibers,  also  derived  from  sympathetic  or  peripheral 
neurons,  pass  into  the  body  of  the  gland  and  ultimately  reach  the 
acini,  on  the  outer  surface  of  which  they  ramify  and  form  a  plexus. 
From  this  plexus  fine  fibers  penetrate  the  acinus  wall  and  end  on 
the  gland-cell.  The  fibers  present  a  varicose  appearance  (Fig.  49). 
The  afferent  nerves  as  they  approach  their  ultimate  terminations 
undergo  similar  changes.  The  end-tufts  become  associated,  in  some 
situations,  with  specialized  end-organs  which  are  extremely  complex; 

e.  g.,  the  retina  in  the  eye,  the 
organ  of  Corti  in  the  ear,  the  taste- 
beakers  in  the  tongue,  the  olfactory 
cells  in  the  nose. 

In  the  skin  and  mucous  mem- 
branes the  mode  of  termination 
varies  considerably.  The  follow- 
ing are  some  of  the  principal 
modes: 

1.  Free  endings  in  the  epithelium. 

2.  Tactile  cells  of  Merkel. 

3.  Tactile  corpuscles  in  the  papillae  of  the  true  skin. 

4.  Pacinian  corpuscles  found  attached  to  the  nerves  of  the  hand  and 

feet,  to  the  intercostal  nerves,  and  to  nerves  in  other  situations. 

5.  End-bulbs  of   Krause  in  the  conjunctiva,  clitoris,  penis,  etc. 

(A  consideration  of  these  end-organs  will  be  found  in  the  chapters 
devoted  to  the  organs  of  which  they  form  a  part.) 

In  the  skeletal  muscles  afferent  fibers  become  associated  with  small 
spindle-shaped  structures  known  as  muscle-spindles  or  neuromuscle 
end-organs.  These  spindles  vary  in  length  from  1  mm.  to  4  mm. 
They  consist  of  a  capsule  of  fibrous  tissue  containing  from  five  to 
twenty  muscle-fibers.  After  penetrating  the  several  layers  of  the 
capsule,  the  nerve-fibers  lose  the  neurilemma  and  myelin  sheaths. 
The  axons  or  axis-cylinders  then  divide  into  several  long  narrow 
branches  which  wind  themselves  in  a  spiral  manner  around  the  con- 
tained muscle-fiber  and  terminate  in  small  oval-shaped  discs.  Similar 
endings  have  been  observed  in  the  tendons  of  muscles. 

Development  and  Nutrition  of  Nerves. — The  efferent  nerve- 
fibers,  which  constitute  some  of  the  cranial  nerves  and  all  the  ventral 
roots  of  the  spinal  nerves,  have  their  origin  in  cells  located  in  the  gray 
matter  beneath  the  aqueduct  of  Sylvius,  beneath  the  floor  of  the  fourth 
ventricle,  and  in  the  anterior  horns  of  the  gray  matter  of  the  spinal 
cord.  These  cells  are  the  modified  descendants  of  independent,  oval, 
pear-shaped  cells — the  neuroblasts — which  migrate  from  the  medullary 
tube.     As   they  approach  the  surface  of  the  cord  their  axons  are 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


105 


Posterior 


Ganglion/ 


directed  toward  the  ventral  surface,  which  eventually  they  pierce. 
Emerging  from  the  cord,  the  axons  continue  to  grow,  and  become 
invested  with  the  myelin  sheath  and  neurilemma,  thus  constituting 
the  ventral  roots. 

The  afferent  nerve-fibers,  which  constitute  some  of  the  cranial 
nerves  and  all  the  dorsal  roots  of  the  spinal  nerves,  develop  outside 
of  the  central  nervous  system  and  only  subsequently  become  con- 
nected with  it.  (See  Fig.  50.)  At  the  time  of  the  closure  of  the 
medullary  tube  a  band  or  ridge  of  epithelial  tissue  develops  near  the 
dorsal  surface,  which,  becoming  segmented,  moves  outward  and  forms 
the  rudimentary  spinal  gang- 
lia. The  cells  in  this  situa- 
tion develop  two  axons,  one 
from  each  end  of  the  cell, 
which  pass  in  opposite  direc- 
tions, one  toward  the  spinal 
cord,  the  other  toward  the 
periphery.  In  the  adult  con- 
dition the  two  axons  shift 
their  position,  unite,  and  form 
a  T  shaped  process,  after 
which  a  division  into  two 
branches  again  takes  place. 
In  the  ganglia  of  all  the 
sensori-cranial  and  sensori- 
spinal  nerves  the  cells  have 
this  histologic  peculiarity. 

The  efferent  fibers  are 
therefore  to  be  regarded  as 
outgrowths  from  the  nerve- 
cells  in  the  ventral  horns  of  the  gray  matter,  and  serve  to  bring  the 
cells  into  anatomic  and  physiologic  relationship  directly  with  the 
skeletal  muscles  and  indirectly,  through  the  intermediation  of  ganglia 
(see  sympathetic  nervous  system),  with  visceral  muscles  and  glands. 

The  afferent  fibers  are  to  be  regarded  as  outgrowths  from  the 
cells  of  the  dorsal  nerve  ganglia,  and  serve  to  bring  the  skin,  mucous 
membrane,  and  certain  visceral  structures  into  relation  with  special- 
ized centers  in  the  central  nerve  system. 

Nerve  Degeneration. — If  any  one  of  the  cranial  or  spinal  nerves 
be  divided  in  any  portion  of  its  course,  the  part  in  connection  with 
the  periphery  in  a  'short  time  exhibits  certain  structural  changes,  to 
which  the  term  degeneration  is  applied.  The  portion  in  connection 
with  the  brain  or  cord  retains  its  normal  condition  with  the  exception 
of  a  few  millimeters  at  its  peripheral  end.  The  degenerative  process 
begins  simultaneously  throughout  the  entire  course  of  the  nerve,  and 
consists  in  a  disintegration  and  reduction  of  the  myelin  and  axis- 
cylinder  into  nuclei,  drops  of  myelin,  and  fat,  which  in  time  disappear 


ntercor 
£oot 


Fig.  50. — Diagram  Showing  the  Mode 
of  Origin  of  the  Ventral  and  Dorsal 
Roots.— (Edinger,  ajter  His) 


io6 


TEXT-BOOK  OF  PHYSIOLOGY. 


through  absorption,  leaving  the  neurilemma  intact.  Coincident  with 
these  structural  changes  there  is  a  progressive  alteration  and  diminu- 
tion in  the  excitability  of  the  nerve.  Inasmuch  as  the  central  portion 
of  the  nerve,  which  retains  its  connection  with  the  nerve-cell,  remains 
histologically  normal,  it  has  been  assumed  that  the  nerve-cells  exert 
over  the  entire  course  of  the  nerve-fibers  a  nutritive  or  a  trophic  in- 
fluence. This  idea  has  been  greatly  strengthened  since  the  discovery 
that  the  axis-cylinder,  or  the  axon,  has  its  origin  in  and  is  a  direct 
outgrowth  of  the  cell.  When  separated  from  the  parent  cell,  the  fiber 
appears  to  be  incapable  in  itself  of  maintaining  its  nutrition. 

The  relation  of  the  nerve-cells  to  the  nerve-fibers,  in  reference  to 
their  nutrition,  is  demonstrated  by  the  results  which  follow  section 
of  the  ventral  and  dorsal  roots  of  the  spinal  nerves.     If  the  anterior 


Fig.  51. — Degeneration  of  Spinal  Nerves  and  Nerve-roots  after  Section. 
A.  Section  of  nerve-trunk  beyond  the  ganglion.  B.  Section  of  anterior  root.  C.  Sec- 
tion of  posterior  root.  D.  Excision  of  ganglion,  a.  Anterior  root.  p.  Posterior  root. 
g.  Ganglion. — (Dalton.) 


root  alone  be  divided,  the  degenerative  process  is  confined  to  the 
peripheral  portion,  the  central  portion  remaining  normal.  If  the 
posterior  root  be  divided  on  the  peripheral  side  of  the  ganglion,  de- 
generation takes  place  only  in  the  peripheral  portion  of  the  nerve. 
(See  Fig.  51.)  If  the  root  be  divided  between  the  ganglion  and  the 
cord,  degeneration  takes  place  only  in  the  central  portion  of  the  root. 
From  these  facts  it  is  evident  that  the  trophic  centers  for  the  ventral 
and  dorsal  roots  lie  in  the  spinal  cord  and  spinal  nerve  ganglia,  re- 
spectively, or,  in  other  words,  in  the  cells  of  which  they  are  an  integral 
part.  The  structural  changes  which  nerves  undergo  after  separation 
from  their  centers  are  degenerative  in  character,  and  the  process  is 
usually  spoken  of,  after  its  discoverer,  as  the  Wallerian  degeneration. 
When  the  nerve-cells  from  which  the  nerve-fibers  arise,  whether 
efferent  or  afferent,  undergo  degeneration  from  any  cause  whatever,  the 
nerve-fiber  becomes  involved  in  the  degenerative  process  and  when  it  is 
completed  the  structures  to  which  they  are  distributed,  especially 
the  muscles,  undergo  an  atrophic  or  fatty  degeneration,  with  a  change 
or  loss  of  their  irritability.  This  is,  apparently,  not  to  be  attributed 
merely  to  inactivity,  but  rather  to  a  loss  of  nerve  influences,  inasmuch 
as  inactivity  merely  leads  to  atrophy  and  not  to  degeneration. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  107 

Reunion  and  Regeneration. — When  a  nerve-trunk  is  divided 
there  is  a  loss  of  function  of  the  parts  to  which  it  is  distributed,  and 
usually  involves  both  motion  and  sensation.  This,  however,  is  not 
necessarily  permanent,  for  after  a  variable  period  of  time  it  not  in- 
frequently happens,  that  the  functions  are  restored  because  of  a  reunion 
of  the  separated  ends  and  a  regeneration  of  the  peripheral  portion. 
A  histologic  study  of  the  nerve-fibers  after  separation  from  the  nerve- 
cells  shows  that  coincidently  with  the  degenerative  process  there 
occurs  a  regenerative  process,  consisting  in  a  multiplication  of  the  nuclei 
lying  just  beneath  the  neurilemma  and  an  accumulation  around  them 
of  a  granular  protoplasm  which  in  due  time  completely  fill  the  neuril- 
emma. At  this  stage  the  fiber  is  known  as  a  band-fiber.  If  now  the 
physical  conditions  are  such  as  to  permit  of  a  reunion  of  the  nerve, 
this  takes  place,  and  under  the  nutritive  influence  of  the  cell  the  axis- 
cylinder  grows  into  the  band-fiber  and  the  protoplasm  becomes  trans- 
formed into  myelin  as  in  the  original  fiber.  The  axis-cylinder  con- 
tinues to  grow  and  extend  itself  forward  until  it  reaches  its  ultimate 
termination. 


CLASSIFICATION  OF  NERVES. 

The  efferent  nerves  may  be  classified,  in  accordance  with  the 
characteristic  form  of  activity  to  which  they  give  rise,  into  several 
groups,  as  follows: 

1.  Skeletal-muscle   or  motor  nerves,  those  which  convey  nerve  energy 

or  nerve  impulses  to  skeletal-muscles  and  excite  them  to  activity. 

2.  Gland   or  secretor  nerves,  those  which  convey  nerve  impulses  to 

glands,  and  cause  the  formation  and  discharge  of  the  secretion 
peculiar  to  the  gland. 

3.  Vascular  or  vaso-motor  nerves,  those  which  convey  nerve  impulses 

to  the  muscle-fibers  of  the  blood-vessels  and  change  in  one  direction 
or  the  other  the  degree  of  their  natural  contraction.  Those  which 
increase  the  contraction  are  known  as  vaso-constrictors  or  vaso- 
augmentors;  those  which  decrease  the  contraction  are  known 
as  vaso-dilatators  or  vaso-inhibitors.  The  nerves  which  pass 
to  that  specialized  part  of  the  vascular  apparatus,  the  heart, 
transmit  nerve  impulses  which  on  the  one  hand  accelerate  its 
rate  or  augment  its  force,  and  on  the  other  hand  inhibit  or  retard 
its  rate  and  diminish  its  force.  For  this  reason  they  are  termed 
cardio-motor  nerves,  one  set  of  which  is  known  as  accelerator 
and  augmentor,  the  other  as  inhibitor  nerves. 

4.  Visceral   or  viscero-motor  nerves,  those  which  transmit  nerve  im- 

pulses to  the  muscle  walls  of  the  viscera  and  change  in  one  direc- 
tion or  another  the  degree  of  their  contraction.  Those  Avhich 
increase  or  augment  the  contraction  are  known  as  viscero-aug- 
mentor,  while  those  which  decrease  or  inhibit  the  contraction, 
are  known  as  viscero-inhibitor  nerves. 


108  TEXT-BOOK  OF  PHYSIOLOGY. 

5.  Hair  bulb  or  pilo-motor  nerves,  those  which  transmit  nerve  im- 
pulses to  the  muscle-fibers  which  cause  an  erection  of  the  hairs. 
Of  the  foregoing  nerves  the  skeletal-muscle  or  motor  nerves  alone 
pass  directly  to  the  muscle.  The  gland,  the  vascular  and  the  visceral 
nerves,  all  terminate  at  a  variable  distance  from  the  peripheral  organ 
around  a  local  sympathetic  ganglion,  which  in  turn  is  connected  with 
the  peripheral  organ.  The  former  are  termed  pre-ganglionic.  The 
latter  post-ganglionic  fibers.     (See  Fig.  61.) 

The  afferent  nerves  may  also  be  classified,  in  accordance  with 
their  distribution  and  the  character  of  the  sensations  or  other  modes 
of  nerve  activity  to  which  they  give  rise,  into  several  groups,  as  fol- 
lows: 

1.  Tegumentary  nerves,  comprising  those  distributed  to  skin,  mucous 
membranes  and  sense  organs  and  which  transmit  nerve  impulses 
from  the  periphery  to  the  nerve  centers.  They  may  be  divided 
into  sensorifacient  and  reflex  nerves. 

A.  Sensorifacient  nerves,  those  which  transmit  nerve  impulses 
to  the  brain  where  they  give  rise  to  conscious  sensations. 
They  may  be  subdivided  into: 

1.  Nerves  of  special  sense — e.  g.,  olfactory,  optic,  auditory, 
gustatory,  tactile,  thermal,  pain,  pressure — which  give 
rise  to  correspondingly  named  sensations. 

2.  Nerves  of  general  sense — e.  g.,  the  visceral  afferent 
nerves — those  which  give  rise  normally  to  vague  and 
scarcely  perceptible  sensations,  such  as  the  general  sen- 
sations of  well-being  or  discomfort,  hunger,  thirst,  fatigue, 
sex,  want  of  air,  etc. 

B.  Reflex  nerves,  those  which  transmit  nerve  impulses  to  the 
spinal  cord  and  medulla  oblongata,  where  they  give  rise  to 
different  modes  of  nerve  activity.    They  may  be  divided  into : 

1.  Reflex  excitator  nerves,  which  transmit  nerve  impulses 
which  cause  an  excitation  of  nerve  centers  and  in  conse- 
quence increased  activity  of  peripheral  organs,  e.  g., 
skeletal  muscles,  glands,  blood-vessels  and  viscera. 

2.  Reflex  inhibitor  nerves,  which  transmit  nerve  impulses 
which  cause  an  inhibition  of  nerve  centers  and  in  conse- 
quence, decreased  activity  of  the  peripheral  organs.  It 
is  quite  probable  that  one  and  the  same  nerve  may  sub- 
serve both  sensation  and  reflex  action,  owing  to  the  col- 
lateral branches  which  are  given  off  from  the  afferent 
roots  as  they  ascend  the  posterior  column  of  the  cord. 

Muscle  nerves,  comprising  those  distributed  to  muscles  and 
tendons  and  which  transmit  nerve  impulses  from  muscle  and 
tendons  to  the  brain  where  they  give  rise  to  the  so-called  muscle 
sensations,  e.  g.,  the  direction  and  the  duration  of  a  movement, 
the  resistance  offered  and  the  posture  of  the  body  or  of  its  indi- 
vidual parts. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  109 

PHYSIOLOGIC  PROPERTIES  OF  NERVES. 

Nerve  Irritability  or  Excitability  and  Conductivity. — These 
terms  are  employed  to  express  that  condition  of  a  nerve  which  enables 
it  to  develop  and  to  conduct  nerve  impulses  from  the  center  to  the 
periphery,  or  from  the  periphery  to  the  center,  in  response  to  the  action 
of  stimuli.  A  nerve  is  said  to  be  excitable  or  irritable  so  long  as  it 
possesses  these  capabilities  or  properties.  For  the  manifestation  of 
these  properties  the  nerve  must  retain  a  state  of  physical  and  chemic 
integrity;  it  must  undergo  no  change  in  structure  or  chemic  composi- 
tion. The  irritability  of  an  efferent  nerve  is  demonstrated  by  the 
contraction  of  a  muscle,  by  the  secretion  of  a  gland,  or  by  a  change 
in  the  caliber  of  a  blood-vessel,  whenever  a  corresponding  nerve  is 
stimulated.  The  irritability  of  an  afferent  nerve  is  demonstrated  by 
the  production  of  a  sensation  or  a  reflex  action  whenever  it  is  stimu- 
lated. The  irritability  of  nerves  continues  for  a  certain  period  of 
time  after  separation  from  the  nerve-centers  and  even  after  the  death 
of  the  animal,  the  time  varying  in  different  classes  of  animals.  In 
the  warm-blooded  animals,  in  which  the  nutritive  changes  take  place 
with  great  rapidity,  the  irritability  soon  disappears — a  result  due  to 
disintegrative  changes  in  the  nerve,  caused  by  the  withdrawal  of  the 
blood-supply  and  other  non-physiologic  conditions.  In  cold-blooded 
animals,  on  the  contrary,  in  which  the  nutritive  changes  take  place 
relatively  slowly,  the  irritability  lasts,  under  favorable  conditions,  for 
a  considerable  time.  Other  tissues  besides  nerves  possess  irritability, 
that  is,  the  property  of  responding  to  the  action  of  stimuli — e.  g., 
glands  and  muscles,  which  respond  by  the  production  of  a  secretion 
or  a  contraction. 

Independence  of  Tissue  Irritability. — The  irritability  of  nerves 
is  distinct  and  independent  of  the  irritability  of  muscles  and  glands, 
as  shown  by  the  fact  that  it  persists  in  each  a  variable  length  of  time 
after  their  histologic  connections  have  been  impaired  or  destroyed  by 
the  introduction  of  various  chemic  agents  into  the  circulation.  Curara, 
for  example,  induces  a  state  of  complete  paralysis  by  modifying  or 
depressing  the  conductivity  of  the  end-organs  of  the  nerves  just  where 
they  come  in  contact  with  the  muscles,  without  impairing  the  irrita- 
bility of  either  nerve-trunks  or  muscles.  Atropin  induces  complete 
suspension  of  gland  activity  by  impairing  the  terminal  organs  of  the 
secretor  nerves  just  where  they  come  into  relation  with  the  gland- 
cells,  without  destroying  the  irritability  of  either  gland-cell  or  nerve. 

Nerve  Stimuli. — Nerves  do  not  possess  the  power  of  spontaneously 
generating  and  propagating  nerve  impulses;  they  can  be  aroused  to 
activity  only  by  the  action  of  an  external  stimulus.  In  the  physiologic 
condition  the  stimuli  capable  of  throwing  the  nerve  into  an  active 
condition  act  for  the  most  part  on  either  the  central  or  peripheral 
end  of  the  nerve.  In  the  case  of  motor  nerves  the  stimulus  to  the 
excitation,  originating  in  some  molecular  disturbance  in  the  nerve- 


no  TEXT-BOOK  OF  PHYSIOLOGY. 

cells,  acts  upon  the  nerve-fibers  in  connection  with  them.  In  the 
case  of  sensor  or  afferent  nerves  the  stimuli  act  upon  the  peculiar 
end-organs  with  which  the  sensor  nerves  are  in  connection,  which  in 
turn  excite  the  nerve-fibers.  Experimentally,  it  can  be  demonstrated 
that  nerves  can  be  excited  by  a  sufficiently  powerful  stimulus  applied 
in  any  part  of  their  extent. 

Nerves  respond  to  stimulation  according  to  their  habitual  function; 
thus,  stimulation  of  a  sensor  nerve,  if  sufficiently  strong,  results  in 
the  sensation  of  pain;  of  the  optic  nerve,  in  the  sensation  of  light; 
of  a  motor  nerve,  in  contraction  of  the  muscle  to  which  it  is  distrib- 
uted; of  a  secretor  nerve,  in  the  activity  of  the  related  gland,  etc. 
It  is,  therefore,  evident  that  peculiarity  of  nerve  function  depends 
neither  upon  any  special  construction  or  activity  of  the  nerve  itself 
nor  upon  the  nature  of  the  stimulus,  but  entirely  upon  the  pecul- 
iarities of  its  central  and  peripheral  end-organs. 

Nerve  stimuli  may  be  divided  into — 
i.  General    stimuli,   comprising    those   agents  which  are   capable  of 

exciting  a  nerve  in  any  part  of  its  course. 
2.  Special   stimuli,  comprising   those  agents  which  act  upon  nerves 
only  through  the  intermediation  of  the  end-organs. 

The  end-organs  are  specialized  highly  irritable  structures  placed 
between  the  nerve-fibers  and  the  surface.  They  are  especially  adapted 
for  the  reception  of  special  stimuli  and  for  the  liberation  of  energy, 
which  in  turn  excites  the  nerve-fiber  to  activity. 

General  stimuli: 
i.  Mechanic:     Sharp  taps,  sudden  pressure,  cutting,  etc. 

2.  Thermic:     Sudden  application  of  heated  object. 

3.  Chemic:     Contact  of  various  substances  which  alter  their  chemic 

composition  quickly,  e.  g.,  strong  acids  or  alkalies,  sol.  sodium 
chlorid  15  per  cent.,  sugar,  urea,  etc. 

4.  Electric:     Either  the  constant  or  induced  current. 
Special  stimuli: 

For  afferent  nerves — 

1.  Light  or  ethereal  vibrations  acting  upon    the  end-organs  of    the 

optic  nerve  in  the  retina. 

2.  Sound  or  atmospheric  undulations  acting  upon  the  end-organs  of 

the  auditory  nerve. 

3.  Heat  or  vibrations  of  the  air  acting  upon  the  end-organs  in  the  skin. 

4.  Chemic  agencies  acting  upon  the  end-organs  of  the  olfactory  and 

gustatory  nerves. 

For  efferent  nerves — 

A  molecular  disturbance  in  the  central  nerve-cells  from  which 
they  arise,  the  nature  of  which  is  unknown. 

Nature  of  the  Nerve  Impulse. — As  to  the  nature  of  the  nerve 
impulse  generated  by  any  of  the  foregoing  stimuli,  either  general  or 
special,  but  little  is  known.  It  has  been  supposed  to  partake  of  the 
nature  of  a  molecular  disturbance,  a  combination  of  physical  and 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  in 

chemic  processes  attended  by  the  liberation  of  energy,  which  propag- 
ates itself  from  molecule  to  molecule.  The  passage  of  the  nerve 
impulse  is  accompanied  by  changes  of  electric  tension,  the  extent  of 
which  is  an  indication  of  the  intensity  of  the  molecular  disturbance. 
Judging  from  the  deflections  of  the  galvanometer  needle  it  is  probable 
that  when  the  nerve  impulse  makes  its  appearance  at  any  given  point 
it  is  at  first  feeble,  but  soon  reaches  a  maximum  development,  after 
which  it  speedily  declines  and  disappears.  It  may,  therefore,  be 
graphically  represented  as  a  wave-like  movement  with  a  definite 
length  and  time  duration.  Under  strictly  physiologic  conditions  the 
nerve  impulse  passes  in  one  direction  only;  in  efferent  nerves  from 
the  center  to  the  periphery,  in  afferent  nerves  from  the  periphery  to 
the  center.  Experimentally,  however,  it  can  be  demonstrated  that 
when  a  nerve  impulse  is  aroused  in  the  course  of  a  nerve  by  an  ade- 
quate stimulus  it  travels  equally  well  in  both  directions  from  the  point 
of  stimulation.  When  once  started,  the  impulse  is  confined  to  the 
single  fiber  and  does  not  diffuse  itself  to  fibers  adjacent  to  it  in  the 
same  nerve-trunk. 

Rapidity  of  Conduction  of  the  Nerve  Impulse. — The  passage 
of  a  nerve  impulse,  either  from  the  brain  to  the  periphery  or  in  the 
reverse  direction,  requires  an  appreciable  period  of  time.  The 
velocity  with  which  the  impulse  travels  in  human  sensory  nerves  has 
been  estimated  at  about  50  meters  a  second,  and  for  motor  nerves  at 
from  28  to  33  meters  a  second.  The  rate  of  movement  is,  however, 
somewhat  modified  by  temperature,  cold  lessening  and  heat  increasing 
the  rapidity;  it  is  also  modified  by  electric  conditions,  by  the  action 
of  drugs,  the  strength  of  the  stimulus,  etc.  The  rate  of  transmission 
through  the  spinal  cord  is  considerably  slower  than  in  nerves,  the 
average  velocity  for  voluntary  motor  impulses  being  only  n  meters 
a  second,  for  sensory  impulses  12  meters,  and  for  tactile  impulses  40 
meters  a  second. 

Nerve  Fatigue. — Inasmuch  as  nerves  are  parts  of  living  cells, 
the  seat  of  nutritive  changes,  it  might  be  supposed  that  the  passage  of 
nerve  impulses  would  be  attended  by  the  disruption  of  energy-holding 
compounds,  the  production  of  waste  products,  the  liberation  of  heat, 
and  in  time  by  the  phenomena  of  fatigue.  Though  it  is  probable  that 
changes  of  this  character  occur,  yet  no  reliable  experimental  data 
have  been  obtained  which  afford  a  clue  as  to  the  nature  or  extent  of 
any  such  changes.  Stimulation  of  motor  nerves  with  the  induced 
electric  current  for  four  hours  appears  to  be  without  influence  either 
on  the  intensity  of  the  nerve  impulse  or  the  rate  of  its  conduction. 

Identity  of  Efferent  and  Afferent  Nerves  and  Nerve  Impulses. 
— Notwithstanding  the  classification  of  nerve-fibers  based  on  differ- 
ences of  physiologic  actions,  there  are  no  characters,  either  histologic 
or  chemic,  which  serve  to  distinguish  them  from  one  another.  More- 
over, as  the  nerve  impulse  is  conducted  through  a  nerve-fiber  equally 
well  in  both  directions,  as  determined  by  experiments,  it  is  probable 


TEXT-BOOK  OF  PHYSIOLOGY. 


that  it  does  not  differ  in  character  in  the  two  classes  of  nerves.  That 
the  efferent  fibers  conduct  the  nerve  impulses  from  the  nerve-centers 
to  the  periphery,  and  the  afferent  nerves  from  the  periphery  to  the 
centers,  is  because  of  the  fact  that  they  receive  their  stimulus  physio- 
logically only  in  the  centers  or  at  the  periphery.  The  fundamental 
reason  for  difference  of  effects  produced  by  stimulation  of  different 
nerves  is  the  character  of  the  organ  to  which  the  nerve  impulse  is 
conducted.  A  nerve  is  merely  the  transmitter  of  the  nerve  impulse, 
which  if  conducted  to  a  muscle  excites  contraction;  to  a  gland,  secre- 
tion; to  a  blood-vessel,  variation  in  caliber;  to 
special  areas  in  the  brain,  sensations  of  light, 
sound,  pain,  etc. 

Electric  Excitation  of  Nerves. — For  the  pur- 
pose of  studying  the  physiologic  activities  of  nerves 
it  has  been  found  convenient  to  employ  the  nerve- 
muscle  preparation  (the  gastrocnemius  muscle  and 
sciatic  nerve)  and  to  use  as  a  stimulus  the  induced 
electric  current.  (See  Fig.  52.)  When  kept  moist, 
this  preparation  is  extremely  sensitive  to  either  the 
galvanic  or  the  induced  current. 

Though  the  development  and  conduction  of  a 
nerve  impulse  may  be  demonstrated  by  the  deflec- 
tion of  the  galvanometer  needle  or  the  movement 
of  the  mercury  in  the  capillary  electrometer,  it  is 
more  conveniently  demonstrated  by  the  contraction 
of  a  muscle,  the  vigor  of  which,  within  limits,  may 
be  taken  as  a  measure  of  the  intensity  of  the  im- 
pulse. The  preparation  should  be  enclosed  in  a 
moist  chamber  and  the  nerve  connected  with  the  inductorium  through 
the  intervention  of  non-polarizable  electrodes.  The  muscle  may  be 
attached  to  the  muscle-lever  and  its  contractions  recorded. 

A  single  shock  of  an  induced  current  develops,  it  is  believed, 
a  single  nerve  impulse  followed  by  a  single  muscle  contraction.  A 
minimal  contraction  following  a  minimal  electric  stimulus  presupposes 
the  development  of  a  nerve  impulse  of  low  intensity.  Within  certain 
limits  a  maximal  contraction  following  a  maximal  electric  stimulus 
presupposes  the  development  of  a  nerve  impulse  of  high  intensity. 
Intermediate  contractions  indicate  nerve  impulses  of  corresponding 
intensity. 

Tetanization  of  a  muscle  indicates  that  the  nerve  impulses  arrive 
at  the  muscle  with  a  frequency  so  great  that  the  muscle  does  not 
succeed  in  relaxing  from  the  effect  of  one  stimulus  before  the  next 
arrives.  Complete  as  well  as  incomplete  tetanus  may  be  developed 
by  gradually  increasing  the  frequency  of  the  stimulus.  The  character 
of  the  contraction  caused  by  indirect  stimulation — i.  e.,  through  the 
nerve — does  not  differ  in  any  essential  respect  from  that  due  to  direct 
stimulation. 


Fig.  52. — Nerve- 
muscle  Prepara- 
tion oe  a  Frog.  F. 
Femur.  S.  Sciatic 
nerve.  I.  Tendo 
AchilHs.  —  (Landois 
and    Stirling.) 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


ELECTRIC  PHENOMENA  OF  NERVES. 

Electric  Currents  from  Injured  Nerves. — It  was  discovered 
by  du  Bois-Reymond  that  electric  currents  can  be  obtained  from 
nerves  as  well  as  from  muscles,  and  that  the  electric  properties  of 
the  former  correspond  in  most  respects  to  those  of  the  latter.  The 
laws  governing  the  development  and  mode  of  action  of  the  currents 
derived  from  muscles  are  equally  applicable  to  the  currents  derived 
from  nerves. 

A  nerve-cylinder  obtained  by  making  two  transverse  sections  of 
any  given  nerve  presents,  as  in  the  case  of  muscles,  a  natural  and 
two  artificial  transverse  surfaces.  A  line 
drawn  around  the  cylinder  at  a  point 
lying  midway  between  the  two  end  sur- 
faces constitutes  the  equator.  From 
such  a  cylinder  strong  currents  are  ob- 
tained when  the  natural  longitudinal 
surface  and  the  transverse  surface  are 
connected  with  the  electrodes  of  the 
galvanometer  circuit.  The  strength  of 
the  current  thus  obtained  will  diminish 
or  increase  according  as  the  electrode  on 
the  longitudinal  surface  is  removed  from 
or  brought  near  to  the  equator.  If  two 
symmetric  points  on  the  longitudinal  sur- 
face equidistant  from  the  equator  are 
united,  no  current  is  obtainable.  When 
asymmetric  points  on  the  longitudinal 
surface  are  connected,  weak  currents  are 
obtained,  in  which  case  the  point  lying 
nearer  the  equator  becomes  positive  to 
the  point  more  distant,  which  becomes 
negative.  From  these  facts  it  is  evident 
that  all  points  on  the  longitudinal  surface 
are  electrically  positive  to  the  transverse 
surface  and  that  the  point  of  greatest 
positive  tension  is  situated  near  the 
equator  (Fig.  53). 

The  electromotive  force  of  the  nerve  current  varies  in  strength 
with  the  length  and  thickness  of  the  nerve.  The  strongest  current 
obtained  from  the  nerve  of  the  frog  is  equal  to  the  0.002  of  a  Daniell 
cell;  that  obtained  from  the  nerve  of  the  rabbit,  0.026  of  a  Daniell. 
The  existence  of  the  nerve  current,  its  strength,  duration,  etc.,  depend 
largely  on  the  maintenance  of  physiologic  conditions.  All  influences 
which  impair  the  nutrition  of  the  nerve  diminish  the  current.  With 
the  death  of  the  nerve  all  electric  phenomena  disappear. 

Negative  Variation  of  the  Nerve  Current. — During  the  pas- 


Fig.  53. — Diagram  to  Illus- 
trate the  Currents  in  Nerves. 
The  arrowheads  indicate  the  direc- 
tion; the  thickness  of  the  lines  in- 
dicates the  strength  of  the  currents. 
— (Landois  and  Stirling.) 


ii4  TEXT-BOOK  OF  PHYSIOLOGY. 

sage  of  the  nerve  impulse  the  resting  nerve  current,  or  the  demarca- 
tion current,  diminishes  more  or  less  completely  in  intensity,  undergoes 
a  negative  variation,  as  shown  by  the  return  of  the  galvanometer 
needle,  due  to  a  change  in  its  electromotive  condition  or  to  a  diminu- 
tion of  the  difference  in  potential  between  the  positive  longitudinal 
and  negative  transverse  sections.  This  negative  variation  of  the  de- 
marcation current  is  observed  equally  well  from  either  the  central  or 
peripheral  end  of  the  nerve.  If  the  two  ends  of  the  nerve  are  con- 
nected with  galvanometers  and  the  nerve  stimulated  in  the  middle, 
the  demarcation  currents  simultaneously  undergo  a  negative  variation. 
This  may  be  taken  as  a  proof  that  the  excitation  process  propagates 
itself  equally  well  in  both  directions.  The  negative  variation  is  inti- 
mately connected  with  changes  in  the  molecular  condition  of  the  nerve 
and  is  not  due  to  any  extraneous  electric  or  other  influence.  And 
du  Bois-Reymond  was  also  enabled  to  obtain  a  negative  variation  of 
the  current  in  the  nerves  of  a  living  frog  which  were  yet  in  connection 
with  the  spinal  cord.  In  this  experiment  the  sciatic  nerve  was  divided 
at  the  knee  and  freed  from  its  connections  up  to  the  spinal  column;  the 
transverse  and  longitudinal  surfaces  were  then  placed  in  connection 
with  the  electrodes  of  the  galvanometer  wires  and  the  current  per- 
mitted to  influence  the  needle.  The  animal  was  then  subjected  to 
the  action  of  strychnin.  Upon  the  appearance  of  the  muscle  spasms 
the  needle  was  observed  to  swing  backward  toward  the  zero  point  to 
the  extent  of  from  i  to  4  degrees,  and  upon  the  cessation  of  the  spasms 
to  return  to  its  previous  position.  In  an  experiment  of  this  nature 
it  is  obvious  that  the  negative  variation  was  the  result  of  a  physiologic 
stimulation  of  the  nerve  arising  within  the  spinal  cord. 

The  question  also  here  arises  as  to  whether  the  negative  variation 
is  due  to  a  steady,  continuous  decrease  of  the  natural  current,  or 
whether  it  is  due  to  successive  and  rapidly  following  variations  in  its 
intensity,  similar  to  that  observed  in  muscles.  Though  this  can  not 
be  demonstrated  with  the  physiologic  rheoscope,  as  was  the  case  with 
the  muscle,  there  can  be  no  doubt,  both  from  experimentation  and 
analogy,  that  the  latter  supposition  is  the  correct  one.  It  has  been 
shown  that  when  non-polarizable  electrodes  connected  with  Siemen's 
telephone  are  placed  in  connection  with  the  longitudinal  and  trans- 
verse sections  of  a  nerve,  low,  sonorous  vibrations  are  perceived 
during  tetanic  stimulation — a  proof  that  the  active  state  of  the  nerve 
is  connected  with  the  production  of  discontinuous  electric  currents. 
The  oscillations  of  the  mercurial  column  of  the  capillary  electrometer 
also  reveal  similar  electric  changes.  It  was  also  demonstrated  by 
Bernstein  with  a  specially  devised  apparatus,  the  repeating  rheotome, 
that  the  negative  variation  is  composed  of  a  large  number  of  single 
variations  which  succeed  each  other  in  rapid  succession  and  sum- 
marize themselves  in  their  effect  on  the  needle. 

Electric  Currents  from  Uninjured  Nerves. — The  pre-existence 
of  electric  currents  in  living  and  wholly  uninjured  nerves  while  at 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  115 

rest  has  also  been  denied  by  Hermann,  who  regards  all  portions  of 
the  nerve  as  isoelectric,  any  difference  of  potential  being  the  result  of 
some  injury  to  its  surface. 

Action  Currents. — For  reasons  to  be  stated  below,  it  is  very  diffi- 
cult to  determine  the  presence  of  diphasic  action  currents  during  the 
passage  of  an  excitatory  impulse  through  the  nerve-fiber.  The  so- 
called  negative  variation  of  the  resting  nerve  current — the  demarca- 
tion current — which  is  occasioned  by  tetanic  stimulation,  Hermann 
regards  as  the  expression  of  an  action  current  which  flows  in  the  nerve 
in  a  direction  opposite  to  the  demarcation  current.  The  origin  of  this 
action  current  is  to  be  sought  for  in  the  continuous  negativity  of  that 
portion  of  the  longitudinal  surface  of  the  nerve  in  contact  with  the 
diverting  electrode,  while  the  dying  substance  of  the  transverse  surface 
takes  no  part  in  the  excitation.  This  tetanic  action  current,  or  nega- 
tive variation,  was  discovered  by  du  Bois-Reymond,  and  Bernstein 
later  succeeded  in  obtaining  this  action  current  during  the  passage 
of  a  single  excitation  process.  That  the  return  of  the  galvanometer 
needle  toward  the  zero  point  is  not  due  to  an  annulment  of  the  demarc- 
ation current  itself,  but  to  the  appearance  of  an  action  current,  is 
shown  by  the  fact  that  if  the  former  be  compensated  by  a  battery 
current  until  the  needle  rests  on  the  zero  point  the  appearance  of  the 
latter  current  will  cause  the  needle  to  swing  in  a  direction  the  opposite 
of  that  caused  by  the  demarcation  current.  The  negative  variation 
and  action  current  may  therefore  be  regarded  as  one  and  the  same 
thing.  It  is  the  expression  of  the  change  the  nerve  is  undergoing 
during  the  passage  of  the  nerve  impulse.  The  rapidity  with  which 
the  negative  variation  or  action  current  travels,  the  variation  in  its 
intensity  from  moment  to  moment,  the  time  required  for  it  to  pass 
a  given  point,  would  express  the  change  in  the  nerve  to  which  the 
term  nerve  impulse  is  given.  From  experiments  made  with  the 
differential  rheotome,  Bernstein  calculated  that  the  speed  of  the 
negative  variation  is  about  28  meters  a  second;  that  it  is  at  first  feeble, 
soon  rises  to  a  maximum,  and  then  declines;  that  it  requires  0.0006 
to  0.0008  of  a  second  to  pass  a  given  point.  From  these  data  it  is 
evident  that  the  negative  variation  or  action  current  has  a  space  value 
of  0.0006  of  28  meters  or  about  18  mm.  Transferring  these  statements 
to  the  nerve  impulse,  it  may  be  said  that  it  is  a  molecular  disturbance, 
traveling  at  the  rate  of  about  28  meters  a  second,  is  wave-like  in  char- 
acter, the  wave  being  18  millimeters  in  length,  and  occupying  from 
0.0006  to  0.0008  of  a  second  in  passing  any  given  point. 

Absence  of  Diphasic  Action  Currents. — When  any  two  points  on 
the  longitudinal  surface  which  do  not  exhibit  a  current  are  connected 
with  the  galvanometer  and  a  single  wave  of  excitation  passes  beneath 
the  electrodes,  it  might  be  expected  that,  as  in  the  case  of  the  muscle, 
a  diphasic  action  current  would  be  observed,  from  the  fact  that  the 
portions  of  the  nerve  beneath  the  electrodes  become  alternately  neg- 
ative with  reference  to  all  the  rest  of  the  nerve.     This,  however,  is 


n6 


TEXT-BOOK  OF  PHYSIOLOGY. 


not  the  case,  the  absence  of  the  two  opposing  phases  of  the  action 
current  being  explained  on  the  supposition  that  the  negativity  of  the 
two  led-off  points  is  of  equal  amount,  and  that,  owing  to  the  great 
rapidity  with  which  the  excitation  wave  travels,  the  two  phases  fall 
together  too  closely  in  time  to  alternately  influence  the  galvanometer 
needle.  During  stimulation  of  the  nerve,  when  two  currentless  or  iso- 
electric points  are  connected,  there  is  also  an  absence  of  the  action 
current,  as  was  observed  first  by  du  Bois-Reymond,  and  which  is  to 
be  explained  on  similar  grounds.  It  is  true  that  an  apparent  action 
current  is  sometimes  seen  when  the  stimulating  current  is  very  power- 
ful or  the  seat  of  stimulation  too  near  the  diverting  electrodes.  This, 
however,  must  be  attributed  to  an  electrotonic  state  of  the  nerve. 

The  Effects  of  a  Galvanic  Current  on  a  Nerve. — When  a  con- 
stant galvanic  current  of  medium  strength  is  made  to  pass  through  a 
portion  of  a  nerve,  several  distinct  effects  are  produced: 


*  POLARIZING  j 
<?    CURRENT     j 


GALVANOME.TER 


ANELECTROTONIC 
CURRENTS 


KATELECTROTONIC 
CURRENTS 


Fig.  54. — Electrotonic  Currents. 


1.  The  development  of  a  nerve  impulse  at  the  moment  the  current 
enters  and  at  the  moment  the  current  leaves  the  nerve,  i.  e.,  at  the 
moment  the  circuit  is  made  and  at  the  moment  it  is  broken.  The 
development  of  the  nerve  impulse  is  made  evident  by  the  contraction 
of  the  muscle  if  the  nerve-muscle  preparation  be  used.  If  the  current 
be  either  very  weak,  or  very  strong,  the  muscle  contraction  may  not 
always  take  place. 

2.  The  development  0}  electric  currents  on  each  side  of  the  positive 
pole  or  anode,  and  the  negative  pole  or  kathode  (see  Fig.  54),  which 
can  be  led  off  by  means  of  wires  into  a  galvanometer  circuit  from 
either  the  artificial  transverse  and  longitudinal  surfaces,  or  from  any 
two  points  on  the  longitudinal  surface  as  shown  by  the  deflection 
of  the  galvanometer  needle.  The  direction  of  these  electric  cur- 
rents in  the  nerve  coincides  with  that  of  the  galvanic  or  "polarizing 
current."  The  "natural  nerve  currents,"  the  currents  of  injury  or 
demarcation  currents,  as  they  are  variously  termed,  are  at  the  same 
time  increased  and  decreased  at  opposite  extremities  of  the  nerve 
according  to  the  direction  of  the  polarizing  current. 

To  this  changed  condition  of  the  electromotive  forces  in  a  nerve 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  117 

the  term  electrotonus  was  given  (du  Bois-Reymond).  The  currents 
themselves  are  known  as  electrotonic  currents;  from  their  relation 
to  the  anode  and  kathode,  they  are  termed  anelectrotonic  and  kat- 
electrotonic  currents.  The  condition  of  the  nerve  around  the  poles 
both  in  the  intra-polar  and  extra-polar  regions  is  known  as  anelectro- 
tonus  and  katelectrotonus. 

The  electrotonic  currents  vary  considerably  in  strength  and  ex- 
tent, according  to  the  intensity  of  the  polarizing  current,  increasing 
steadily  with  the  intensity  of  the  latter  up  to  the  point  at  which  the 
polarizing  current  begins  to  destroy  the  physical  and  chemic  integrity 
of  the  nerve.  The  electrotonic  currents  are  strongest  in  the  imme- 
diate neighborhood  of  the  electrodes,  but  gradually  diminish  in  strength 
as  the  distance  between  the  polarized  and  led-off  portions  is  increased. 
The  distance  to  which  the  electrotonic  currents  extend  along  the  nerve 
will  depend  very  largely  upon  the  strength  of  the  polarizing  current, 
though  it  is  conditioned  by  the  physical  state  of  the  nerve;  for  if  it  be 
ligated  or  injured  beyond  the  polarized  portion,  the  electrotonic  cur- 
rents are  abolished.  The  electrotonic  currents  have  no  necessarv 
connection  with  the  natural  nerve  currents,  nor  are  they  to  be  regarded 
as  branchings  of  the  galvanic  current.  They  are  in  all  probability  of 
artificial  origin,  due  to  an  inner  positive  and  negative  polarization  of 
the  nerve  which  extends  for  a  variable  distance  on  each  side  of  the 
poles,  and  due  to  the  action  of  the  polarizing  or  the  galvanic  current. 

3.  An  alteration  in  the  excitability  and  conductivity  of  the  nerve 
in  the  neighborhood  of  the  poles,  whereby  the  results  of  nerve  stimu- 
lation— that  is,  muscle  contraction,  sensation,  and  inhibition — are 
increased  or  decreased  according  to  the  strength  and  direction  of  the 
current.  To  this  condition  the  term  electrotonus  was  also  given 
(Pfluger).  This  word  has  thus  been  employed  to  express  two  distinct 
series  of  effects  exhibited  by  a  nerve  through  a  portion  of  which  a  con- 
stant galvanic  current  is  passing.  It  appears  desirable,  for  the  sake  of 
clearness,  to  limit  the  term  electrotonus  to  the  electric  or  electrotonic 
currents  which  can  be  led  off  from  either  extremity  of  the  nerve,  and 
to  apply  to  the  modifications  of  irritability  which  accompany  electro- 
tonus the  expression,  electrotonic  alteration  of  excitability  and  con- 
ductivity. 

During  the  passage  of  the  current  the  excitability  of  the  intra- 
polar  as  well  as  the  extra-polar  regions  undergoes  a  change  which, 
as  shown  on  examination,  is  found  to  be  diminished  in  the  neigh- 
borhood of  the  anode  or  positive  pole  and  increased  in  the  neighbor- 
hood of  the  kathode  or  negative  pole.  These  alterations  in  the  excita- 
bility are  most  marked  in  the  immediate  vicinity  of  the  electrodes-; 
though  they  extend  for  some  distance  into  both  the  extra-polar  and 
intra-polar  regions,  though  with  gradually  diminishing  intensity, 
until  they  finally  disappear.  Between  the  electrodes  there  is  a  point 
where  the  excitability  is  unchanged  and  known  as  the  neutral  or 
indifferent  point  (Fig.  55).     The  extent  to  which  the  excitability  is 


nS 


TEXT-BOOK  OF  PHYSIOLOGY. 


modified  as  well  as  the  position  of  the  neutral  point  will  depend  largely 
on  the  strength  of  the  polarizing  or  galvanic  current. 

The  electrotonic  alterations  of  excitability  and  conductivity  can 
be  experimentally  demonstrated  on  the  muscle-nerve  preparation  in 
the  following  manner: 
i.  With  a  descending  current  of  medium  strength.     Previous  to  the 


N     f 


Fig.  55. — Sch&me  of  the  Electrotonic  Excitability. — (Landois  and  Stirling.) 

closure  of  the  polarizing  current,  the  nerve  is  stimulated  first 
in  the  extra-polar  anodic  region  and  the  extra-polar  kathodic 
region  with  an  induction  shock  of  medium  intensity  and  the 
height  of  the  contraction  recorded.  On  repeating  the  stimulation 
after  closure  of  the  polarizing  current  the  contraction  resulting 


'Miimmiii, 


ANODE 


^.REGION    OF 

J\  INCREASED  EXCITABILITY 


KATHODE 


SECONDARY  COIL 

Fig.  56. — Diagram  Showing  the  Region  of  Increased  Excitability  Caused 
j-.y  the  Passage  of  a  Galvanic  Current,  Stimulation  of  which  Gtves  Rise  to  In- 
creased Contraction. 


from  .stimulation  of  the  anodic  region  will  be  enfeebled  or  may  be 
entirely  wanting,  while  the  contraction  from  stimulation  of  the 
kathodic  region  will  be  decidedly  increased.     (See  Fig.  56.) 
With  an  ascending  current  of  the  same  strength.     After  prelim- 
inary testing  of  the  excitability  and  the  subsequent  closure  of 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


119 


the  polarizing  current,  it  will  be  found  that  stimulation  of  the 
extra-polar   anodic   region  will   provoke   a   much  less  energetic 
contraction  or  perhaps  none  at  all.     Stimulation  of  the  extra- 
kathodic  region,  though  of  increased  excitability,  as  shown  by 
the  previous  experiment,  may  also  fail  to  provoke  a  contraction, 
owing  to  the  diminished  conductivity  of  the  region  in  the  neighbor- 
hood  of  the   anode.     The  impulse  on  reaching  this   region   is 
blocked  in  its  passage.     A  similar  if  not  more  marked  decrease 
in  the  conductivity  may  be  developed  in  the  region  of  the  kathode 
if  the  current  strength  be  very  great.     (See  Fig.  57.) 
The  Law  of  Contraction;  Polar  Stimulation. — It  was  stated 
in  a  previous  paragraph  that  when  a  galvanic  current  of  medium 
strength  is  made  to  enter  a  nerve,  and  when  it  is  withdrawn  from  the 
nerve,  there  is  a  contraction  of  its  related  muscle.     These  are  generally 


REGION    OF 
DECREASED    EXCITABILITY 


Fig.  57. — Diagram  Showing  the  Region  of  Decreased  Excitability  Caused  by 
the  Passage  of  a  Galvanic  Current,  Stimulation  of  which  Gives  Rise  to  De- 
creased Contraction. 


known  as  the  make  and  break  effects.  During  the  actual  passage 
of  the  current  no  effect  is  observed  so  long  as  its  strength  remains 
uniform.  Any  sudden  variation  in  the  strength  of  the  current  at 
once  arouses  the  nerve  to  activity,  as  shown  by  a  muscle  contraction. 

The  muscle  response  to  the  make  and  break  of  the  constant  current 
is  more  or  less  variable  unless  the  direction  of  the  current  as  well  as 
its  strength  be  taken  into  consideration.  If  the  current  is  made  to 
flow  from  the  central  toward  the  peripheral  end  of  the  nerve  it  is 
termed  a  direct,  descending,  or  centrifugal  current;  if  it  is  made  to 
flow  in  the  reverse  direction,  it  is  termed  an  indirect,  ascending,  or 
centripetal  current.  The  strength  of  the  current  is  determined  and 
regulated  by  means  of  a  rheocord. 

The  make  and  break  of  currents  of  different  but  known  strengths 
and  directions  give  rise  to  contractions  which  occur  with  more  or  less 
regularitv.     The   order    in    which    thev    occur   under   these   van-ins: 


TEXT-BOOK  OF  PHYSIOLOGY, 


conditions   of  experimentation  has  been  determined   and   tabulated 
as  follows  by  Pfluger,  and  is  termed  the  law  of  contraction: 


Ascending  Current. 

Descending  Current. 

Current  Intensity. 

Make. 

Break. 

Make. 

Break. 

Ccii  rraction. 
Cc'1  ili  action. 
Res- 

Rest. 

Contraction. 
Contraction. 

Contraction. 
Contraction. 
Contraction. 

Rest. 

Contraction. 

Rest    or    weak 

contraction. 

The  results  as  above  tabulated  are  sometimes  complicated  on  the 
opening  of  the  circuit  by  a  series  of  irregular  pulsations  of  the  muscle, 
an  apparent  tetanus,  and  long  known  as  the  opening  tetanus  of  Ritter, 
which  is  attributed  to  rapid  changes  in  the  irritability  of  the  nerve, 
in  the  region  of  the  anode.  A  similar  tetanic  contraction  of  the 
muscle  is  sometimes  observed  on  the  closure  of  the  circuit  due  to 
continued  excitation  in  the  region  of  the  kathode.  This  is  known 
as  the  closing  tetanus  of  Wundt  or  of  Pfluger.  All  the  phenomena 
of  the  law  of  contraction  were  explained  by  Pfluger  on  the  assumption 
that  the  current  stimulates  the  nerve  only  at  the  one  electrode,  at  the 
kathode  on  closing,  and  at  the  anode  on  opening;  or,  in  other  words,  by 
the  appearance  of  katelectrotonus  or  by  the  disappearance  of  anelectro- 
tonus,  both  conditions  being  attended  by  a  rise  of  excitability — not, 
however,  by  the  opposite  changes.  It  is  further  assumed  that  the 
appearance  of  katelectrotonus  is  more  effective  as  a  stimulus  than  the 
disappearance  of  anelectrotonus.  For  these  reasons  the  term  polar 
stimulation  is  generally  employed  in  discussing  the  make  and  break 
effects  of  the  galvanic  current.  The  law  of  contraction  may  then  be 
explained  as  follows:  Very  feeble  currents,  either  ascending  or  de- 
scending, produce  contraction  only  upon  the  closure  of  the  circuit,  the 
sudden  increase  of  the  excitability  in  the  katelectrotonic  area  being 
alone  sufficient  to  generate  an  impulse.  The  contraction  which 
follows  the  closing  of  the  weak  ascending  current  depends  upon  the 
fact  that  the  decrease  of  excitability  and  conductivity  at  the  anode  is 
insufficient  to  interfere  with  the  conduction  of  the  kathodal  stimulus. 
Medium  currents,  either  ascending  or  descending,  produce  contrac- 
tion both  on  closing  and  opening  the  circuit.  The  appearance  of 
katelectrotonus  and  the  disappearance  of  anelectrotonus  are  both 
sufficiently  powerful  to  generate  an  impulse  without,  however,  seri- 
ously impairing  the  conductivity  of  the  nerve. 

Very  strong  currents  produce  contraction  only  upon  the  opening 
of  the  ascending  and  closure  of  the  descending  currents,  or  upon  the 
passage  of  the  excitability  in  the  former  from  the  marked  anelectro- 
tonic  decrease  to  the  normal  condition,  and  in  the  latter  from  the 
normal  to  that  of  katelectrotonic  increase.  The  absence  of  contraction 
upon  the  closure  of  the  ascending  current  is  dependent  upon  the 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  121 

blocking  of  the  kathodal  stimulus  by  the  decrease  of  the  excitability 
and  conductivity  at  the  anode.  With  the  opening  of  the  descending 
current  the  disappearance  of  anelectrotonus  should  also  be  followed 
by  contraction,  which  would  indeed  be  the  case  if  the  stimulus  so 
generated  was  not  blocked  by  the  decrease  of  the  conductivity  at  the 
kathode  in  consequence  of  the  fall  of  a  high  state  of  katelectrotonus  to 
the  normal  condition. 

The  order  in  which  the  contractions  occur  may  be  tabulated  as 
follows : 

With  Ascending  Current.  With  Descending  Current. 

Weak, 1.  K.  C.  C*  .  .  K.  C.  C. . 

Medium, 2.  K.  C.  C.  A.  O.  C.f  K.  C.  C.  A.  O.  C. 

Strong, 3.         ..  A.  O.  C.  K.  C.  C.  A.  O.  C.(?) 

Polar  Stimulation  of  Human  Nerves. — The  preceding  state- 
ments as  to  changes  in  the  excitability  caused  by  the  passage  of  a 
constant  current,  as  well  as  to  the  law  of  contraction,  are  based  en- 
tirely on  experiments  made  with  the  isolated  nerve  of  the  frog.  It 
is  probable,  however,  that  the  same  phenomena  would  have  been 
observed  had  the  nerve  of  a  mammal  been  used  and  its  excitability 
been  maintained. 

If  the  electrodes  connected  with  the  wires  of  a  sufficiently  strong 
galvanic  battery  be  applied  to  the  skin  over  the  course  of  a  superficially 
lying  nerve,  e.  g.,  the  brachial,  it  will  be  found  that  there  occurs  on 
the  closure  of  the  circuit  an  increase  in  the  excitability  in  the  extra- 
polar  anelectrotonic  region  and  a  decrease  in  the  excitability  in  the 
extra-polar  katelectrotonic  region,  as  shown  by  stimulating  the  nerve 
in  the  extra-polar  regions  with  the  induced  current — results  which  are 
in  apparent  contradiction  to  those  obtained  with  the  isolated  nerve. 
This  want  of  accordance  in  the  results  of  the  two  classes  of  experi- 
ments arises  from  a  failure  to  recognize  the  fact  that  the  physiologic 
anode  and  kathode  do  not  coincide  with  the  physical  anode  and 
kathode. 

It  has  been  experimentally  demonstrated  that  owing  to  the  large 
amount  of  readily  conducting  tissue  by  which  the  nerve  is  surrounded, 
the  current  density,  though  great  immediately  under  the  electrode, 
quickly  decreases  at  a  short  distance  from  it,  so  that  for  the  nerve  it 
becomes  almost  nil.  The  current,  therefore,  shortly  after  entering, 
again  leaves  the  nerve  at  various  points  which  become  physiologic 
kathodes.  Stimulation  of  this  physiologic  kathode  with  the  induced 
current  gives  rise,  therefore,  to  the  phenomenon  of  increased  excita- 
bility in  the  region  of  the  anode.  If,  however,  the  galvanic  and 
stimulating  current  be  combined  in  one  circuit  and  both  be  applied 
to  the  same  tract  of  nerve,  results  will  be  obtained  which  harmonize 
with  those  obtained  with  the  frog's  nerve. 

The  changes  in  the  excitability  of  a  nerve  of  a  living  man  and  the 

*K.  C.  C,  kathodal  closing  contraction.  fA.  O.  C.  anodal  opening  contraction. 


122  TEXT-BOOK  OF  PHYSIOLOGY. 

contractions  which  follow  the  closing  and  opening  of  the  constant 
current  have  been  thoroughly  studied  by  Waller  and  de  Watteville. 
These  observers  employed  a  method  similar  to  that  of  Erb,  conjoin- 
ing in  one  circuit  the  testing  and  polarizing  currents.  By  the  graphic 
method  they  recorded  first  the  contraction  produced  by  an  induc- 
tion shock  alone;  and,  secondly,  the  contraction  produced  by  the 
same  stimulus  under  the  influence  of  the  polarizing  current.  As  a 
result  of  many  experiments,  they  also  demonstrated  an  increase  of 
the  excitability  in  the  polar  region  when  it  is  cathodic,  and  a  decrease 
when  it  is  anodic.  Following  the  suggestion  of  Helmholtz,  that  the 
current  density  quickly  decreases  with  the  distance  from  the  elec- 
trodes, they  recognize,  at  the  point  of  entrance  and  exit  of  the  current 
from  the  nerve,  two  regions — a  polar,  having  the  same  sign  as  the 
electrode,  and  a  peripolar,  having  the  opposite  sign  (Figs.  58  and  59). 


Fig.  58. — Anode  of  Battery. 
Polar  region  of  nerve  is  anodic.  Peri- 
polar  region   of   nerve   is   cathodic. 


Fig.  59. — Cathode  of  Battery. 
Polar  region  of  nerve  is  cathodic.  Peri- 
polar region  of  nerve  is  anodic. — (Walk)-.) 


The  peripolar  regions  also  experience  similar  alterations  of  excita- 
bility, though  less  in  degree,  according  as  they  are  kathodic  or  anodic. 

As  it  is  impossible  to  confine  the  current  to  the  trunk  of  the  nerve 
when  surrounded  by  living  tissues,  as  is  easily  the  case  when  experi- 
menting with  the  frog's  nerves,  it  is  incorrect  to  speak  of  either  as- 
cending or  descending  currents.  Waller,*  who  has  thoroughly 
studied  the  electrotonic  effects  of  the  galvanic  current  from  this  point 
of  view,  sums  up  his  conclusions  in  the  following  words:  "We  must 
apply  one  electrode  only  to  the  nerve  and  attend  to  its  effects  alone, 
completing  the  circuit  through  a  second  electrode,  which  is  applied 
according  to  convenience  to  some  other  part  of  the  body. 

"Confining  our  attention  to  the  first  electrode,  let  us  see  what 
will  happen  according  as  it  is  anode  or  kathode  of  a  galvanic  current 
(Figs.  58  and  59).  If  this  electrode  be  the  anode  of  a  current,  the 
latter  enters  the  nerve  by  a  scries  of  points  and  leaves  it  by  a  second 
series  of  points;  the  former,  or  proximal  series  of  points,  collectively 
constitutes  the  polar  zone  or  region;  the  latter,  or  distal  scries  of 
points,  collectively  constitutes  the  peripolar  zone  or  region.  In  such 
case  the  polar  region  is  the  seat  of  entrance  of  current  into  the  nerve — 

*"  Human  Physiology,"  p.  363,  1891. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  123 

i.  e.,  is  anodic;  the  peripolar  region  is  the  seat  of  exit  of  current  from 
the  nerve — i.  e.,  is  kathodic.  If,  on  the  contrary,  the  electrode  under 
observation  be  the  kathode  of  a  current,  the  latter  enters  the  nerve 
by  a  series  of  points  which  collectively  constitute  a  'peripolar'  region, 
and  it  leaves  the  nerve  by  a  series  of  points  which  collectively  con- 
stitute a  'polar'  region.  The  current,  at  its  entrance  into  the  body, 
diffuses  widely,  and  at  its  exit  it  concentrates;  its  'density'  is  greatest 
close  to  the  electrode,  and,  the  greater  the  distance  of  any  point  from 
the  electrode,  the  less  the  current  density  at  that  point;  hence  it  is 
obvious  that  the  current  density  is  greater  in  the  polar  than  in  the 
peripolar  region.  These  conditions  having  been  recognized,  we  may 
apply  to  them  the  principles  learned  by  study  of  frogs'  nerves  under 
simpler  conditions. 

"  Seeing  that,  with  either  pole  of  the  battery,  whether  anode  or  kath- 
ode, the  nerve  has  in  each  case  points  of  entrance  (constituting  a 
collective  anode)  and  points  of  exit  to  the  current  (constituting  a  col- 
lective kathode),  and  admitting  as  proved  that  make  excitation  is 
kathodic,  break  excitation  anodic,  we  may,  with  a  sufficiently  strong 
current,  expect  to  obtain  a  contraction  at  make  and  at  break  with 
either  anode  or  kathode  applied  to  the  nerve;  and  we  do  so,  in  fact. 
When  the  kathode  is  applied,  and  the  current  is  made  and  broken,  we 
obtain  a  kathodic  make  contraction  and  a  kathodic  break  contraction; 
when  the  anode  is  applied,  and  the  current  is  made  and  broken,  Ave 
obtain  an  anodic  make  contraction  and  an  anodic  break  contraction. 
These  four  contractions  are,  however,  of  very  different  strengths;  the 
kathodic  make  contraction  is  by  far  the  strongest;  the  kathodic  break 
contraction  is  by  far  the  weakest;  the  kathodic  make  contraction  is 
stronger  than  the  anodic  make  contraction;  the  anodic  break  con- 
traction is  stronger  than  the  kathodic  break  contraction.  Or,  other- 
wise regarded,  if,  instead  of  comparing  the  contractions  obtained  with 
a  sufficiently  strong  current,  we  observe  the  order  of  their  appearance 
with  currents  gradually  increased  from  weak  to  strong,  we  shall  find 
that  the  kathodic  make  contraction  appears  first,  that  the  kathodic 
break  contraction  appears  last,  and  the  formula  of  contraction  for  man 
reads  as  follows: 


Weak  current,  .  .  . 

.   K.  C.  C. 

Medium  current,    . 

.   K.  C.  C. 

A.  C  C. 

A.  0.  C. 

Strong  current,  .  . 

.  K.  C.  C. 

A.  C.  C. 

A.  O.  C. 

K.  O.    C." 

The  constant  or  the  galvanic  current  is  frequently  used  for  thera- 
peutic and  diagnostic  purposes.  In  accordance  with  the  statements 
above  quoted,  one  electrode  should  be  applied  to  the  part  to  be  in- 
vestigated, the  other  to  some  indifferent  region.  The  electrode  con- 
veying the  current  to  or  from  this  part  should  be  of  a  size  sufficient 
to  localize  the  current  and  to  increase  its  density.  It  was  discovered 
by  Duchenne  that  there  are  certain  points  all  over  the  body  stimula- 
tion of  which  is  more  quickly  followed  by  muscle  contraction  than 
others.     It  was  subsequently  discovered  by  Remak  that  these  points 


124 


TEXT-BOOK  OF  PHYSIOLOGY. 


coincide  with  the  entrance  of  the  nerve  into  the  muscle.  It  is  to 
these  motor  points  that  the  one  electrode  should  be  applied.  The 
position  of  some  of  these  points  on  the  forearm  is  shown  in  Fig.  60. 
Reactions  of  Degeneration. — In  consequence  of  the  degen- 
eration and  changes  in  irritability  which  occur  in  nerves  when  separ- 
ated from  their  centers  and  in  muscles  when  separated  from  their 
related  nerves,  either  experimentally  or  as  the  result  of  disease,  the 
response  of  these  structures  to  the  induced,  and  the  make  and  break 
of  the  constant  current,  differs  from  that  observed  in  the  physiologic 
condition.     The  facts  observed  under  the  application  of  these  two 

M.  biceps  brachii. 

brach.  anticus. 
N.  medianus. 

pronator  teres. 
M.  flex,  digitor.  commun.  profund. 
M.  flex,  carpi  radialis. 

M.  flex,  digitor.  sublim. 

M.  flex.  dig.  subl.  (dig.  ind.  et  min.) 
tM.  flex.  poll, 
longus. 
N.med 
ianus. 


N.  ulnaris. 


M.  flexor  carpi  ulnaris. 


N.  ulnaris. 


Fig.  60. — Motor  Points  of  the  Median  and  Ulnar  Nerves,  with  the  Muscles 
Supplied  by  Them. — (Landois  and  Stirling.) 


forms  of  electricity,  are  of  importance  in  the  diagnosis  and  thera- 
peutics of  the  precedent  lesions.  The  principal  difference  of  behavior 
is  observed  in  the  muscles,  which  exhibit  diminished  or  abolished 
excitability  to  the  induced  current,  while  at  the  same  time  manifesting 
an  increased  excitability  to  the  constant  current;  so  much  so  is  this  the 
case  that  a  closing  contraction  is  just  as  likely  to  occur  at  the  positive 
as  at  the  negative  pole.  This  peculiarity  of  the  muscle  response  is 
termed  the  reaction  0}  degeneration.  The  synchronous  diminished 
excitability  of  the  nerves  is  the  same  for  either  current.  The  term 
"partial  reaction  of  degeneration"  is  used  when  there  is  a  normal 
reaction  of  the  nerves,  with  the  degenerative  reaction  of  the  muscles. 
This  condition  is  observed  in  progressive  muscular  atrophy. 

Reflex  Action. — Inasmuch  as  many  of  the  muscle  movements  of 
the  body,  as  well  as  the  formation  and  discharge  of  secretions  from 
glands,  variations  in  the  caliber  of  blood-vessels,  inhibition  and  ac- 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


125 


celeration  in  the  activity  of  various  organs,  are  the  result  of  stimu- 
lations of  the  terminal  organs  of  afferent  nerves,  they  are  termed,  for 
convenience,  reflex  actions,  and,  as  they  take  place  for  the  most  part 
through  the  spinal  cord  and  medulla  oblongata  and  independently 
of  the  brain  or  of  volitional  influences,  they  are  also  termed  involun- 
tary actions.  A  reflex  action  of  skeletal  muscles,  glands,  or  non- 
striated  muscles  of  blood-vessels  or  of  viscera,  therefore,  may  be  defined 
as  an  action  which  takes  place  independent  of  volition  and  in  response  to 
peripheral  stimulation.     As  many  of  the  processes  to  be  described  in 


sp.c 


Fig.  61. — Diagpaai  Showing  the  Structures  Involved  in  the  Production 
of  Reflex  Actions.  {G.  Bachman.)  r.s.  Receptive  surface;  a).n.  afferent  nerve;  ex. 
emissive  or  motor  cells  in  the  anterior  horn  of  the  gray  matter  of  the  spinal  cord,  sp.c; 
ej.  n.  efferent  nerves  distributed  to  responsive  organs,  e.g.,  directly  to  skeletal  muscles,  sk.n., 
and  indirectly  through  the  intermediation  of  sympathetic  ganglia,  sym.g.,  to  blood-vessels, 
b.v.,  and  to  glands,  g.     The  nerves  distributed  to  viscera  are  not  represented. 


succeeding  chapters  are  of  this  character,  requiring  for  their  per- 
formance the  cooperation  of  several  organs  and  tissues  associated 
through  the  intermediation  of  the  nerve  system,  it  seems  advisable  to 
consider  briefly,  in  this  connection,  the  parts  involved  in  a  reflex 
action,  as  well  as  their  mode  of  action.  As  shown  in  Fig.  61,  the 
necessary  structures  are  as  follows : 

1.  A  receptive  surface,  skin,  mucous  membrane,  sense-organ,  etc. 

2.  An  afferent  nerve-fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises — 

4.  An  efferent  nerve,  distributed  to  a  responsive  organ,  as 

5.  Skeletal  muscle,  gland,  blood-vessel,  etc. 

Such  a  combination  of  structures  constitutes  a  reflex  mechanism 
or  arc,  the  nerve  portion  of  which,  in  the  case  of  skeletal  muscles,  is 
composed  of  but  two  neurons — an  afferent  and  an  efferent.     In  the 


126 


TEXT-BOOK  OF  PHYSIOLOGY. 


case  of  glands  and  non-striated  muscles,  whether  of  blood-vessels  or 
viscera,  the  efferent  neuron  instead  of  passing  direct  to  the  responsive 
organ,  arborizes  around  the  nerve-cells  of  a  peripheral  sympathetic 
ganglion.  The  reflex  arc  is  then  continued  by  the  processes  of  the 
ganglion  cells.  An  arc  of  this  simplicity  would  of  necessity  sub- 
serve but  a  simple  movement.  The  majority  of  reflex  activities, 
however,  are  extremely  complex,  and  involve  the  cooperation  and 
coordination  of  a  number  of  nerve  centers  situated  at  different 
levels  of  the  spinal  cord  on  the  same  and  opposite  side,  and  of  re- 
sponsive organs  frequently  situated  at  distances  more  or  less  remote 
from  one  another.  This  implies  that  a  number  of  neurons  are  as- 
sociated in  function.  The  transference 
of  nerve  impulses  coming  from  a  localized 
area  of  a  sentient  surface  to  emissive  cells 
situated  at  different  levels  is  accom- 
plished by  the  intercalation  of  a  third 
neuron  situated  in  the  gray  matter  which 
is  in  connection,  on  the  one  hand,  with 
the  central  terminals  of  the  afferent 
neuron,  and,  on  the  other  hand,  through 
its  collateral  branches  with  the  dendrites 
of  the  efferent  neurons  situated  at  differ- 
ent levels  of  the  cord.     (Fig.  62.) 

For  the  excitation  of  a  reflex  action 
it  is  essential  that  the  stimulus  applied 
to  the  sentient  surface  be  of  an  intensity 
sufficient  to  develop  in  the  terminals  of 
the  afferent  nerve  a  series  of  nerve  im- 
pulses, which,  traveling  inward,  will  be 
distributed  to  and  received  by  the  den- 
drites of  the  emissive  or  motor  cell.  With 
the  reception  of  these  impulses  there  is 
apparently  a  disturbance  of  the  equilib- 
rium of  its  molecules,  a  liberation  of 
energy,  and,  in  consequence,  a  transmis- 
sion outward  of  impulses  through  the  efferent  nerve  to  muscle,  gland,  or 
blood-vessel,  separately  or  collectively,  with  the  production  of  muscle 
contraction,  a  secretion,  vascular  dilatation  or  contraction,  etc.  The 
reflex  actions  take  place,  for  the  most  part,  through  the  spinal  cord 
and  medulla  oblongata,  which,  by  virtue  of  their  contained  centers, 
coordinate  the  various  organs  and  tissues  concerned  in  the  performance 
of  the  organic  functions.  The  movements  of  mastication;  the  secretion 
of  saliva;  the  muscle,  gland,  and  vascular  phenomena  of  gastric  and 
intestinal  digestion;  the  vascular  and  respiratory  movements;  the 
mechanism  of  micturition,  etc.,  arc  illustrations  of  reflex  activity. 


Fig.  62. — Diagram  Showing 
the)  Relation  of  the  Third 
Neuron  a,  to  the  Afferent 
Neuron  b,  and  to  the  Efferent 
Neurons  c,  c,  c. — {After  Kolliker.) 


CHAPTER  IX. 
FOODS. 

The  functional  activity  of  every  organ  and  tissue  of  the  body  is 
accompanied  by  a  more  or  less  active  disintegration  of  the  living 
material,  the  bioplasm,  of  which  it  is  composed,  as  well  as  of  the  food 
materials  circulating  in  its  interstices.  The  complex  molecules  of 
the  living  material  and  of  the  non-living  food  materials  are  continually 
undergoing  disruption  and  falling  into  less  complex  and  more  stable 
compounds;  these,  through  oxidative  processes,  are  eventually  re- 
duced through  a  series  of  descending  chemic  stages  to  a  small  number 
of  simpler  compounds  which,  being  of  no  further  value  to  the  organ- 
ism, are  eliminated  by  the  various  eliminating  or  excretory  organs, 
the  lungs,  skin,  kidney,  and  liver.  Among  these  excreted  compounds 
derived  from  tissue  and  from  food  metabolism  the  more  important 
are  urea,  uric  acid,  and  carbon  dioxid.  Many  other  compounds, 
organic  as  well  as  inorganic,  are  also  eliminated  from  the  body  in  the 
various  excretions,  though  they  are  present  in  but  small  amounts. 
Coincident  with  this  metabolic  process,  there  is  a  transformation  of 
potential  into  kinetic  energy,  which  manifests  itself  for  the  most  part 
as  heat  and  mechanic  motion. 

In  order  that  the  organs  and  tissues  may  continue  in  the  per- 
formance of  their  functions,  it  is  essential  that  they  be  supplied  with 
nutritive  materials  similar  to  those  which  enter  into  their  own  com- 
position: viz.,  proteids,  fat,  carbohydrates,  water,  and  inorganic 
salts.  These  compounds,  though  originally  derived  from  the  food, 
are  immediately  derived  from  the  blood  as  it  flows  through  the  capil- 
lary blood-vessels.  The  blood  is  therefore  to  be  regarded  as  a  reser- 
voir of  nutritive  material  in  a  condition  to  be  absorbed  and  trans- 
formed into  utilizable  and  living  material.  Inasmuch  as  the  materials 
lost  to  the  body  daily,  through  disintegration  and  oxidation,  though 
considerable,  are  supplied  by  the  blood,  it  is  evident  that  this  fluid 
would  diminish  rapidly  in  volume,  with  a  corresponding  decline 
in  functional  activity,  were  it  not  restored  by  the  introduction  into 
the  body  of  new  material  in  the  food.  With  the  diminution  of  the 
volume  of  the  blood  and  an  insufficient  supply  to  the  tissues,  there 
arise  the  sensations  of  hunger  and  thirst,  which  lead  to  the  consump- 
tion of  food  and  the  subsequent  restoration  of  the  physiologic  condi- 
tion of  the  tissues.  These  two  sensations  are  also  partially  dependent 
on  the  empty  condition  of  the  stomach  and  the  dryness  of  the  mucous 
membrane  of  the  mouth  and  throat. 

127 


i28  TEXT-BOOK  OF  PHYSIOLOGY. 

The  foods  which  are  consumed  daily  in  response  to  the  sensations 
of  hunger  and  thirst  are  complex  in  composition  and  contain,  though 
in  varying  amounts,  proteins,  fats,  carbohydrates,  water  and  inor- 
ganic salts,  which,  in  contradistinction  to  foods,  are  termed  food 
principles  or  nutritive  principles.  In  these  compounds  is  also  to  be 
found  the  potential  energy  necessary  to  maintain  the  dynamic  equi- 
librium of  the  body  and  which  will  become  manifest  as  heat  and 
mechanic  motion  in  the  transformations  of  the  material  underlying 
the  nutritive  processes. 

The  animal  body  may  be  therefore  regarded  as  a  machine  capable 
each  day  of  performing  a  certain  amount  of  work  by  the  expendi- 
ture of  a  definite  amount  of  energy.  In  the  performance  of  its  work, 
whether  it  be  the  raising  of  weights  against  gravity,  the  overcoming 
of  friction,  cohesion,  or  elasticity,  the  machine  suffers  disintegration 
and  metabolizes  a  portion  of  the  food  materials  and  loses  a  portion  of 
its  available  energy.  Unlike  other  machines,  however,  it  possesses  the 
power,  within  limits,  of  self-renewal,  self-adjustment,  when  supplied 
with  foods  in  proper  quantity  and  quality. 


QUANTITIES  OF  FOOD  PRINCIPLES  REQUIRED  DAILY. 

In  order  that  the  body  may  continue  in  the  performance  of  its 
work  and  yet  retain  a  given  weight,  it  is  essential  that  the  loss  to  the 
body  daily  shall  be  exactly  compensated  by  the  introduction  and 
assimilation  of  a  corresponding  amount  of  food  principles.  If  this 
condition  is  realized,  the  body  neither  gains  nor  loses,  but  remains  in 
a  condition  of  nutritive  equilibrium.  The  determination  of  the  exact 
quantities  of  the  different  food  principles  required  daily  and  their 
ratio  one  to  another  is  made  from  an  examination  of  the  quantity  and 
composition  of  the  daily  excretions.  Since  the  proteins  disintegrated 
are  represented  in  the  excretions  by  urea  and  similar  nitrogen-holding 
compounds  and  the  fats  and  carbohydrates  by  carbon  dioxid,  it 
becomes  possible  to  determine  from  them  the  quantities  required  to 
restore  equilibrium  under  any  given  condition.  But  as  the  activity 
of  the  nutritive  changes  will  vary  in  accordance  with  climatic  condi- 
tions, work  done,  etc.,  and  as  the  excreted  products  will  vary  in  the 
same  ratio,  it  is  obvious  that  the  required  amounts  of  food  will  vary 
in  accordance  with  these  varying  conditions,  if  equilibrium  is  to  be 
maintained. 

Various  estimates  have  been  made  by  different  investigators  as 
to  the  amounts  of  the  excreted  products  and  the  food  principles  re- 
quired daily,  which,  though  differing  to  some  extent,  have,  neverthe- 
less, an  average  nutritive  and  energy-producing  value.  The  follow- 
ing table  shows  the  diet  scale  of  Vierordt  and  the  excretions  to  which 
it  would  give  rise.  As  the  income  and  outgo  practically  balance, 
there  would  be  no  change  in  the  weight. 


FOODS. 
COMPARISON  OF  THE  INCOME  AND  OUTGO. 


129 


Income. 


Protein, 

Fat 

Carbohydrates, 

Salts, 

Water, 

Oxygen, 


Grams. 

Ounces. 

120 

4-25 

90 

3-!7 

33° 

11.64 

32 

i-i3 

2818 
756 

99-3° 
26.66 

4146 

146.13 

Outgo. 


Grams.     Ounces. 


Water, 2818 

Urea, 40 

Feces,  dry, 38 

Salts, 32 

Carbon  dioxid, 922 

Water  formed  in  body,  .  296 


99-3° 
1.40 

1.60 

"3 

32-37 
i°-33 


4146        146.15 


Other  estimates  as  to  the  amounts  of  the  organic  food  principles 
required  daily  are  as  follows: 

Ranke.  Voit.  Moleschott.        Atwater.  Hultgren. 

Grams.  Grams.  Grams.  Grams.  Grams. 

Protein, 100  118  130  125  134 

Fat, 100  56  84  125  79 

Starch, 250       500       550       400       522 

In  arranging  tables  showing  the  relation  between  the  income  and 
the  outgo,  it  is  generally  customary  to  state  merely  the  amounts  by 
weight  of  the  nitrogen  and  carbon  each  contains.  This  method 
furnishes  sufficiently  accurate  information  regarding  the  metabolism 
of  the  body,  for  the  reason  that  the  nitrogen  represents  the  protein, 
and  the  carbon,  with  the  exception  of  that  contained  in  the  protein, 
the  fat  and  carbohydrates  which  have  undergone  disintegration  or 
metabolism. 

The  following  balance  table,  as  given  by  Ranke,  shows  the  rela- 
tion of  the  nitrogen  to  the  carbon  in  the  average  mixed  diet  and  in  the 
excretions  of  a  man  weighing  70  kilograms,  in  a  condition  of  nutritive 
equilibrium : 


Income. 


Grams. 


Protein, I       100 

Fat, :       100 

Carbohydrates, ;      250 


15-5 


53-° 

79.0 

93-o 


-- 
15-5 

225.0 

Outgo. 

Grams. 

N. 

c. 

Urea, 

3i-5  \ 
°-5J 

14.4 

1.1 

6.16 

Feces, 

10. S4 

co2, 

J5-5 

225.00 

i3o  TEXT-BOOK  OF  PHYSIOLOGY. 

From  the  above  it  will  be  observed  that  the  daily  discharge  for 
each  kilogram  of  body-weight  is  0.21  gram  nitrogen  and  3.03  grams 
of  carbon;  the  relation  of  the  two  being  ^  =  14.5.  On  a  diet  in 
which  there  is  an  excess  of  either  proteid  or  carbohydrates  this  ratio 
necessarily  changes. 

CLASSIFICATION  OF  FOOD  PRINCIPLES. 

Though  the  food  principles  are  grouped  as  proteids,  fats,  carbo- 
hydrates, etc.,  the  members  of  each  group  differ  somewhat  in  chemic 
composition,  digestibility,  and  nutritive  value.  These  groups  are  as 
follows : 

1.  Proteins. 

Principle.  Where  found. 

Myosin, Flesh  of  animals. 

Albumin,  vitellin, White  of  egg,  yolk  of  egg. 

Caseinogen,   Milk. 

Serum-albumin,  fibrin, Blood  contained  in  meat. 

Glutin, Grain  of  wheat  and  some  other  cereals. 

Vegetable  albumin, Soft -growing  vegetables. 

Legumin, Peas,  beans,  lentils,  etc. 

2.  Fats. 

Animal  fats, In  adipose  tissue  of  animals. 

Vegetable  oils, In  seeds,  grains,  nuts,  fruits,  and  other 

vegetable  tissues. 

3.  Carbohydrates. 

Dextrose  or  grape-sugar, \  In  fmits> 

Levulose  or  fruit-sugar, / 

Lactose  or  milk-sugar, .  Milk. 

Saccharose  or  cane-sugar, Sugar-cane,  beet  roots. 

Maltose, Malt  and  malted  foods. 

c        ,  1  Cereals,  tuberous  roots,  and  legumin- 

btarch' ]      ous  plants. 

Glycogen, Liver,  muscles. 

4.  Inorganic. 

Water, ] 

Sodium  and  potassium  chlorid, I  T  ,         „  ,  ,  .   ,  , 

„    ,.  '  j        ■  1  ■  In     nearly    all    animal    and    vegetable 


Sodium,     potassium,     and      calcium  \      ?      , 

phosphates  and  carbonates, | 

Iron, J 

5.  Vegetable  Acids. 

Citric,  tartaric,  acetic,  malic, In  fruit  and  vegetables. 

6.  Accessory  Foods. 
Coffee,  Tea,  Cocoa,  Alcohol. 

Disposition  of  Food. — The  protein  principles  of  the  food,  after 
undergoing  all  those  changes  which  are  embraced  under  the  term  di- 


FOODS.  131 

gestion,  are  absorbed  from  the  intestine.  During  the  act  of  absorption 
they  are  transformed  into  the  forms  of  protein  characteristic  of  blood. 
After  being  distributed  by  the  blood-stream  to  the  tissues,  they  are 
brought  into  relation  with  the  living  cells.  The  disposition  made  of 
the  protein  material  by  the  bioplasm  of  the  cell  has  not  been  definitely 
determined.  According  to  Voit,  of  the  protein  thus  brought  into  con- 
tact with  the  living  tissues,  only  a  small  percentage  is  utilized  and  assim- 
ilated for  tissue  repair.  This  he  terms  tissue  or  organ  protein.  The 
remaining  large  percentage  circulating  in  the  interstices  of  the  tissues, 
though  not  forming  an  integral  part  of  them,  is  acted  on  directly  by 
them,  merely  in  virtue  of  contact— split  up,  oxidized,  and  reduced  to 
simpler  compounds.     This  he  terms  circulating  protein. 

According  to  Pfluger  and  others,  this  view  is  not  tenable.  Pfliiger 
asserts  that,  as  material  changes  or  metabolism  can  only  take  place 
within  living  cells,  all  the  protein  must  first  be  assimilated  and  organ- 
ized by  the  cells  before  it  can  undergo  metabolic  changes.  Metab- 
olism by  contact  action  is  denied,  and  the  division  of  proteins  into 
organ  and  circulating  protein  is  not  justifiable. 

In  the  process  of  metabolism  the  protein  suffers  disintegration,  giv- 
ing rise  through  oxidation  to  some  carbon-holding  compound,  possibly 
fat,  and  to  some  nitrogen-holding  compounds,  which  eventually  give 
rise  to  urea.  The  intermediate  stages,  however,  are  not  definitely 
known;  the  immediate  antecedents  of  urea  are  probably  carbamate  and 
carbonate  of  ammonia.  The  disintegration  of  the  proteins  is  attended 
by  the  disengagement  of  heat,  thus  contributing  to  the  general  store 
of  the  energy  of  the  body. 

The  fat  principles,  after  digestion,  are  absorbed  by  the  lymphatic 
vessels  and  discharged  by  the  thoracic  duct  into  the  blood,  from  which 
they  rapidly  disappear.  Though  it  is  possible  that  a  portion  of  the 
fat  enters  directly  into  the  formation  of  the  living  material,  it  is  gener- 
ally believed  that  it  is  at  once  oxidized  and  reduced  to  carbon  dioxid 
and  water  with  the  liberation  of  energy.  The  natural  supposition 
that  a  portion  of  the  ingested  fat  was  directly  stored  up  in  the  cells  of 
the  areolar  connective  tissue,  thus  giving  rise  to  adipose  tissue,  has 
been  a  subject  of  much  controversy,  though  modern  experimentation 
renders  this  very  probable.  The  body-fat,  under  physiologic  con- 
ditions, is  also  a  product  of  the  metabolic  activity  of  connective-tissue 
cells  and  is  a  derivative  of  both  proteids  and  carbohydrates. 

The  carbohydrate  principles,  after  digestion,  are  absorbed  into 
the  blood  as  dextrose.  This  compound  is  then  stored  up  in  the  liver 
and  muscles  as  glycogen.  The  intermediate  stages  which  glycogen 
passes  through  before  it  is  reduced  to  carbon  dioxid  and  water  are 
only  imperfectly  known.  Though  a  large  part  of  the  carbohydrate 
material  is  at  once  oxidized,  it  is  now  well  established  that  another 
portion  contributes  to  the  formation  of,  if  it  is  not  directly  converted 
into,  fat.  As  the  carbohydrates  form  a  large  portion  of  the  food,  they 
contribute  materially  to  the  production  of  energy. 


i32  TEXT-BOOK  OF  PHYSIOLOGY. 

The  inorganic  principles,  though  not  playing  apparently  as  active 
a  part  in  the  metabolism  of  the  body  as  the  organic,  are  nevertheless 
essential  to  its  physiologic  activity. 

Water  is  promptly  absorbed  after  ingestion  and  becomes  a  part 
of  the  circulating  fluids- — blood  and  lymph.  In  the  digestive  appa- 
ratus it  favors  the  occurrence  of  those  chemic  changes  in  the  food 
necessary  for  their  absorption,  it  promotes  absorption  of  the  food, 
holds  various  constituents  of  the  blood  and  other  fluids  in  solution, 
hastens  the  general  metabolism  of  the  body,  holds  in  solution  various 
products  of  metabolic  activity,  and,  leaving  the  body  through  the 
excretory  organs,  promotes  their  elimination. 

Sodium  chlorid  is  absorbed  into  the  blood  and,  unless  taken  in 
excess,  is  utilized  in  replacing  that  which  is  lost  to  the  organism  daily. 
The  exact  role  which  sodium  chlorid  plays  in  the  nutritive  process 
is  unknown;  but,  as  it  is  present  as  a  necessary  constituent  in  all  the 
fluids  and  solids  of  the  body,  and  as  it  is  instinctively  employed  as 
a  condiment,  it  may  be  assumed  to  have  a  more  or  less  important 
function. 

When  taken  as  a  condiment,  it  imparts  sapidity  to  the  food  and 
excites  the  flow  of  the  digestive  fluids;  it  ultimately  furnishes  the 
chlorin  for  the  hydrochloric  acid  of  the  gastric  juice.  Judging  from 
the  impairment  of  the  nutrition  as  observed  in  animals  after  depriva- 
tion of  salt  for  a  long  period  of  time,  if  favorably  influences  the  growth 
and  functional  activity  of  all  tissues. 

It  is  well  known  that  herbivorous  animals,  races  of  men  as  well  as 
individuals  who  live  largely  on  vegetable  foods,  require  a  larger  addi- 
tional amount  of  sodium  chlorid  than  carnivorous  animals,  or  human 
beings  who  live  largely  on  animal  foods,  even  though  the  two  classes 
of  foods  contain  relatively  the  same  amounts.  The  explanation  is 
that  the  vegetable  foods  contain  potassium  salts  which,  meeting  in 
the  blood  with  sodium  chlorid,  undergo  decomposition  into  potassium 
chlorid  and  sodium  carbonate  or  phosphate,  all  of  which,  when  in 
excess,  are  at  once  eliminated  by  the  kidneys.  The  blood,  therefore, 
becomes  poorer  in  sodium  chlorid,  one  of  its  necessary  constituents. 

Potassium  phosphate  and  carbonate  are  also  essential  to  the  nor- 
mal composition  of  the  solids  and  fluids.  They  impart  a  certain 
degree  of  alkalinity  to  the  blood  and  lymph,  one  of  the  conditions 
necessary  to  the  life  and  activity  of  the  tissue-cells  bathed  by  them. 
When  administered  in  small  doses,  they  increase  the  force  of  the 
heart,  raise  the  arterial  pressure,  and  increase  the  activity  of  the 
circulation. 

Calcium  phosphate  and  carbonate  are  partly  utilized  in  maintain- 
ing the  solidity  of  the  bones  and  teeth,  replacing  the  amount  metab- 
olized daily.  Inasmuch  as  the  metabolism  of  these  two  tissues  is 
slight,  there  is  not  much  need  in  the  adult  for  lime  as  an  article  of 
food.  In  young  animals  lime  is  essential  to  the  solidification  and 
development  of  bone.     When  deprived  of  it,  the  skeleton  undergoes 


FOODS.  133 

a  defective  development  similar  to  the  pathologic  condition  known 
as  rickets.  Lime  is  present  in  milk  to  the  extent  of  0.15  per  cent.,  as 
well  as  in  eggs  and  peas  in  relatively  large  quantities. 

Iron  is  contained  in  both  animal  and  vegetable  foods,  not,  how- 
ever, in  the  form  of  inorganic  iron,  nor  in  the  form  of  an  organic  salt, 
but  as  a  compound  with  nuclein,  thus  forming  an  integral  part  of  the 
proteid  molecule.  After  absorption  the  iron  is  utilized  in  the  forma- 
tion of  the  coloring-matter  of  the  blood-corpuscles — hemoglobin. 
The  organic  compounds  of  iron  and  the  nucleins  have  been  termed 
hematogens.  The  amount  of  iron  ingested  has  been  estimated  at 
from  10  to  90  milligrams,  the  larger  part  of  which  is  eliminated  in 
the  feces.  The  relatively  small  part  eliminated  by  the  kidneys  and 
liver  is  usually  taken  as  the  amount  metabolized,  though  it  is  probable 
that  this  is  not  wholly  true,  as  there  is  evidence  that  iron  can  be  re- 
tained in  the  body  and  utilized  again  in  the  formation  of  newr  hemo- 
globin. Contrary  to  what  might  be  expected,  milk  contains  but  a 
very  small  quantity  of  iron,  not  more  than  3  or  4  milligrams  in  1000 
grams  (human  milk) — an  amount  insufficient  for  the  development  of 
the  necessary  hemoglobin.  This  is  compensated  for,  however,  by 
the  accumulation  of  iron  in  the  liver  during  intrauterine  life.  Ac- 
cording to  Bunge,  the  liver  of  a  newly  born  rabbit  contains  as  much 
as  18.2  milligrams  per  100  grams  of  body-weight,  while  at  the  end 
of  twenty-four  days  it  only  contains  3.2  milligrams  per  100  grams  of 
body-weight. 

Vegetable  acids  increase  the  secretions  of  the  alimentary  canal,  and 
are  apt,  in  large  amounts,  to  produce  flatulence  and  diarrhea.  After 
entering  into  combination  with  bases  to  form  salts,  they  stimulate  the 
action  of  the  kidneys  and  promote  a  greater  elimination  of  all  the 
urinary  constituents.  In  some  unknown  way  they  influence  nutrition; 
when  deprived  of  these  acids,  the  individual  becomes  scorbutic. 

The  accessory  foods — coffee,  tea,  and  cocoa — when  taken  in 
moderation  have  a  stimulating  influence  on  the  nervous  system,  as 
shown  by  the  removal  of  both  mental  and  physical  fatigue,  by  an 
increased  capacity  for  sustained  mental  work,  by  the  persistent  wake- 
fulness among  those  unaccustomed  to  their  use.  Coffee  more  especially 
increases  the  frequency  and  force  of  the  heart-beat,  raises  the  arterial 
pressure,  and  hastens  the  general  blood-flow.  It  has  no  influence 
either  in  the  way  of  increasing  or  decreasing  proteid  metabolism. 

Tea  frequently  acts  as  an  astringent  on  the  alimentary  canal  on 
account  of  the  tannin  which  passes  into  the  water  when  the  infusion 
is  made.  Inasmuch  as  tannin  also  coagulates  peptones,  the  excessive 
use  of  tea  as  a  beverage  is  apt  to  derange  the  digestive  organs  and 
the  general  process  of  digestion. 

Cocoa  is  more  nutritive  than  either  coffee  or  tea,  on  account  of  the 
large  amount  of  fat  and  proteid  it  contains.  It  is,  however,  less 
stimulating. 

The  active  principles  in  coffee,  tea,  and  cocoa,  and  to  which  their 


i34  TEXT-BOOK  OF  PHYSIOLOGY. 

effects  are  to  be  attributed,  are  caffein,  thein,  and  theobromin  respec- 
tively. These  alkaloids  are  chemically  closely  related  one  to  the 
other  and  to  the  compound  xanthin.  They  are  present  in  the  coffee 
seeds,  the  tea  leaves,  and  the  cocoa  bean  to  the  extent  of  1.7  per  cent., 
1.4  per  cent.,  and  1.6  per  cent,  respectively.  When  prepared  as  a 
beverage,  however,  there  is  three  times  as  much  caffein  in  coffee  as  thein 
in  tea. 

Alcohol  when  taken  in  small  quantities  stimulates  the  digestive 
glands  to  increased  activity  and  thus  promotes  digestive  power.  Its 
absorption  into  the  blood  is  followed  by  increased  action  of  the  heart, 
dilatation  of  the  cutaneous  blood-vessels,  a  sensation  of  warmth, 
and  an  excitation  of  the  brain.  In  large  quantities  it  acts  as  a  paraly- 
zant, depressing  more  especially  the  vaso-constrictor  nerve-centers  and 
certain  areas  of  the  brain,  as  shown  by  an  impairment  in  the  power  of 
sustained  attention,  clearness  of  judgment,  and  muscle  coordination. 

Alcohol  is  undoubtedly  oxidized  in  the  body,  as  only  about  2 
per  cent,  can  be  obtained  from  the  urine  and  expired  air.  It  thus 
contributes  to  the  store  of  the  body-energy.  As  to  whether  for  this 
reason  it  can  be  regarded  as  a  food — that  is,  whether  it  can  be  sub- 
stituted in  part  at  least  for  fat  or  carbohydrate  material  without  im- 
pairing the  proteid  metabolism — is  at  present  a  subject  of  experimen- 
tation and  discussion.  According  to  some  investigators,  alcohol  does 
not  retard  proteid  metabolism,  for  when  it  is  introduced  into  the  body 
in  amounts  equivalent  to  the  carbohydrates  withdrawn  from  the  food 
there  is  at  once  a  rise  in  the  amount  of  nitrogen  excreted.  Hence  it 
cannot  be  regarded  as  a  food.  According  to  other  investigators, 
alcohol  retards  or  protects  proteid  metabolism  just  as  effectually  as 
an  equivalent  amount  of  starch  or  sugar.  Many  more  experiments 
are  required  to  decide  this  question.  When  taken  habitually  in  large 
quantities,  alcohol  deranges  the  activities  of  the  digestive  organs, 
lowers  the  body  temperature,  impairs  muscle  power,  lessens  the 
resistance  to  depressing  external  conditions,  diminishes  the  capacity 
for  sustained  mental  work,  and  leads  to  the  development  of  structural 
changes  in  the  connective  tissues  of  the  brain,  spinal  cord,  and  other 
organs.  In  zymotic  diseases  and  in  cases  of  depression  of  the  vital 
powers  it  is  most  useful  as  a  restorative  agent. 

THE  ENERGY  OR  HEAT  VALUE  OF  FOOD  PRINCIPLES. 

The  food  consumed  not  only  restores  the  material  metabolized 
and  discharged  from  the  body,  but  also  the  energy  which  has  been 
expended  as  heat  and  mechanic  motion.  The  food  principles  are 
products  of  the  constructive  processes  taking  place  in  the  vegetable 
world  during  the  period  of  growth  and  activity.  At  the  time  of  their 
formation  there  is  an  absorption  and  storing  of  the  sun's  energy  which 
then  exists  in  a  potential  condition.  During  the  metabolism  of  the 
animal  body  these  compounds  are  reduced  through  oxidation  to  rela- 


FOODS.  135 

tively  simple  bodies,  such  as  carbon  dioxid,  water,  urea,  etc.,  with  the 
liberation  of  their  contained  energy.  All  of  the  energy  of  the  body, 
whatever  its  manifestations  may  be,  can  be  traced  to  chemic  changes 
going  on  in  the  tissues,  and  more  particularly  to  those  changes  in- 
volved in  the  oxidation  of  the  food  principles. 

The  amount  of  heat  or  energy  which  any  given  food  principle  will 
yield  can  be  determined  by  burning  a  definite  amount  (e.  g.,  1  gram) 
to  carbon  dioxid  and  water  and  ascertaining  the  extent  to  which  the 
heat  thus  liberated  will  raise  the  temperature  of  a  given  amount  of 
water  (e.  g.,  1  kilogram).  The  amount  of  heat  may  be  expressed  in 
gram  or  kilogram  degrees  or  calories,  a  gram  calorie  or  kilogram 
calorie  being  the  amount  of  heat  required  to  raise  the  temperature  of 
a  gram  or  a  kilogram  (1000  grams)  of  water  i°  C.  The  apparatus 
employed  for  this  purpose  is  termed  a  calorimeter,  and  consists  es- 
sentially of  a  closed  chamber  in  which  the  oxidation  takes  place, 
surrounded  by  a  water  jacket,  the  rise  in  temperature  of  the  water 
indicating  the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calorim- 
eters and  different  food  principles  of  the  same  group  vary,  though 
within  certain  limits:  e.  g.,  1  gram  of  casein  yields  5.867  kilogram 
calories;  1  gram  of  lean  beef,  5.656  calories;  1  gram  of  fat  yields 
9.353,  9.423,  9.686  calories;  1  gram  of  carbohydrate,  4.182,  4.479, 
etc.,  calories.  These  numbers  represent  the  physical  heat  values  of 
these  food  principles. 

In  the  human  body  as  determined  by  calorimetric  methods  the 
oxidation  of  the  food  principles  yields  practically  the  same  amount 
of  heat  they  yield  when  oxidized  outside  the  body,  with  the  excep- 
tion of  the  proteins,  which  are  oxidized  only  to  the  stage  of  urea.  As 
this  compound  is  capable  of  further  reduction  in  the  calorimeter  to 
carbon  dioxid  and  water  with  the  liberation  of  heat,  the  quantity  of 
heat  it  contains  must  therefore  be  deducted  from  the  calorimetric 
heat  value  of  the  protein.  According  to  Rubner,  1  gram  of  urea  will 
yield  2.523  kilogram  calories.  As  the  urea  which  results  from  the 
oxidation  of  1  gram  of  protein  is  about  -J  of  a  gram,  the  amount  of 
heat  to  be  deducted  from  the  heat  value  of  the  protein  is  ^  of  2.523,  or 
0.841  calories.  It  has  also  been  shown  that  some  of  the  ingested 
protein  escapes  in  the  feces,  the  heat  value  of  which  must  also  be 
determined  and  deducted.  This  having  been  done,  the  physiologic 
heat  value  becomes  4.124  calories. 

The  following  estimates  give  approximately  the  number  of  kilo- 
gram calories  produced  when  the  food  is  burned  to  carbon  dioxid, 
water,  and  urea  in  the  body: 

1  gram  of  protein  yields,, 4-124  calories. 

1  "fat  "        9.353 

1      "      "  carbohydrate  yields, 4. 116         " 

The  total  number  of  kilogram  calories  or  kilogram  degrees  of  heat 
yielded  by  any  of  the  previously  given  diet  scales  can  be  readily  deter- 


136  TEXT-BOOK  OF  PHYSIOLOGY. 

mined  by  multiplying  the  quantities  of  food  principles  consumed  by 
the  above-mentioned  factors.  The  diet  scale  of  Vierordt,  for  example, 
yields  the  following: 

120  grams  of  protein  yields, 494.88  calories. 

90       "       "fat  "        841.77         " 

330       "      "  starch       "        1358.28         " 

2694.93 

The  total  calories  obtained  from  other  diet  scales  would  be  as  follows : 
Ranke,  2335;  Voit,  3387;  Moleschott,  2984;  Atwater,  3331;  Hultgren, 

3436- 

Starvation. — The  relation  of  the  .different  food  principles  to  the 
general  nutritive  process  becomes  more  apparent  from  an  examination 
of  the  excretions  from  the  body  during  the  process  of  starvation  com- 
bined with  an  examination  of  the  organs  and  tissues  after  death.  If 
an  animal  be  deprived  entirely  of  food,  a  decline  in  body-weight  at 
once  sets  in,  which  continues  until  about  40  per  cent,  of  the  weight 
has  been  lost,  when  death  generally  ensues.  This  results  from  the 
fact  that  the  active  tissue  cells  consume,  for  the  purpose  of  maintain- 
ing the  normal  temperature  of  the  body,  not  only  their  own  reserve 
food  material,  but  that  of  the  less  active  or  storage  tissues  as  well;  and, 
in  consequence,  there  is  a  progressive  diminution  in  weight. 

The  phenomena  which  characterize  this  non-physiologic  con- 
dition are  as  follows:  hunger,  intense  thirst,  gastric  and  intestinal 
uneasiness  and  pain,  diminished  pulse-rate,  and  respiration,  muscular 
weakness  and  emaciation,  a  lessening  in  the  amount  of  urine  and  its 
constituents,  diminished  exhalation  of  carbon  dioxid,  an  exhalation 
of  a  fetid  odor  from  the  body,  vertigo,  stupor,  delirium,  at  times  con- 
vulsions, a  sudden  fall  in  body-temperature,  and  finally  death.  The 
duration  of  life  after  complete  deprivation  of  food  varies  from  eight  to 
thirteen  days  or  more,  though  this  period  can  be  prolonged  if  the  ani- 
mal be  supplied  with  water,  this  being  more  essential  under  the  cir- 
cumstances than  the  organic  materials  which  can  be  supplied  by  the 
organism  itself.  The  duration  of  the  starvation  period  will  vary  in 
accordance  with  the  previous  condition  of  the  animal  and  the  amount 
of  reserved  food  the  body  contains.  The  excretion  of  urea  declines 
very  rapidly  during  the  first  two  days — a  fact  which  has  been  attrib- 
uted to  a  rapid  consumation  of  the  surplus  proteid  food.  After  this 
period,  when  the  tissues  begin  to  metabolize  their  own  proteid,  the 
excretion  remains  fairly  constant  until  toward  the  close,  when  the 
am6unt  eliminated  falls  very  rapidly.  As  proteids  contain  about  16 
per  cent,  of  nitrogen,  1  part  of  nitrogen  equals  6.25  parts  of  protein. 
Hence,  for  every  1  gram  of  nitrogen  or  2.14  grams  urea  excreted, 
it  may  be  assumed  that  6.25  grams  of  protein  or,  according  to  Voit,  30 
grams  of  flesh  have  been  metabolized.  The  daily  excretion  of  urea, 
therefore,  indicates  the  extent  of  the  protein  metabolism. 

It  has  been  observed  also  that  there  is  a  steady  diminution  in  the 


FOODS. 


137 


excretion  of  carbon  dioxid,  though  this  is  greatest  in  the  last  few  days. 
As  fat  contains  about  76  per  cent,  of  carbon,  1  part  of  carbon  equals  1.3 1 
parts  of  fat.  Hence,  for  every  1  gram  of  carbon  or  3.66  grams  carbon 
dioxid  excreted  it  may  be  assumed  that  1.3 1  grams  of  fat  have  been 
metabolized.  The  daily  excretion  of  carbon,  therefore,  indicates  the 
extent  of  fat  metabolism.  The  carbohydrates  are  here  left  out  of  con- 
sideration, as  they  constitute  only  about  1  per  cent,  of  the  body-weight. 
It  must  be  borne  in  mind,  however,  that  in  the  metabolism  of  protein  a 
certain  quantity  of  fat  is  produced  which  also  undergoes  oxidation. 
The  amount  of  the  carbon  or  the  fat  that  the  protein  would  give  rise  to, 
as  previously  determined,  must  therefore  be  subtracted  from  that  elim- 
inated by  the  lungs,  etc.,  in  order  to  determine  the  amount  of  body- 
fat  metabolized.  Observations  of  human  beings  in  the  fasting  con- 
dition show  that  for  a  period  of  ten  days  there  is  a  daily  excretion  of 
about  21  grams  of  urea,  equivalent  to  about  70  grams  of  protein.  This 
amount,  however,  may  be  reduced  to  from  50  to  60  per  cent,  if  the  in- 
dividual has  a  surplus  of  body-fat. .  Human  beings  under  similar 
circumstances  may  lose  during  the  first  few  days  200  grams  of  fat 
daily. 

The  following  table  shows  the  excretion  of  nitrogen  and  carbon  and 
the  calculated  amounts  of  protein  and  fat  metabolized  from  an  experi- 
ment made  by  Ranke  on  himself  during  a  fast  of  twenty-hours,  be- 
ginning; twentv-four  hours  after  the  last  meal: 


Disintegration  of  Tissue. 
(Calculated.) 

Expenditure. 

Nitrogen. 

Carbon. 

Nitrogen. 

Carbon. 

7.8 
O.O 

26.5 

157-5 

7.2 
0.0 

Protein,  50  gm., 

Fat,  199.6  gm., 

Uric  acid,  0.2  gm.,  . . .  J 
Carbon  dioxid,   

3-4 
180.6 

7.8 

184.0 

7.2 

184.0 

Coincidently  with  these  losses  to  the  body  there  is  also  a  gradual 
loss  of  inorganic  salts,  and  toward  the  termination  of  the  period  a 
sudden  fall  in  temperature  of  several  degrees  centigrade,  in  conse- 
quence of  the  final  consumption  of  all  available  foods,  when  death 
ensues,  in  all  probability,  from  a  cessation  in  the  action  of  the  heart. 

Post-mortem  Appearances. — It  has  been  experimentally  determined 
that  animals  die  when  the  body-weight  has  declined  to  about  40  per 
cent.  Post-mortem  examination  shows  that  the  loss  of  material, 
though  very  generally  distributed  throughout  the  body,  is  greatest 
in  organs  and  tissues  least  essential  to  life. 

The  results  of  an  analysis  of  the  organs  and  tissues  of  a  cat  after 
a  thirteen-day  period  of  starvation,  during  which  the  animal  lost  1017 


138 


TEXT-BOOK  OF  PHYSIOLOGY. 


grams  in  weight,  are  given  in  the  following  table,  based  on  data  fur- 
nished by  Voit: 


Organ. 


Adipose  tissue, . 

Spleen, 

Liver,    

Testes. 

Muscles, 

Blood, 

Kidneys, 

Skin  and  hair, 

Lungs, 

Intestines, 

Pancreas, 

Bones, 

Heart, 

Nervous  system 


Percentage. 

Actual  Loss 
of  Tissue. 

Grams. 

97 

267 

67 

6 

54 

49 

40 

1 

3i 

429 

27 

37 

26 

7 

21 

S9 

18 

3 

18 

21 

17 

1 

14 

55 

3 

0 

3 

1 

It  will  be  observed  from  this  table  that  the  adipose  tissue  suffers 
the  greatest  loss,  the  entire  amount  disappearing  with  the  exception 
of  a  small  portion  in  the  posterior  part  of  the  orbital  cavity  and  around 
the  kidneys.  The  muscles,  though  only  losing  31  per  cent,  of  their 
weight,  yet  furnish  429  grams  of  presumably  proteid  material,  for 
nutritive  purposes.  The  heart  and  nervous  system  experience  but 
slight  loss. 

Mixed  Diet. — The  chemic  composition  of  the  tissues,  taken  in 
connection  with  their  metabolism  during  starvation,  implies  that  no 
one  article  of  food  is  sufficient  for  tissue  repair  and  heat  production; 
but  that  all  classes  of  foods — in  other  words,  a  mixed  diet — are  essential 
to  the  maintenance  of  a  normal  nutrition.  Experimental  investiga- 
tion has  also  conclusively  established  this  fact.  Moreover,  the  amounts 
of  nitrogen  and  carbon  eliminated  daily,  and  the  ratio  existing  between 
them,  indicate  the  amounts  of  proteid,  fat,  and  carbohydrate  which 
are  required  to  cover  the  loss. 

Metabolism  on  a  Purely  Protein  Diet. — Notwithstanding 
the  chemic  composition  of  the  proteins  and  the  possibility  of  their 
giving  rise  to  both  fat  and  a  carbohydrate  during  their  metabolism 
it  has  been  found  extremely  difficult  to  maintain  the  normal  nutrition 
for  any  length  of  time  on  a  pure  protein  or  fat-free  flesh  diet.  This, 
however,  has  been  accomplished  with  dogs.  It  was  found,  however, 
that,  in  order  to  maintain  the  equilibrium,  it  was  necessary  to  increase 
the  proteins  from  two  to  three  times  the  usual  amount.  Thus,  a  dog 
weighing  30  to  35  kilograms  required  from  1500  to  1800  grams  of 
flesh  daily  in  order  to  get  the  requisite  amount  of  carbon  to  prevent 
consumption  of  its  own  adipose  tissue.  Under  similar  circumstances, 
a  human  being  weighing  70  kilograms  would  require  more  than  2000 
grams  of  lean  beef — an  amount  which,  from  the  nature  of  the  digestive 


FOODS. 


!39 


apparatus,  it  would  be  practically  impossible  to  digest  and  assimilate 
for  any  length  of  time.  Even  the  slight  habitual  excess  beyond  the 
amount  normally  required  is  imperfectly  assimilated  and  gives  rise 
to  the  production  of  nitrogen-holding  compounds  which,  on  account 
of  the  difficulty  with  which  they  are  eliminated  by  the  kidneys,  ac- 
cumulate within  the  body  and  develop  the  gouty  diathesis,  with  all 
its  protean  manifestations. 

Metabolism  on  a  Fat  and  Carbohydrate  Diet. — As  nitrogen 
is  an  indispensable  constituent  of  the  tissues,  it  is  evident  that  neither 
fat  nor  carbohydrates  can  maintain  nutritive  equilibrium  except  for 
very  short  periods.  On  such  a  diet  the  tissues  consume  their  own 
proteids,  as  shown  by  the  continuous  excretion  of  urea,  though  the 
amount  is  less  than  during  starvation.  An  excess  of  fat  retards  the 
metabolism  of  proteids.     The  same  holds  true  for  the  carbohydrates. 

Thus,  in  any  well-arranged  dietary  there  should  be  a  combina- 
tion of  proteins,  fats,  and  carbohydrates  in  amounts  sufficient  to  main- 
tain nutritive  equilibrium;  in  other  words,  to  repair  the  loss  of  tissue 
and  to  furnish  the  requisite  amount  of  heat  in  accordance  with  work 
done,  as  well  as  with  climatic  and  seasonal  variations. 

COMPOSITION  OF  FOODS. 

The  food  principles  essential  to  the  maintenance  of  the  nutrition 
of  the  body  are  contained  in  varying  proportions  in  compound  sub- 
stances termed  foods;  e.  g.,  meat,  milk,  wheat,  potatoes,  etc.  Their 
nutritive  value  depends  partly  on  the  amounts  of  their  contained  food 
principles  and  partly  on  their  digestibility.  The  dietary  of  civilized 
man  embraces  foods  derived  from  both  the  animal  and  vegetable 
worlds. 

Composition  of  Animal  Foods. — The  following  table  shows  the 
average  percentage  composition  of  various  kinds  of  meats,  cow's 
milk,  and  eggs: 


In  ioo  Parts 


Beef. 


Water, [  76.25 

Protein, 20.24 

Fat, 1. 68 

Carbohydrates, 0.50 

Salts, 1.38 


Veal. 


Mut- 
ton. 


77.S2 

19.86 

0.82 

0.80 

0.70 


75-59 
17. 11 

5-47 
0.60 

i-23 


Pork.     Fowl.      Fish. 


72-57 

I9-3I 

5.82 

0.60 

1.70 


70.80 

22.70 

4.10 

1.20 

1.20 


Cow's 

Milk. 


79-3° 

S6.S7 

18.30 

4-75 

0.70 

3-5° 

0.90 

4.00 

0.80 

0.17 

Eggs. 


73.67 

12.55 
12. 11 

°-55 
i-i3 


Meats. — It  will  be  observed  from  these  analyses  that  the  meats 
contain  from  18  to  20  per  cent,  of  a  protein  which  belongs  in  virtue 
of  its  chemic  relations  to  the  group  of  globulins.  In  the  living  con- 
dition this  body,  known  as  myosinogen,  is  in  a  semifluid  condition, 
but  shortly  after  death  undergoes  coagulation,  giving  rise  to  solid 
myosin  and  a  soluble  albumin.     There  are  also  present  in  meat  small 


140  TEXT-BOOK  OF  PHYSIOLOGY. 

percentages  of  other  forms  of  protein;  e.  g.,  myoalbumin,  myoglob- 
ulin,  paramyosinogen,  etc.  After  being  subjected  to  the  cooking 
process,  meats  contain  the  albuminoid  body  gelatin,  a  product  of  the 
transformation  of  the  proteids  of  the  connective  tissue. 

The  percentage  of  fat,  contained  within  the  meat  substance,  is 
very  small  except  in  mutton  and  pork,  where  it  rises  to  5.4  per  cent, 
and  5.8  per  cent,  respectively.  The  fat-globules  in  these  meats  are 
packed  closely  between  the  muscle-fibers,  and  prevent  the  easy  entrance 
of  the  digestive  fluids,  and  hence  they  are  more  difficult  of  digestion 
than  beef. 

The  carbohydrates  vary  from  0.5  to  1  per  cent.,  and  are  represented 
by  glycogen.  The  principal  inorganic  salts  are  potassium  phosphate 
and  sodium  chlorid. 

Cooking,  when  properly  done,  not  only  makes  the  meat  more 
palatable  and  appetizing  from  the  development  of  agreeable  flavors, 
but  converts  the  connective  tissue,  which,  in  old  animals  especially, 
is  tough  and  resisting,  into  gelatin,  thus  rendering  it  more  easy  of 
mastication  and  digestion.  At  the  same  time  parasitic  organisms, 
such  as  the  embyronic  forms  of  tenia  or  tapeworm,  trichina  spiralis, 
as  well  as  bacterial  growths,  which  frequently  infest  the  bodies  of  an- 
imals, are  destroyed  and  made  harmless. 

Milk  is  the  natural  food  of  the  young  of  all  mammals,  and  is  usu- 
ally regarded  as  typical  on  account  of  the  ratio  existing  among  its  nu- 
tritive principles.  The  analysis  given  above  is  that  of  cow's  milk. 
Examined  microscopically,  milk  is  seen  to  consist  of  a  clear  fluid,  the 
milk  plasma,  holding  in  suspension  an  enormous  number  of  small, 
highly  refractive  oil- globules,  which  measure  on  the  average  about  TT}F¥ 
of  an  inch  in  diameter.  Each  globule  is  supposed  by  some  observers 
to  be  surrounded  by  a  thin  albuminous  envelope,  which  enables  it  to 
maintain  the  discrete  form.  Others  deny  the  existence  of  such  a  mem- 
brane. The  chief  protein  constituent  of  milk,  caseinogen,  is  held  in 
solution  by  the  presence  of  phosphate  of  lime.  On  the  addition  of 
acetic  acid  or  sodium  chlorid  up  to  the  point  of  saturation  the  casein- 
ogen is  precipitated  as  such  and  may  be  collected  by  appropriate  chemic 
methods.  When  taken  into  the  stomach,  caseinogen  is  coagulated; 
that  is,  it  is  separated  into  casein  or  tyrein  and  a  small  quantity  of  a 
new  soluble  proteid.  This  change  is  brought  about  by  the  presence  in 
the  gastric  juice  of  a  special  ferment  known  as  rennin  or  pexin. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures. 
It  is  a  combination  of  olein,  palmitin,  and  stearin,  with  a  small  quan- 
tity of  butyrin  and  caproin.  When  milk  is  allowed  to  stand  for  some 
time,  the  fat-globules  rise  to  the  surface  and  form  a  thick  layer  known 
as  cream.  When  subjected  to  the  churning  process,  fat-globules 
run  together  and  form  a  coherent  mass — butter. 

Lactose  is  the  particular  form  of  sugar  found  in  milk.  In  the 
presence  of  Bacillus  acidi  lactici,  the  lactose  is  decomposed  into  lactic 
acid  and  carbon  dioxid,  the  former  of  which  not  only  imparts  a  sour 


FOODS. 


141 


taste  to  the  milk,  but  causes  a  precipitation  of  the  caseinogen.  The 
chief  salt  found  in  milk  is  phosphate  of  lime,  and  this  is  the  chief  source 
of  this  agent  in  the  formation  of  bones. 

Eggs  are  also  to  be  regarded  as  complete  natural  foods,  inasmuch 
as  they  contain  all  the  necessary  food  principles.  The  analysis  given 
in  the  above  table  represents  the  composition  of  the  entire  egg.  The 
white  of  the  egg  contains  12  per  cent,  of  proteid  and  2  per  cent,  of  fat. 
The  yolk,  however,  contains  15  per  cent,  of  proteid  and  30  per  cent, 
of  fat. 

Composition  of  Cereal  Foods. — The  average  composition  of 
the  principal  cereals  is  shown  in  the  following  table : 


In  100  Parts. 


Water, 

Protein, 

Fat, 

Carbohydrate 
Cellulose,  .  .  . 
Salts, 


Wheat. 

Rye. 

Barley. 

Oats. 

Corn. 

Rice. 

I3-56 

12.65 

13-77 

12-37 

13.10 

13.12 

12-35 

12-55 

11. 14 

10.41 

9-«5 

7.88 

i-75 

I.97 

2.16 

5-23 

4-57 

0.85 

67.90 

67-95 

64-93 

57-78 

68.42 

76-55 

2.63 

3.00 

5-3i 

II. 19 

2-5° 

°-55 

1.81 

1.88 

2.69 

3.02 

1.56 

1.05 

Buck- 
wheat. 


12.62 

10.02 

2.24 

64-43 
8.67 
2.02 


That  the  cereals  are  most  important  and  useful  articles  of  diet  is 
evident  from  their  composition,  consisting,  as  they  do,  of  proteins 
and  carbohydrates  in  large  proportion.  Owing  to  the  cellulose  or 
woody  fiber  which  envelops  and  penetrates  the  grain,  they  are  some- 
what difficult  of  digestion.  A  section  of  a  grain  of  wheat  shows  the 
external  cellulose  envelope,  the  husk,  beneath  which  is  a  layer  of  large 
cells  containing  the  chief  protein — the  gluten.  The  interior  of  the 
grain  consists  of  small  cavities,  the  walls  of  which  are  formed  of  cellu- 
lose and  which  contain  the  granules  of  starch,  fat,  small  quantities  of 
proteid,  and  inorganic  salts.     All  other  cereals  have  a  similar  structure. 

In  the  preparation  of  white  flour  from  wheat  it  is  customary  to 
remove  the  husk,  a  process  which  involves  the  removal  also  of  a  por- 
tion, if  not  all,  of  the  gluten  cells,  so  that  such  flour  contains  less  nitrog- 
enized  material  than  the  original  grain.  It  is  possible,  however,  in 
the  milling  of  wheat,  to  remove  only  the  husk  and  retain  the  gluten  in 
the  flour,  as  in  the  preparation  of  whole  wheat  flour. 

Bread  is  an  artificially  prepared  food  made  either  of  wheat  or 
rye.  Owing  to  the  fact  that  the  proteids  of  the  other  cereals  do  not 
possess  the  same  adhesive  properties  when  kneaded  with  water,  they 
can  not  be  used  for  bread-making  purposes.  In  the  making  of  bread, 
the  flour  is  kneaded  with  water  until  a  glutinous  mass — dough — is 
formed.  During  this  process,  salt,  sugar,  and  yeast  are  added.  It 
is  then  placed  in  a  temperature  of  about  ioo°  F.  In  the  presence  of 
heat  and  moisture  the  natural  ferment  of  the  flour — diastase — con- 
verts a  portion  of  the  starch  into  sugar,  which  in  turn  is  split  up  into 


142 


TEXT-BOOK  OF  PHYSIOLOGY. 


carbon  dioxid  and  alcohol  by  the  yeast  plant.  The  bubbles  of  carbon 
dioxid,  becoming  entangled  in  the  dough,  cause  it  to  swell  or  rise  and 
subsequently  give  the  porous  or  spongy  character  to  the  bread.  When 
baked  at  a  temperature  of  400 °  F.,  the  alcohol  is  driven  off;  yeast  cells 
and  other  organisms  are  destroyed;  the  starch,  particularly  that  on  the 
surface,  is  dextrinized.  Thus  prepared,  white  bread  consists  of  water, 
32  per  cent.;  protein,  8.8  per  cent.;  fat,  1.7  per  cent.;  carbohydrate, 
56.3  per  cent.;  salts,  0.9  per  cent.  The  principal  salts  are  potassium 
and  magnesium  phosphate.  Whole  wheat  bread  consists  of  water,  40 
per  cent.;  protein,  12.2  per  cent.;  fat,  1.2  per  cent.;  carbohydrate,  43.5 
per  cent.;  salts,  1.3  per  cent.;  cellulose,  1.8  per  cent. 

Composition  of  Vegetable  Foods.  —The  average  composition 
of  some  of  the  principal  vegetables  is  shown  in  the  following  table : 


In  100  Parts. 


Water, 

Protein, 

Fat, 

Carbohydrates, 

Cellulose, 

Salts, 


Beans, 


^3-74 

23.21 

2.14 

53-67 

3-69 

3-55 


Peas. 


14.99 

22.85 

1.79 

52-36 

5-43 
2.58 


Pota- 
toes. 


75-47 
i-95 
0.15 

20.69 
0.76 
0.98 


Beets, 


0.30 

13.00 

1.60 


Tur- 
nips. 


89.42 

1-35 
0.18 

7-36 
0.94 
0.75 


Toma- 
toes. 


96.30 
0.90 

0.50 
2.80 

0.40 


Aspa- 
ragus. 


93-75 
1.79 
0.25 
2.63 
1.04 
0-54 


Cab- 
bage. 


89.97 
1.89 
0.20 
4.87 
1.84 
1.23 


The  vegetable  foods,  as  a  class,  vary  considerably  in  nutritive 
value  and  digestibility,  the  latter  depending  on  the  amount  of  cellu- 
lose they  contain.  A  section  of  a  vegetable  shows  not  only  the  pres- 
ence of  an  external  cellulose  envelope,  but  also  an  inner  framework 
which  penetrates  its  substance  in  all  directions.  The  nutritive  prin- 
ciples are  contained  in  small  cavities,  the  walls  of  which  are  formed 
by  the  framework.  Nearly  all  vegetables  require  cooking  before 
being  eaten.  When  subjected  to  heat  and  moisture,  not  only  is  the 
texture  of  the  vegetable  softened  and  disintegrated,  but  the  starch 
grains  are  hydrated  and  partially  prepared  for  conversion  into  dextrin 
and  sugar.  At  the  same  time  various  savory  substances  are  set  free, 
which  make  the  food  more  palatable. 

Beans  and  peas  contain  large  quantities  of  a  proteid,  legumin, 
and  starch,  and  hence  are  especially  valuable  as  nutritive  foods.  The 
presence  of  the  cellulose  envelope,  especially  in  ripe  beans  and  peas, 
combined  with  rather  a  dense  texture,  renders  them  somewhat  difficult 
of  digestion.  Potatoes,  though  largely  employed  as  food,  are  ex- 
tremely poor  in  protein,  2  per  cent.,  and  carbohydrates,  20  per  cent. 
When  sufficiently  cooked  they  are  easily  digested,  owing  to  the  small 
amount  of  cellulose  they  contain. 

Green  vegetables, — e.  g.,  lettuce,  spinach,  tomatoes,  asparagus, 
onions,  etc.,  though  containing  food  principles  in  small  amounts,  are, 
nevertheless,  valuable  adjuncts  to  the  dietary,  for  the  reason  that  they 
contain  inorganic  as  well  as  organic  salts,  which  appear  to  be  necessary 


FOODS. 


M3 


to  the  maintenance  of  the  normal  nutrition.  The  want  of  green  veget- 
ables has  been  supposed  to  be  the  cause  of  scurvy. 

Ripe  fruits,  grapes,  cherries,  apples,  pears,  peaches,  strawberries, 
lemons,  oranges,  etc.,  though  consumed  largely,  possess  but  little  nutri- 
tive value.  They  consist  largely  of  water,  75  to  85  per  cent.,  proteins 
a  trace,  sugar  from  5  to  13  per  cent.,  organic  acids  (citric,  malic,  tar- 
taric), pectose,  and  various  inorganic  salts. 

Relative  Value  of  Animal  and  Vegetable  Foods. — Though 
both  animal  and  vegetable  foods  contain  the  different  classes  of  food 
principles,  it  is  not  a  matter  of  entire  indifference  as  to  which  are  con- 
sumed. It  has  been  found  by  experiment  that  animal  proteids  are 
more  easily  and  completely  digested  and  absorbed  than  vegetable 
proteids;  that  cellulose  is  not  only  highly  indigestible,  but  by  its  pres- 
ence in  large  quantities  retards  the  digestive  process  and  impairs  the 
activity  of  the  entire  digestive  mechanism,  though  in  moderate  quan- 
tity it  undoubtedly  aids  digeston  indirectly  by  mechanically  promot- 
ing peristalsis.  The  following  table  shows  the  relative  digestibility 
of  the  two  classes  of  foods: 


Vegetable. 

Animal. 

Digested. 

Undigested. 

Digested. 

Undigested. 

Of  100  parts  of  solids,    

Of  100  parts  of  protein, 

Of  100  parts  of  fats  or  carbohy- 
drates  

75-5 
46.6 

9°-3 

24-5 
53-4 

9-7 

89.9 
81.2 

96.9 

II. 1 
18.S 

3-1 

Construction  of  Dietaries. — Inasmuch  as  neither  animal  nor 
vegetable  foods  contain  the  food  principles  in  proper  quantities  and 
proportions,  the  instinctive  choice  of  mankind  has  led  to  a  combina- 
tion of  the  two  classes  of  foods.  From  the  analyses  tabulated  above 
it  becomes  comparatively  easy  to  construct  a  suitable  dietary,  com- 
posed of  different  articles  of  food,  in  which  the  food  principles  shall 
bear  the  proper  ratio  one  to  the  other — a  ratio  based  on  the  total  quan- 
tity of  nitrogen  (15  to  20  grams)  and  carbon  (225  to  300  grams)  elim- 
inated from  the  body  daily. 

It  is  only  necessary,  therefore,  to  combine  two  or  more  foods,  the 
composition  of  which  is  known,  in  quantities  sufficient  to  furnish 
the  requisite  amount  of  nitrogen  and  carbon,  or  their  equivalents  in 
proteid,  fat,  and  carbohydrates.  As  illustrations  of  such  combina- 
tions the  following  examples  are  given: 


Foods. 

Meat, 250  gm.,    8.8  oz. 

Bread, 400  gm.,  14.2  oz. 

Fat, 100  gm.,    3.5  oz. 

Sugar, 70  gm.,     2.5  oz. 


Food  Principles. 
Protein, 100  gm. 

Fat, 100  gm. 

Carbohydrates, 2^0  gm. 


N.       C. 


144  TEXT-BOOK  OF  PHYSIOLOGY. 

Foods.                                                                                   N.  C. 

Meat, 225  gm.,  %  lb.              7.5  gm.  34  gm. 

Bread 450  gm.,   1  lb.              5.5  gm.  117  gm. 

Fats, 113  gm.,  J  lb.                  ...  84  gm. 

Potatoes, 450  gm.,   1  lb.              1.3  gm.  45  gm. 

Milk, 225  gm.,  J  pint           1.7  gm.  20  gm. 

Eggs,    113  gm.,  1  lb.             2.0  gm.  15  gm. 

Cheese,    56  gm.,  J  lb.              3.0  gm.  20  gm. 

2i-o  335 

Waller. 
DAILY  RATION  OF  THE  UNITED  STATES  SOLDIER. 

Fresh  beef, 20  oz. 

or  pork, 12  oz. 

or  bacon, 12  oz. 

Flour, 18  oz. 

or  soft  bread, 18  oz. 

or  hard  bread, 16  oz. 

Potatoes, 16  oz. 

or  potatoes  ni  and  tomatoes  45 16  oz. 

Beans  or  peas, 2.4  oz. 

Rice  or  hominy, 1.6  oz. 

Coffee, 1 . 6  oz. 

Sugar, 2.00  oz. 

Vinegar, 0.32  gill 

Salt,    0.60  gill 


CHAPTER    X. 
DIGESTION. 

Foods  are  heterogeneous  compounds  consisting  of  organic  and 
inorganic  nutritive  principles  associated  with  a  varying  amount  of 
non-nutritive  material,  such  as  the  dense  parts  of  the  connective  tissue 
of  the  animal  foods  and  the  woody  fiber  or  cellulose  of  the  vegetable 
foods.  Before  the  nutritive  principles  can  be  utilized  they  must  be 
dissociated  from  the  non-nutritive  material.  Even  when  consumed 
in  the  free  state,  the  food  principles  are  seldom  in  a  condition  to  be 
absorbed  into  the  blood  and  assimilated  by  the  tissues.  When  foods 
are  consumed  in  their  natural  state  or  after  they  have  been  subjected 
to  the  cooking  process,  they  are  subjected  while  in  the  food  canal  to  the 
solvent  action  of  various  fluids  by  which  they  are  disintegrated  and 
reduced  to  the  liquid  condition.  The  nutritive  principles  freed  from 
their  combinations  are  changed  in  chemic  composition  and  transformed 
into  substances  capable  of  absorption.  To  all  the  physical  and  chemic 
changes  which  foods  undergo  in  the  food  canal  the  term  digestion  has 
been  given. 

The  digestive  apparatus  comprises  the  entire  alimentary  or  food 
canal  and  its  various  appendages:  the  teeth,  the  tongue,  the  mouth, 
the  gastric  and  intestinal  glands,  the  pancreas,  and  the  liver  (Fig.  63). 

The  canal  itself  is  a  musculo-membranous  tube  about  thirty-two 
feet  in  length,  and  extends  from  the  mouth  to  the  anus.  It  may  be 
subdivided  into  several  distinct  portions,  as  mouth,  pharynx,  esoph- 
agus, stomach,  small  and  large  intestines.  The  mouth  is  provided 
(1)  with  teeth,  by  which  the  food  is  divided,  (2)  with  the  tongue,  and 
(3)  with  glands,  by  which  a  solvent  fluid,  the  saliva,  is  secreted.  The 
glands,  though  situated  for  the  most  part  outside  the  mouth,  are  con- 
nected with  it  by  means  of  ducts.  Posteriorly  the  mouth  opens  into 
the  pharynx  or  throat,  a  somewhat  pyramidal-shaped  structure  about 
five  inches  in  length,  which  in  turn  is  followed  bv  the  esophagus  or 
gullet,  a  tube  about  nine  inches  in  length.  As  the  esophagus  passes 
through  the  diaphragm  it  expands  into  the  stomach,  a  curved  pyri- 
form  organ,  which  serves  as  a  reservoir  for  the  reception  and  retention 
of  the  food  for  a  varying  length  of  time.  The  small  intestine  is  that 
portion  of  the  alimentary  canal  extending  from  the  end  of  the  stomach 
to  the  beginning  of  the  large  intestine;  owing  to  its  length,  about  twenty- 
two  feet,  it  presents  a  very  convoluted  appearance  in  the  abdominal 
cavity.  Embedded  in  its  walls  arc  the  intestinal  glands  which  open  on 
its  surface  and  secrete  the  intestinal  fluid.     In  the  upper  portion  of 

10  M5 


146 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  small  intestine,  within  five  inches  of  the  stomach,  there  are  generally 
two  orifices,  the  terminations  of  the  ducts  of  the  liver  and  pancreas, 
organs  which  secrete  the  bile  and  pancreatic  juice  respectively.  The 
large  intestine  is  from  five  to  six  feet  in  length  and  extends  from  the 
end  of  the  small  intestine  to  the  anus.  Its  walls  contain  a  large  num- 
ber of  glands. 


■U'A I  Salivary  Gland 


Vermiform  Appendix 

Fjg.  63. — Diagram  of  the  Alimentary  Canal. — (Modified  from  Landois.) 


The  general  process  of  digestion  is  largely  accomplished  by  the 
chemic  action  of  the  digestive  fluids :  the  saliva,  the  gastric,  intestinal, 
and  pancreatic  juices,  and  the  bile.  Though  taking  place  through- 
out a  large  portion  of  the  food  canal,  the  process  may  be  subdivided 
into  several  stages:  viz.,  mouth  digestion,  gastric  digestion,  and  intesti- 
nal digestion. 


DIGESTION.  147 

As  a  result  of  the  action  of  these  fluids  the  nutritive  principles  are 
prepared  for  absorption  into  the  blood;  the  non-nutritive  principles, 
along  with  certain  waste  products,  pass  into  the  large  intestine  to  be 
finallv  extruded  from  the  bodv. 


MOUTH  DIGESTION. 

The  digestion  of  the  food  as  it  takes  place  in  the  mouth  comprises 
a  series  of  physical  and  chemic  changes,  the  result  of  the  action  of 
the  teeth,  the  tongue,  and  the  saliva.  The  mechanic  division  of  the 
food  and  the  incorporation  of  the  saliva  with  it  are  termed  respectively 
mastication  and  insalivation. 


MASTICATION. 

Mastication  is  the  mechanic  division  of  the  food,  and  is  accom- 
plished by  the  teeth  and  the  movements  of  the  lower  jaw  under  the 
influence  of  muscle  contractions.  Complete  mechanic  disintegra- 
tion of  the  food  is  essential  to  its  subsequent  solution  and  chemic 
transformation;  for  when  finely  divided  it  presents  a  larger  surface 
to  the  action  of  the  digestive  fluids  and  thus  enables  them  to  exert 
their  respective  actions  more  effectively  and  in  a  shorter  period  of 
time. 

The  Teeth. — In  man  passing  from  childhood  to  adult  life  two 
sets  of  teeth  make  their  appearance.  The  first  set  constitute  the  tem- 
porary, deciduous,  or  milk  teeth;  the  second  set  constitute  the  per- 
manent teeth,  which  should  last  with  proper  care  through  life  or  to  an 
advanced  age. 

The  temporary  teeth,  twenty  in  number,  ten  in  each  jaw,  though 
smaller  than  the  permanent  teeth,  have  the  same  general  conforma- 
tion. They  are  divided  into  four  incisors,  two  cuspids  or  canines, 
and  four  molars  for  each  jaw. 

The  permanent  teeth,  thirty-two  in  number,  sixteen  in  each  jaw, 
are  divided  into  four  incisors,  two  cuspids  or  canines,  four  bicuspids  or 
premolars,  and  six  molars  for  each  jaw. 

Each  tooth  may  be  said  to  consist  of  three  portions :  (i)  the  crown, 
or  that  portion  which  projects  above  the  gums;  (2)  the  root  or  fang, 
that  portion  embedded  in  the  alveolar  socket;  (3)  the  constricted  por- 
tion or  neck,  which  is  surrounded  by  the  free  margin  of  the  gum.  The 
teeth  are  firmly  secured  in  their  sockets  by  a  fibrous  membrane,  the 
peridental  membrane,  which  is  attached,  on  the  one  hand,  to  the  alveo- 
lar process,  and,  on  the  other,  to  the  cementum. 

A  vertical  section  of  a  tooth  shows  that  it  consists  of  three  distinct 
solid  structures,  the  enamel,  the  dentine,  and  the  cementum,  which 
have  the  anatomic  relationship  as  represented  in  Fig.  64.  In  the 
center  of  the  dentine  there  is  a  cavity  the  general  shape  of  which  varies 


TEXT-BOOK  OF  PHYSIOLOGY. 


in  different  teeth,  and  which  is  occupied  during  the  living  condition  by 
the  tooth  pulp. 

Microscopic  examination  of  the  tooth  reveals  the  presence  of  ir- 
regular stellate  spaces,  the  interglobular  spaces,  between  the  dentine 
and  the  cementum,  which  are  occupied  by  connective-tissue  cells. 
Clefts  of  varying  size  are  also  observed  at  the  junction  of  the  dentine 
and  the  enamel,  and  which  extend  for  some  distance  into  the  latter. 
The  enamel  is  composed  of  dense  hard  cylinders  which,  on  account 
of  their  small  size  and  close  relationship,  appear  to  be  hexagonal  in 
shape.     These  cylinders  are  held  together  by  cement  substance.     The 

free  border  of  the  enamel  is  covered, 
in  early  life  at  least,  by  a  thin  mem- 
brane known  as  the  cuticle  or  mem- 
brane of  Nasmyth. 

The  dentine  is  somewhat  less  dense 
than  the  enamel.  It  is  composed  of 
connective-tissue  fibers  embedded  in  a 
ground-substance,  both  of  which  have 
undergone  calcification  in  the  course 
of  development.  The  dentine  is  pen- 
etrated by  a  series  of  fine  canals,  the 
dentine  canals  or  tubules,  which  begin 
by  open  mouths  on  the  pulp  side. 
From  this  point  the  tubules  pass  out- 
ward to  the  cementum  and  enamel, 
where  their  terminal  branches  com- 
municate with  and  terminate  in  the 
interglobular  spaces  and  clefts.  In 
their  course  the  tubules  give  off  a 
series  of  branches  which  communicate 
freely  with  one  another.  The  dentine 
bordering  the  tubule  is  somewhat 
more  dense  than  the  intertubular  por- 
tion and  constitutes  what  is  known  as 
the  dentinal  sheath  or  Neumann's 
sheath. 

The  cementum  resembles  bone  be- 
cause  it   contains   both   lacunae    and 
canaliculi,  though  it  is,  as  a  rule,  devoid  of  Haversian  canals. 

The  pulp  consists  of  a  framework  of  connective  tissue  which  affords 
support  to  blood-vessels  and  nerves,  both  of  which  enter  the  pulp 
chamber  through  a  small  foramen  at  the  apex  of  the  root.  The  outer 
surface  of  the  pulp  is  covered  with  a  layer  of  large  spheric  cells,  the 
odontoblasts.  Each  cell  presents  on  its  inner  surface  short  processes 
which  pass  into  the  pulp;  on  its  outer  surface  it  presents  a  long  process 
which  enters  a  dentine  tubule  and  extends  as  far  as  its  ultimate  ter- 
minations.     Collectively  these  processes  are  known  as   the  dentine 


Fig 
Tooth 
Dentine.      P. 
brane.      P.C. 


ment. 
Vein. 
ling.) 


64. — Vertical  Section  of 
in     Jaw.     E.    Enamel.     D. 
M.  Periodontal     mem- 
Pulp    cavity.      C.  Ce- 


Bone    of    lower   jaw.     V. 
a.  Artery.    N.    Nerve. — {Stir- 


DIGESTION.  149 

fibers.  Inasmuch  as  the  fibers  do  not  completely  occupy  the  lumen 
of  the  tubule,  it  is  probable  that  there  is  a  free  circulation  of  lymph 
from  the  pulp  chamber  through  the  dentine  tubules  into  the  enamel 
clefts,  into  the  interglobular  spaces,  and  possibly  into  the  lacunas  of  the 
cementum. 

The  peridental  membrane  is  composed  of  connective-tissue  fibers 
abundantly  supplied  with  blood-vessels,  and  nerves. 

Movements  of  the  Lower  Jaw. — The  lower  jaw  is  capable  of  a 
downward  and  upward,  an  antero-posterior,  and  a  lateral  movement, 
all  dependent  on  the  peculiar  construction  of  the — 

Temporo-maxillary  Articulation. — This  articulation  is  formed  by 
the  anterior  portion  of  the  glenoid  cavity,  the  eminentia  articularis 
and  the  condyle  of  the  inferior  maxilla,  all  of  which  are  united  by 
means  of  ligaments.  Situated  between  the  glenoid  cavity  and  the 
condyle  is  a  plate  of  fibro-cartilage  oval  in  shape  and  biconcave.  This 
cartilage  divides  the  joint  into  two  cavities — one  above,  the  other 
below — each  of  which  is  provided  with  a  synovial  membrane.  The 
function  of  the  cartilage  is  to  present  constantly  an  articulating  surface 
to  the  condyle  in  the  various  movements  of  the  lower  jaw,  which  it  is 
enabled  to  do  by  virtue  of  its  mobility. 

In  the  downward  movement  of  the  lower  jaw  each  condyle  glides 
forward,  carrying  with  it  the  interarticular  fibro-cartilage  the  upper 
concave  surface  of  which  is  applied  to  the  convex  surface  of  the  emin- 
entia articularis.  In  the  upward  movement  of  the  jaw  both  the  con- 
dyles and  the  cartilages  pass  backward  and  resume  their  normal  posi- 
tion. The  movements  of  depression  and  elevation  are  made  possible 
by  the  transverse  direction  of  the  condyle.  In  the  carnivorous  ani- 
mals, whose  food  requires  considerable  cutting,  these  movements  are 
especially  well  developed.  In  the  antero-posterior  movement  the  jaw 
moves  in  a  horizontal  direction  and  the  condyles  and  the  articular  car- 
tilages glide  forward  and  backward  in  the  glenoid  fossae.  In  the  rodent 
animals  the  long  axis  of  the  condyle  runs  in  the  antero-posterior  direc- 
tion, which  allows  of  a  considerable  gliding  movement.  When  the 
jaw  performs  a  lateral  movement,  the  condyle  and  cartilage  of  one  side 
remain  in  their  normal  position,  while  the  opposite  condyle  and  cartil- 
age glide  forward  in  the  glenoid  fossa,  directing  the  symphysis  of  the 
jaw  to  the  opposite  side  of  the  median  line.  The  lateral  movements 
are  well  exhibited  by  the  herbivorous  animals,  in  which  they  are  quite 
extensive,  and  made  possible  by  the  small  size  of  the  condyle  and  the 
large  extent  of  articulating  surface.  In  man  the  structure  of  the  joint 
is  such  as  to  admit  of  all  these  possibilities,  and  the  lower  jaw  acquires 
in  consequence  great  freedom  of  movement. 

The  Functions  of  the  Muscles  of  Mastication.  —The  move- 
ments of  the  lower  jaw  are  caused  by  the  action  of  numerous  muscles, 
which,  having  a  fixed  point  of  origin,  are  attached  to  various  points  on 
its  surface.  The  muscles  concerned  in  the  movements  of  mastication 
are  presented  in  the  following  table : 


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TEXT-BOOK  OF  PHYSIOLOGY. 


Anterior  belly  of  digastic 
Mylohyoid 
Geniohyoid 
Temporal 

Internal  portion  of  masseter 
Internal  pterygoid 
External  pterygoids 
External  portion  of  masseter 
Anterior  fibers  of  temporal 
Posterior  fibers  of  temporal 
Internal  portion  of  masseter 
Digastric,    mylohyoid,    and    genio- 
hyoid 
Internal  pterygoids 
External  pterygoids 
Pterygoids,  external  and  internal 
Temporal 
Masseter 


Depress  the  lower  jaw  and  open  the 
mouth. 

Elevate  the  lower  jaw  and  close  the 

mouth. 

Draw  the  lower  jaw  forward  and 
cause  the  lower  teeth  to  project 
beyond  the  upper. 

Draw  the  lower  jaw  back  to  its  nor 
mal  position. 

Contracting  alternately,  draw  the 
jaw  to  the  opposite  side. 

Produce  grinding  movements  of  the 
lower  jaw. 


The  action  of  the  depressor  muscles  becomes  apparent  when  their 
points  of  origin  and  insertion  are  considered.  The  anterior  belly 
of  the  digastric,  the  mylohyoid,  the  geniohyoid  muscles,  agree  in  having 
a  similarity  of  origin— the  hyoid  bone — and  a  common  area  of  inser- 
tion, the  anterior  portion  of  the  inferior  maxillary.  Their  anatomic 
relation  is  such  that  their  combined  action  will  depress  the  lower  jaw 
and  open  the  mouth. 

The  elevator  muscles  arise  from  various  points  on  the  side  of  the 
head,  and  are  inserted  into  the  coronoid  process,  ramus,  and  internal 
surface  of  the  angle  of  the  lower  jaw.  When  the  mouth  has  been 
opened,  the  simultaneous  contraction  of  these  muscles  elevates  the  jaw 
and  closes  the  mouth  with  considerable  force.  The  power  of  these 
muscles  is  very  great,  and  depends  on  the  shortness  and  thickness  of 
the  muscle-bundles. 

The  action  of  the  rotator  muscles,  those  which  give  rise  to  the  lateral 
movements  of  the  jaw,  depends  in  like  manner  on  their  origin  and  in- 
sertion. Arising  from  the  superior  maxillary  and  sphenoid  bones, 
they  are  inserted  into  the  neck  of  the  condyle  and  angle  of  the  lower 
jaw  respectively.  When  they  contract,  the  condyle  on  the  correspond- 
ing side  is  drawn  forward,  while  the  opposite  condyle  remains  stationary. 
As  a  result,  the  symphysis  of  the  jaw  is  directed  to  the  opposite  side. 
The  grinding  movements  of  the  jaw  are  produced  by  the  coordinated 
action  of  all  the  groups  of  muscles  acting  more  or  less  successively. 

For  the  proper  mastication  of  the  food  it  is  essential  that  it  be  kept 
between  the  opposing  surfaces  of  the  teeth.  This  is  accomplished  by 
the  contraction  of  the  orbicularis  oris  and  buccinator  muscles  from 
without  and  the  tongue  muscles  from  within. 

The  Nerve  Mechanism  of  Mastication.*— The  movements  of 
mastication,  though  originating  in  efforts  of  the  will  and  under  its 
control,  are  for  the  most  part  of  an  automatic  or  reflex  character;  for 

*  By  this  term  is  meant  a  combination  of  nerves,  afferent  and  efferent,  and  nerve 
centers  which  when  stimulated  coordinates  the  actions  of  the  organs  with  which  it  is 
associated. 


DIGESTION.  151 

when  once  initiated  by  a  voluntary  effort  they  continue  for  an  indef- 
inite period — so  long,  in  fact,  as  the  impressions  which  the  food  makes 
upon  the  afferent  nerves  are  received  by  the  nerve-centers  which 
regulate  and  control  them.  That  the  masticatory  movements  are  of 
this  reflex  nature  is  shown  by  the  fact  that  they  will  be  maintained 
even  though  the  voluntary  effort  which  called  them  forth  has  sub- 
sided and  the  attention  has  been  directed  to  some  entirely  different 
subject.  It  would  appear  that  all  that  is  necessary  under  such  con- 
ditions is  the  exciting  action  of  the  food  upon  the  periphery  of  the 
afferent  nerves  distributed  to  the  tongue  and  mouth. 

The  nerves  involved  in  this  reflex  are  shown  in  the  following  table : 

Afferent  Nerves.  Efferent  Nerves, 

i.  Lingual  branch  of  fifth  nerve.  1.  Inferior  maxillary  division  of  fifth  nerve. 

2.   Glossopharyngeal.  2.  Hypoglossal  or  sublingual. 

3.  Facial  or  portio  dura. 

The  nerve-center  coordinating  the  movements  of  mastication  is 
situated  in  the  medulla  oblongata.  The  afferent  or  excitor  nerves 
which  receive  the  impressions  of  the  food  are  distributed  largely  to 
the  mucous  membrane  of  the  tongue.  When  these  impressions  are 
received  by  the  center  in  the  medulla  oblongata,  it  discharges  nerve 
impulses,  which,  passing  outward  through  motor  nerves,  excite  con- 
traction in  the  masticatory  muscles.  The  motor  nerves  innervating 
the  muscles  are :  (1)  The  small  root  of  the  fifth  nerve,  which,  after  emerg- 
ing from  the  cavity  of  the  cranium  through  the  foramen  ovale,  joins 
the  inferior  maxillary  division  of  the  large  sensor  root.  It  then  is  dis- 
tributed to  the  masseter,  temporal,  internal,  and  external  pterygoids, 
anterior  belly  of  the  digastric,  and  mylohyoid  muscles,  and  controls 
their  movements.  (2)  The  hypoglossal  nerve,  which,  after  emerging 
through  the  anterior  condyloid  foramen,  passes  downward  and  forward 
to  be  distributed  to  the  extrinsic  and  intrinsic  muscles  of  the  tongue. 
(3)  The  facial  or  portio  dura,  which,  after  emerging  from  the  stylo- 
mastoid foramen,  is  distributed  to  the  muscles  of  the  face.  Irritation 
of  any  one  of  these  nerves  produces  convulsive  movements  in  the  mus- 
cles to  which  it  is  distributed,  while  their  division  is  followed  by  paral- 
ysis of  these  muscles.  The  medulla  not  only  generates  the  impulses 
which  are  directly  responsible  for  the  movements  of  mastication,  but 
also  coordinates  them  in  such  a  manner  that  the  movements  of  mastica- 
tion may  be  directed  toward  the  accomplishment  of  a  definite  purpose. 

INSALIVATION. 

Insalivation  is  the  incorporation  of  the  saliva  with  the  food,  and 
takes  place  for  the  most  part  during  mastication.  The  saliva  ordi- 
narily present  in  the  mouth  is  a  complex  fluid  composed  of  the  various 
secretions  of  the  parotid,  submaxillary,  and  sublingual  glands  and 
the  muciparous  follicles  of  the  mouth,  which  collectively  constitute 
the  salivary  apparatus  (Fig.  65). 


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TEXT-BOOK  OF  PHYSIOLOGY. 


The  parotid  gland  is  situated  in  front  of  and  partly  below  the 
external  ear,  where  it  is  held  in  position  by  the  fascia  and  skin.  From 
the  anterior  border  of  the  gland  there  emerges  a  duct  (Stensen's), 
which,  after  crossing  the  masseter  muscle  to  its  anterior  border,  turns 
inward,  pierces  the  buccinator,  and  opens  on  the  surface  of  the  cheek 
opposite  the  second  upper  molar  tooth. 

The  submaxillary  gland  is  situated  below  the  jaw  in  the  anterior 
part  of  the  submaxillary  triangle.     From  the  gland  there  emerges  a 

duct  (Wharton's)  which  opens 
into  the  mouth  by  a  minute 
orifice  on  the  surface  of  a 
papilla  by  the  side  of  the 
tongue. 

The  sublingual  gland  is 
situated  just  beneath  the  mu- 
cous membrane  in  the  anterior 
part  of  the  mouth,  where  it 
forms  a  projection  between 
the  gums  and  tongue.  The 
posterior  part  of  the  gland 
gives  origin  to  a  duct  (the 
duct  of  Rivinus,  described  also 
by  Bartholin)  which  opens 
into  the  mouth  with  or  very 
near  to  the  duct  of  Wharton. 
The  anterior  part  of  the  gland 
gives  origin  to  a  varying  num- 
ber of  ducts  (Walthers)  which 
open  separately  along  the  edge 
of  the  sublingual  plica  of  the 
mucous  membrane. 

Histologic  Structure  of 
the  Salivary  Glands.— In 
their  ultimate  structure  the 
salivary  glands  bear  a  close 
resemblance  to  one  another. 
They  are  compound  tubulo-alveolar  glands  composed  of  small 
irregularly  shaped  lobules  united  by  areolar  tissue  and  connected 
by  branches  of  the  salivary  ducts.  Each  lobule  is  made  up  of  a 
number  of  small  alveoli  or  acini  more  or  less  tubular  in  shape  which 
are  the  terminal  expansions  of  the  smallest  ducts.  (See  Fig.  66.) 
The  wall  of  the  acinus  is  formed  by  a  reticulated  basement  mem- 
brane, surrounded  externally  by  blood-vessels,  the  spaces  between 
which  constitute  lymph-spaces  or  channels.  The  inner  surface  of 
the  acinus  membrane  supports  a  single  layer  of  irregular  spheric 
or  polygonic  epithelial  cells.  The  cells  do  not  entirely  fill  up  the 
cavity  of  the  acinus,  but  leave  an  intercellular  space,   the  lumen, 


Fig.  65. — Salivary  Glands,  i,  2.  Parotid. 
3.  DuctofSteno.  4.  Submaxillary.  5.  Sublin- 
gual. 6.  Mylohyoid  muscle.  7.  Lingual  branch 
of  the  fifth  nerve.  8.  Duct  of  Wharton.  9.  Di- 
gastric muscle.  10.  Sternomastoid  muscle.  11. 
External  jugular  vein.  12.  Facial  vein.  13. 
Temporal  vein.  14,  15.  Internal  jugular  vein. 
16.  Branch  of  the  cervical  plexus.  17.  Sublin- 
gual nerve. — (Le  Bon.) 


DIGESTION. 


i53 


which  constitutes  the  beginning  of  the  duct  for  the  transmission  of 
the  secretion  to  the  mouth.  From  each  acinus  there  passes  a  narrow 
intercalary  duct  lined  by  a  layer  of  flattened  cells.  The  common 
excretory  duct— formed  by  the  union  of  the  intralobular  and  inter- 
lobular ducts— consists  also  of  a  basement  membrane,  lined,  however, 
by  tall  columnar  epithelial  cells.  The  salivary  glands  are  abundantly 
supplied  with  blood-vessels  and  nerves  which  are  closely  related  to 
their  activity. 

Based  partly  on  the  character  of  their  secretions  and  partly  on  the 
microscopic  appearance  of  their  secreting  cells,  the  salivary  glands 
have  been  divided  by  Heidenhain  into  two  classes:  viz.,  serous  or  al- 
buminous, and  mucous  glands.  To  the  first  class  belong  the  parotid, 
a  portion  of  the  submaxillary,  and  a 
portion  of  the  glands  of  the  tongue. 
To  the  second  class  belong  a  portion 
of  the  submaxillary  gland,  the  sub- 
lingual, a  portion  of  the  glands  of  the 
tongue,  the  glands  of  the  cheeks,  lips, 
palate,  and  pharynx.  It  is  possible 
that  a  single  alveolus  of  any  gland  may 
contain  both  albuminous  and  mucous 
cells. 

In  the  serous  glands  the  cells  are 
more  or  less  spheric  in  shape,  nu- 
cleated, and  almost  completely  filled 
with  dark  granular  material  (Fig.  67). 
In  the  mucous  glands  the  cells  are 
large,  clear  in  appearance,  and  loaded 
with  a  highly  refracting  material  re- 
sembling mucin  (Fig.  68).  Between 
the  basement  membrane  and  the  clear 
cells  are  to  be  found  in  the  acini  of 

the  submaxillary  glands  small  crescentic  shaped  cells  filled  with 
granular  material  which  stains  deeply  with  various  coloring-matters. 
These  are  known  as  the  demilunes  of  Heidenhain.  At  one  time  it 
was  supposed  that  they  were  young  cells  destined  to  take  the  place 
of  the  clear  cells  which  were  believed  to  be  exhausted  and  to  have 
undergone  disintegration.  At  the  present  time  they  are  regarded  as 
albuminous  or  serous  cells  which  exhibit  changes  similar  to  the  cells 
of  the  parotid  gland. 

The  glands  embedded  in  the  mucous  membrane  covering  the 
tongue,  lips,  cheek,  palate,  and  pharynx  are  for  the  most  part  lined 
with  epithelial  cells  which  contain  a  highly  refracting  matter  similar 
to,  if  not  identical  with,  that  found  in  the  cells  of  the  submaxillary 
and  sublingual  glands. 

Nerve-supply. — Histologic  investigation  has  demonstrated  that 
the  cells  and  blood-vessels  of  the  salivary  glands  are  supplied  with 


Secretory 
tubule. 


Intercalated 
pieces. 


Acini. 


Fig.  66. — Scheme  of  the  Human 
Submaxillary  Gland. — (Stdhr.) 


154 


TEXT-BOOK  OF  PHYSIOLOGY. 


nerve-fibers  directly  from  ganglion  cells  situated  in  their  immediate 
neighborhood.  Thus  the  cells  and  blood-vessels  of  the  submaxillary 
and  sublingual  glands  receive  nerve-fibers  from  the  submaxillary, 
sublingual  and  superior  cervical  ganglia,  while  the  cells  and  blood- 
vessels of  the  parotid  gland  receive  nerve-fibers  from  the  otic  and  the 
superior  cervical  ganglia.  From  their  ultimate  distribution  it  may  be 
inferred  that  some  of  the  ganglion  cells  and  fibers  influence  the  pro- 
duction of  the  secretions  (secretor  nerves),  while  others  influence  the 
caliber  of  the  blood-vessels  causing  either  constriction  or  dilatation 
(vaso-constrictor  and  vaso-dilatator  nerves).  (Fig.  69.)  The  secretor 
fibers  penetrate  the  basement  membrane  enclosing  the  gland  acinus 
and  finally  terminate  between  and  on  the  surface  of  the  secretory  cells. 
The  vaso-motor  fibers  terminate  between  and  on  the  muscle  cells  in 
the  walls  of  the  blood-vessels. 

The  local  ganglion  cells,  however,  are  in  anatomic  relation  with 
nerve-trunks  coming  directly  from  the  medulla  oblongata  and  the  spinal 


Fig.  67. — Section  of  Human  Par- 
otid Gland  Including  Several  Acini. 
d.  Cut  intralobular  duct.— (Piersol.) 


Fig.  68. — Section  of  Human  Sub- 
lingual Gland.  Among  the  clear  cells 
lining  the  mucous  acini  are  nests  (g,  g) 
of  granular  elements  which  constitute  the 
demilunes  of  Heidenhain. — (Piersol.) 


cord.  As  they  approach  the  ganglia,  their  terminal  branches  arborize 
around  and  closely  invest  the  cells  of  the  ganglia  and  come  into  in- 
timate histologic  and  physiologic  connection  with  them.  The  nerve- 
fibers  coming  from  the  central  nerve  system  are  known  as  pre-ganglionic 
fibers,  while  those  coming  from  the  ganglia  are  known  as  post-gangli- 
onic  fibers.  Through  the  intermediation,  therefore,  of  the  ganglion 
cells,  the  secretor  cells  of  the  salivary  glands  and  the  blood-vessels 
surrounding  them,  are  brought  into  relation  with  the  central  organs  of 
the  nerve  system  and  become  susceptible  of  being  influenced  by  them. 
The  Parotid  Saliva. — The  parotid  saliva,  as  it  flows  from  the 
orifice  of  Stensen's  duct,  is  clear,  limpid,  free  from  viscidity,  distinctly 
alkaline  in  reaction,  with  a  specific  gravity  of  1.003.  Chemic  analysis 
shows  that  it  consists  of  water,  a  small  quantity  of  proteid  matter,  a 
trace  of  a  sulphocyanogen  compound,  and  inorganic  salts.  The  secre- 
tion is  increased  during  mastication,  and  especially  on  the  side  engaged 
in  mastication.  Dry  food  causes  a  larger  flow  than  moist  food.  The 
situation  of  the  orifice  of  the  parotid  duct  is  such  that  as  the  secretion 


DIGESTION. 


i55 


is  poured  into  the  mouth  it  is  at  once  incorporated  with  the  food  by 
the  movements  of  the  lower  jaw,  and  thus  fulfils  the  physical  function 
of  softening  and  moistening  it. 

The  Submaxillary  Saliva.— The  submaxillary  saliva  is  clear, 
slightlv  viscid,  alkaline  in  reaction,  and  has  a  specific  gravity  of  1.002. 
It  consists  of  water,  proteid  matter  (mucin),  and  inorganic  salts. 

The  Sublingual  Saliva.— The  sublingual  saliva  is  clear,  extremely 
viscid,  and  strongly  alkaline  in  reaction.  It  consists  of  water,  proteid 
matter  (chiefly  mucin),  and  inorganic  salts. 


5-Aeroe 
7tJiMeney 

Glosso-Pharyngtal 


Otic  Ganglion 
rarotid  Gland 

Jacoiseris  jVen/e 


Sub-Maxillary  Ganglion^   ~?£ 


dub-Maxillary  Glancfr— 

CAorJaTympaniMw  Sympathetic  Nerves 

Fig.  69. — Scheme  of  the  Nerves  Involved  em  the  Secretion  of  Saliva. 


The  small  racemose  glands  embedded  in  the  mucous  membrane 
on  the  inner  surface  of  the  cheeks  and  lips,  on  the  hard  and  soft 
palate,  on  the  tongue  and  pharynx,  secrete  a  fluid  which  is  grayish 
in  color,  extremely  viscid  and  ropy.  It  contains  a  large  amount  of 
mucin. 

Mixed  Saliva.— The  saliva  of  the  mouth  is  a  complex  fluid  com- 
posed of  the  secretions  of  all  the  salivary  glands.  As  obtained  from 
the  mouth  it  is  frothy,  opalescent,  slightly  turbid,  and  somewhat  viscid. 
The  specific  gravity  is  low,  ranging  from  1.003  t0  1-006.  The  reac- 
tion is  usually  distinctly  alkaline.  It  may,  however,  be  neutral  or 
even  acid  in  reaction  if  there  is  any  fermentation  of  food  particles  in 
the  mouth  or  certain  disorders  of  the  aliment  an*  canal.     When  ex- 


156  TEXT-BOOK  OF  PHYSIOLOGY. 

amined  with  the  microscope,  the  saliva  is  seen  to  contain  epithelial 
cells,  salivary  corpuscles  resembling  leukocytes,  particles  of  food, 
various  microorganisms,  and  especially  Leptothrix  buccalis. 

The  chemic  composition  of  the  saliva  is  shown  in  the  following 
table: 


COMPOSITION  OF  HUMAN  SALIVA. 

Water, 995-!6  994.20 

Epithelium, 1.62  2.20 

Soluble  organic  matter, 1.34  1.40 

Potassium  sulphocyanid, 0.06  0.04 

Inorganic  salts, 1.82  2.20 

1000.00  1000.04 
(Jacubowitsch.)      (Hammerbacher.) 

Water  constitutes  the  main  ingredient,  amounting  to  99.5  per  cent. 
The  soluble  organic  matter  is  proteid  in  character  and  is  a  mixture 
of  mucin,  globulin,  and  serum-albumin.  The  potassium  sulpho- 
cyanid is  mainly  derived  from  the  parotid  gland.  Its  presence  can  be 
demonstrated  by  the  addition  of  a  few  minims  of  a  dilute  solution  of 
slightly  acidulated  ferric  chlorid,  when  a  characteristic  red  color  is 
developed.  The  inorganic  constituents  comprise  the  sodium,  calcium 
and  magnesium  phosphates,  sodium  carbonate,  sodium  and  potassium 
chlorids. 

The  relative  amounts  of  the  different  constituents  of  the  saliva 
will  depend  on  the  relative  degree  of  activity  of  the  different  glands, 
and  this  in  turn  will  -be  determined  by  the  character  of  the  food. 
When  the  food  is  dry,  there  will  be  an  excess  of  the  parotid  secretion; 
when  it  partakes  of  the  consistence  of  meat,  there  will  be  a  larger 
secretion  of  the  submaxillary  saliva.  The  glands  apparently  adapt 
their  activity  to  the  character  of  the  food. 

Quantity  of  Saliva. — The  estimation  of  the  total  quantity  of 
mixed  saliva  secreted  in  twenty-four  hours  is  exceedingly  difficult,  and 
the  results  obtained  must  be  only  approximative.  It  is,  of  course, 
subject  to  considerable  variation,  depending  upon  habit,  the  nature  of 
the  food,  etc.  The  experiments  of  Professor  Dalton  and  the  results 
obtained  by  him  are  eminently  trustworthy,  and  in  all  probability 
represent  as  nearly  as  possible  the  exact  amount  secreted.  He  found 
that  without  any  artificial  stimulus  he  was  enabled  to  collect  from 
the  mouth  about  36  grams  (540  grains)  of  saliva  per  hour,  but  that 
upon  the  introduction  of  any  stimulating  substance  into  the  mouth 
the  quantity  could  be  greatly  increased.  During  mastication  the  saliva 
was  poured  out  in  greater  abundance,  the  amount  depending  upon 
the  relative  dryness  of  the  food.  He  found  that  wheaten  bread  ab- 
sorbed 55  per  cent,  of  its  weight,  and  fresh  cooked  meat  48  per  cent. 
If,  therefore,  the  average  quantity  of  bread  and  meat  required  daily 
by  a  man  of  ordinary  physical  development  and  activity  be  assumed 
to  be  540  grams  (19  oz.)  of  the  former  and  450  grams  (16  oz.)  of  the 


DIGESTION. 


i57 


latter,  these  two  substances  would  absorb  respectively  297  grams 
(4573.8  grains)  and  216  grams  (3326.4  grains),  making  a  total  of  513 
grams  (7900  grains).  If,  therefore,  the  amount  secreted  and  mixed 
with  the  food  during  an  estimated  two  hours  of  mastication  be  added 
to  the  amount  secreted  during  the  remaining  twenty-two  hours,  sup- 
posing that  it  continues  at  the  rate  of  36  grams  per  hour,  we  have  a 
total  amount  of  513  -f-  792  grams,  or  1305  grams  (19,780  grains), 
or  about  2.8  pounds. 

Histologic  Changes  in  the  Salivary  Glands  during  Secretion. 
— During  and  after  secretion  very  remarkable  changes  take  place  in 
the  cells  lining  the  acini,  which  are  in  some  way  connected  with  the 
production  of  the  essential  constituents  of  the  salivary  fluids.  In  the 
case  of  the  parotid  gland,  which  may  be  regarded  as  the  type  of  a 
serous  or  albuminous  gland,  the  following  changes  have  been  observed 
by  Langley  (Fig.  70).  During  the  period  of  rest  and  just  previous  to 
secretory  activity,  the  epithelial  cells  are  enlarged  and  swollen,  and 


Fig.  70. — Cells  of  the  Alveoli  of  a  Serous  or  Watery  Salivary  Gland.  A 
After  rest.  B.  After  a  short  period  of  activity-  C.  After  a  prolonged  period  of  activitv. 
— (Yeo's  "Text-Book  of  Physiology.") 


encroach  on  the  lumen  of  the  acinus.  The  protoplasm  of  the  cells 
is  so  completely  filled  with  dark  fine  granules  as  not  only  to  obscure 
the  nucleus,  but  to  almost  obliterate  the  line  of  union  of  the  cells. 
As  soon  as  secretion  becomes  active,  however,  the  granules  begin  to 
disappear  from  the  outer  region  of  the  cell  and  move  toward  the  inner 
border  and  into  the  lumen  of  the  acinus.  From  these  observations 
it  might  be  inferred  that  during  rest  the  protoplasm  of  the  cells  gives 
rise  to  granular  material,  and  that  during  and  after  secretory  activity 
there  is  an  absorption  of  new  material  from  the  lymph  and  a  recon- 
struction of  the  granular  material. 

In  the  submaxillary  gland,  a  portion  of  which  may  be  taken  as 
a  type  of  a  mucous  gland,  similar  changes  have  been  observed  (Fig. 
71).  During  rest  the  epithelial  cells  are  large,  clear  in  appearance, 
highly  refractive,  and  loaded  with  small  globules  resembling  mucin. 
The  nucleus,  surrounded  by  a  small  quantity  of  protoplasm,  lies  near 
the  margin  of  the  cell.  That  the  granules  are  not  protoplasmic  in 
character  is  shown  by  the  fact  that  they  do  not  stain  on  the  addition 
of  carmine.  When  treated  with  water  or  dilute  acids,  the  globules 
swell  up,  coalesce,  and  form  a  uniform  mass.     The  chemic  relations 


i58  TEXT-BOOK  OF  PHYSIOLOGY. 

of  this  substance  indicate  that  it  is  the  precursor  of  mucin — namely, 
mucigen.  During  secretory  activity  the  cells  discharge  these  mucigen 
granules  into  the  lumen  of  the  acinus  where  they  are  transformed  into 
mucin.  Though  the  appearance  of  the  gland-cell  appears  to  in- 
dicate it,  there  is  no  evidence  for  the  view  that  the  cell  itself  undergoes 
disintegration  in  the  process. 

The  Physiologic  Actions  of  Saliva. — The  constant  presence  of 
salivary  glands  in  the  different  classes  of  animals  and  the  large  amount 
of  secretion  they  pour  daily  into  the  alimentary  canal  point  to  the 
conclusion  that  this  mixed  fluid»plays  an  important  role  in  the  general 
digestive  process.  Experiment  has  demonstrated  that  it  has  a  two- 
fold action,  physical  and  chemical. 

Physically,  saliva  softens  and  moistens  the  food,  unites  its  par- 
ticles into  a  consistent  mass  by 
means   of  its   contained  mucin, 
and  thus  facilitates  swallowing. 
Chemically  it  converts  starch 
into  sugar.     This  action  is  more 
marked  with  boiled   than  with 
raw  starch,  a  fact  which  depends 
on  the  physical  structure  of  the 
starch    grain.      In    the    natural 
condition  each  starch  grain  con- 
Fig.  71.— The  Appearance  Presented      s}sts   0f  a   cellulose  envelope    or 
by  the  Cells  of  the  Submaxillary  Gland        ,  +1  1  f      ■>•  1 

of  the  Dog  after  Prolonged  Secretion.  Stroma  in  tlie  mesnes  01  wnicn 
— (Modified  from  Landois  and  Stirling.)  is  contained  the  true  starch  ma- 

terial, the  granulose.  When 
boiled  for  some  minutes,  the  starch  grain  absorbs  water,  the  granulose 
swells  and  ruptures  the  cellulose  envelope,  after  which  it  passes  into 
an  imperfect  opalescent  solution  more  or  less  viscid,  depending  on 
the  relative  amounts  of  water  and  starch.  This  is  the  change  largely 
brought  about  by  the  process  of  cooking.  If  a  portion  of  this  hydrated 
starch  be  kept  in  the  mouth  for  a  few  minutes  it  will  be  converted  into 
sugar,  a  fact  made  evident  by  the  sense  of  taste. 

The  chemic  action  of  saliva  in  converting  starch  into  sugar,  as 
well  as  the  intermediate  stages,  can  be  experimentally  shown  in  the 
following  manner:  To  5  volumes  of  a  thin  starch  solution  in  a  test- 
tube  add  two  volumes  of  filtered  saliva.  Place  the  mixture  in  a 
water-bath  at  a  temperature  of  35 °  C.  In  a  few  minutes  the  starch 
passes  into  a  soluble  condition  and  the  fluid  becomes  clear  and  trans- 
parent. On  testing  the  solution  from  time  to  time  with  iodin  the 
characteristic  blue  reaction  will  be  found  to  gradually  disappear,  the 
color  passing  from  blue  to  violet,  to  red,  to  yellow.  If  now  the  solu- 
tion be  boiled  with  a  solution  of  cupric  hydroxid  (Fehling's  solution) 
a  (  opious  red  or  yellow  precipitate  of  cuprous  oxid  is  formed,  which 
indicates  the  presence  of  sugar.  The  polariscope  shows  that  this 
sugar  is   maltose,  CI2H220„.     During  the  conversion  of  the  starch 


DIGESTION.  159 

intermediate  substances  are  formed  to  which  the  term  dextrin  is 
applied.  After  the  starch  has  been  rendered  soluble  it  undergoes 
a  cleavage  into  maltose  and  a  dextrin,  which,  as  it  gives  rise  to  a  red 
color  with  iodin,  is  termed  erythrodextrin.  At  a  later  stage  this 
erythrodextrin  also  undergoes  a  cleavage  into  maltose  and  a  second 
variety  of  dextrin,  which,  as  it  does  not  give  rise  to  any  color  with 
iodin,  is  termed  achroodextrin.  It  is  claimed  by  some  investigators 
that  this  form  can  also  in  time  be  transformed  into  sugar.  It  is  pos- 
sible that  a  small  quantity  of  dextrose  is  also  formed. 

The  successive  stages  of  the  conversion  of  starch  into  sugar  may 
be  represented  by  the  following  schema: 

,  -r,     ...       ,        .         f  Achroodextrin. 
Starch  =  Soluble  Starch  =  {^0™  I  MaltOSe- 

This  change  consists  in  the  assumption  by  the  starch  of  a  molecule 
of  water,  and  for  this  reason  the  process  is  termed  hydrolysis.  The 
nature  of  the  chemic  change  is  shown  in  the  following  formula: 

3(C6HI005)  +  H20  =  CI2H22On  +  C6HioOs 
Starch        +  Water  =      Maltose  +    Dextrin. 

The  amylolytic  or  starch-changing  action  of  saliva  depends  on 
the  presence  of  an  unorganized  ferment  or  enzyme  known  as  -ptyalin. 
This  enzyme  is  present  in  the  secretion  of  each  of  the  salivary  glands. 
The  chemic  character  of  ptyalin  is  unknown,  though  there  are  reasons 
for  believing  that  it  partakes  of  the  nature  of  a  proteid.  It  is  a  prod- 
uct in  all  probability  of  the  katabolic  activity  of  the  secretory  cells. 
According  to  Latimer  and  Warren,  ptyalin  is  a  derivative  of  the 
zymogen,  ptyalogen.  This  latter  compound  has  been  shown  to  be 
present  in  the  glands  of  the  dog,  cat,  and  sheep.  Ptyalin  effects  the 
transformation  of  starch  merely  by  its  presence,  and  undergoes  no 
perceptible  consumption  in  the  process.  The  activity  of  this  enzyme 
is  very  great,  and  unless  interfered  with  by  an  excess  of  sugar  and 
dextrin,  it  acts  practically  indefinitely. 

The  activity  of  ptyalin  is  modified  by  various  external  conditions, 
among  which  may  be  mentioned  the  chemic  reaction  of  the  medium 
in  which  it  is  placed.  It  is  most  active  when  the  medium  is  moder- 
ately alkaline.  Its  activity  is  arrested  either  by  strong  alkalies  or 
acids,  though  the  presence  of  a  small  percentage  of  an  acid  does  not 
appear  to  have  any  effect  in  either  hastening  or  retarding  the  process. 
This  fact  has  a  bearing  upon  the  question  as  to  whether  the  action  of 
the  saliva  is  interfered  with  in  the  stomach  by  the  presence  of  the 
gastric  juice.  At  present  it  is  a  disputed  matter,  but  the  weight  of 
authority  is  in  favor  of  the  view  that  the  transforming  action  may 
continue  for  almost  half  an  hour  during  the  early  stages  of  gastric 
digestion.  The  temperature  also  influences  the  rapidity  with  which 
the  transformation  of  the  starch  is  effected.  At  a  temperature  of 
950  to  1060  F.  the  ptyalin  acts  most  energetically,  while  its  activity 


i6o  TEXT-BOOK  OF  PHYSIOLOGY. 

is  entirely  arrested  by  reducing  the  temperature  to  the  freezing-point 
or  raising  it  to  the  boiling-point. 

The  Nerve  Mechanism  of  the  Secretion  of  Saliva. — The 
secretion  of  the  saliva  is  a  complex  act  and  involves  the  cooperation 
of  gland-cells,  blood-vessels,  and  nerves.  During  the  intervals  of 
mastication  the  glands  are  practically  at  rest  as  far  as  the  discharge 
of  saliva  is  concerned.  The  cells,  however,  are  actively  engaged  in 
absorbing  from  the  surrounding  lymph-spaces  materials  derived 
from  the  blood  from  which  they  construct  their  characteristic  con- 
tents. The  blood-vessels  possess  that  degree  of  dilatation  necessary 
for  nutritive  purposes. 

With  the  beginning  of  mastication  the  blood-vessels  suddenly 
dilate,  the  blood-supply  is  increased,  and  the  gland-cells  begin  to  dis- 
charge water,  inorganic  salts,  and  their  organic  constituents  into  the 
lumen  of  the  acinus.  This  continues  until  mastication  ceases,  when 
all  the  structures  return  to  their  former  condition  of  relative  inactivity. 
The  entire  process  is  reflex  in  character  and  takes  place  through  the 
medulla  oblongata.  It  requires  the  usual  mechanism  necessary  for 
all  reflex  acts — viz.,  a  sentient  surface,  afferent  nerves,  emissive 
cells,  efferent  nerves,  and  the  responsive  organs. 

With  the  introduction  of  food  into  the  mouth  impressions  are 
made  on  the  terminal  branches  of  the  afferent  nerves  distributed  in 
the  mucous  membrane.  Nerve  impulses  developed  by  the  mechanic 
and  chemic  action  of  the  food  are  then  transmitted  to  the  medulla 
oblongata  and  received  by  emissive  cells.  These  in  turn  discharge 
nerve  impulses  which  are  transmitted  through  efferent  nerves  to  the 
structures,  producing  the  vascular  and  secretory  effects  already  stated. 

The  nerves  and  nerve-centers  which  constitute  the  reflex  mechan- 
ism for  the  secretion  of  saliva  are  shown  in  the  following  table : 

Afferent  Nerves.  Nerve-centers.  Efferent  Nerves. 

i.  Lingual  branch  of  fifth        Medulla  oblongata.        Chorda  tympani  for  the  submax- 
nerve.  illary    and    sublingual    glands, 

auriculo-temporal  branch  of  the 

2.  Taste  fibers  in  the  chorda  fifth  nerve  for  the  parotid  gland. 

tympani. 

3.  Glossopharyngeal.  Sympathetic  nerve  for  all  glands. 

That  the  secretion  of  the  saliva  is  regulated  by  the  above  mechan- 
ism, and  that  the  lingual  branches  of  the  fifth  nerves  and  the  glosso- 
pharyngeal are  the  afferent  nerves,  can  be  demonstrated  by  exposing 
the  glands  and  their  nerve  connections  and  subjecting  them  to  ex- 
periment. Under  such  circumstances,  if  a  cannula  be  placed  in  the 
duct  of  the  submaxillary  gland,  and  the  lingual  nerve  stimulated  by 
an  induced  electric  current  of  moderate  strength,  a  copious  flow  of 
saliva  at  once  takes  place.  If  now  the  glossopharyngeal  nerve  be 
stimulated  in  a  similar  manner,  the  effect  on  the  secretion  will  be  the 
same.     Division  of  these  two  nerves  in  an  animal,  in  such  a  wav  as  to 


DIGESTION.  161 

prevent  the  nerve  impulses  from  reaching  the  medulla  oblongata,  is 
followed  by  a  marked  diminution  in  the  amount  of  saliva  secreted. 
The  reflex  centers,  however,  may  receive  impulses  and  be  excited  to 
activity  by  impulses  coming  through  other  nerves — e.  g.,  the  pneumo- 
gastric,  when  the  mucous  membrane  of  the  stomach  is  stimulated; 
the  sciatic,  when  after  division  its  central  end  is  stimulated. 

The  central  mechanism  which  causes  through  its  efferent  nerves  the 
discharge  of  saliva  is  also  capable  of  being  excited  to  activity  by  psychic 
influences.  It  is  well  known  that  ideas  and  emotional  states  developed 
by  the  sight  and  the  odor  of  foods,  especially  after  long  abstinence, 
cause  a  discharge  of  saliva  into  the  mouth.  The  watering  of  the 
mouth  under  such  circumstances  is  a  demonstration  of  the  influence 
of  such  psychic  states.  This  fact  was  experimentally  demonstrated 
on  dogs  by  Pawlow.  This  investigator  caused  the  ducts  of  the  glands 
to  be  brought  to  the  surface  in  such  a  manner  that  they  healed  into  the 
edges  of  the  skin  wounds.  By  means  of  suitable  receivers  applied 
over  the  orifices  of  the  ducts,  the  saliva  could  be  readily  collected. 
When  the  dog,  under  such  circumstances,  was  tempted  by  the  sight 
of  food  there  was  at  once  a  free  discharge  of  saliva. 

Whenever  the  central  mechanism  is  stimulated,  either  by  nerve 
impulses  coming  through  afferent  nerves,  from  the  periphery  or  from 
the  brain,  impulses  are  generated  which  pass  outward  through  efferent 
nerves — the  chorda  tympani  nerve  to  the  submaxillary  and  sublingual 
glands,  the  auriculo-temporal  nerve  to  the  parotid  gland,  and  the 
sympathetic  nerve  to  all  three. 

The  Chorda  Tympani.— The  chorda  tympani  nerve  is  a  branch 
of  the  facial,  the  trunk  of  which  it  leaves  in  the  aqueduct  of  Fallopius. 
It  then  crosses  the  tympanic  cavity,  emerges  through  the  Glaserian 
fissure,  and  joins  the  lingual  branch  of  the  inferior  maxillary  division 
of  the  fifth  nerve.  After  passing  forward  as  far  as  the  sublingual 
gland,  nearly  all  of  the  original  fibers  leave  the  lingual  nerve  by  four 
or  five  strands  to  become  connected  by  terminal  branches  with  nerve- 
ganglion  cells  in  relation  with  the  submaxillary  and  sublingual  glands. 
(See  Fig.  69.) 

The  effects  on  the  secretion  and  flow  of  saliva  from  the  submaxil- 
lary gland  which  follow  division  and  stimulation  of  the  chorda  tym- 
pani nerve  are  shown  in  the  following  way:  a  cannula  is  inserted  into 
Wharton's  duct  and  the  rate  of  flow  estimated;  the  nerve  is  then 
divided,  after  which  the  flow  ceases.  The  peripheral  end  of  the 
nerve  is  then  stimulated  with  the  induced  electric  current,  when  a 
copious  secretion  of  a  thin  saliva  takes  place,  accompanied  by  a 
marked  dilatation  of  the  blood-vessels  of  the  gland.  The  quantity  of 
blood  passing  through  the  vessels  is  so  great  as  to  give  to  the  venous 
blood  an  arterial  hue  and  to  the  small  veins  a  distinct  pulsation.  It 
would  appear  from  these  effects  that  the  chorda  contains  two  sets  of 
fibers,  one  of  which  inhibits  the  action  of  a  local  vaso-motor  mechan- 
ism permitting  the  blood-vessels  to  dilate  (vaso-dilatator  fibers),  the 


i62  TEXT-BOOK  OF  PHYSIOLOGY. 

other  of  which  stimulates  the  secretor  cells  to  activity,  either  directly 
or  through  the  intermediation  of  local  ganglia.  That  local  ganglia 
are  involved  is  shown  by  the  effects  which  follow  the  injection  of 
nicotin  into  the  circulation.  After  a  sufficient  dose — 10  milligrams 
for  the  cat — stimulation  of  the  chorda  has  no  effect.  Stimulation  of 
the  nerve-plexus  beyond  the  ganglion,  however,  is  at  once  followed 
by  vascular  dilatation  and  secretion. 

It  might  be  inferred  that  the  increase  in  the  flow  of  saliva  is  due 
to  filtration,  the  result  of  the  increased  blood-supply  to  the  gland,  and 
not  to  the  influence  of  any  true  secretor  fibers  stimulating  the  activities 
of  the  secretor  cells.  That  this  is  not  the  case,  however,  can  be 
demonstrated  in  several  ways:  First,  the  pressure  in  the  duct  of  the 
submaxillary  gland,  as  shown  by  the  mercurial  manometer,  rises, 
when  the  gland  is  secreting,  considerably  above  the  pressure  in  the 
carotid  artery,  which  could  not  be  the  case  if  it  were  clue  to  a  mere 
filtration;  for  if  pressure  alone  were  the  cause,  the  flow  of  saliva  would 
cease  as  soon  as  the  pressure  in  the  tube  equaled  the  pressure  in  the 
blood-vessels.  Second,  even  in  the  absence  of  blood  the  gland  can 
be  made  to  yield  a  secretion,  as  shown  by  stimulating  the  nerve  in  a 
recently  killed  animal.  Third,  after  the  injection  of  atropin  into  the 
circulation  the  secretion  is  abolished,  but  the  local  vaso-motor  mechan- 
ism is  unimpaired,  for  stimulation  of  the  nerve,  as  in  the  previous 
instance,  gives  rise  to  a  dilatation  of  the  vessels  and  an  increased 
blood-supply.  There  is  thus  abundant  proof  that  the  chorda  tym- 
pani  contains  two  sets  of  fibers — one  regulating  the  blood-supply  to 
the  gland,  the  other  stimulating  the  secretor  cells. 

The  Auriculo-temporal  Nerve. — The  nerve-fibers  which  conduct 
nerve  impulses  outward  from  the  medulla  to  the  parotid  gland  are 
believed  to  pass  through  the  glossopharyngeal  nerve,  through  the 
tympanic  branch  or  nerve  of  Jacobson,  to  the  otic  ganglion,  with  which 
they  become  connected.  From  this  ganglion  new  nerve-fibers  arise 
which  pass  into  the  fifth  nerve  and  reach  the  secretor  cells  of  the 
parotid  gland  through  the  auriculo-temporal  nerve. 

The  influence  of  the  auriculo-temporal  branch  of  the  fifth  nerve 
on  the  parotid  gland  is  similar  to  the  action  of  the  chorda  tympani 
on  the  submaxillary  gland.  The  active  fibers  of  this  nerve  are  prob- 
ably derived  from  the  ninth  nerve  or  glossopharyngeal.  If  the  nerve 
be  stimulated  by  the  induced  electric  current,  there  follows  a  dilata- 
tion of  the  blood-vessels  and  an  abundant  discharge  of  a  thin  saliva, 
rich  in  water  and  salts,  but  containing  a  small  amount  of  organic 
matter.  Division  of  the  nerve,  extirpation  of  the  otic  ganglion,  or 
division  of  Jacobson's  nerve,  is  followed  by  a  loss  of  reflex  secretion. 
Stimulation  of  Jacobson's  nerve,  as  shown  by  Hcidenhain,  gives  rise 
to  the  secretion. 

The  Sympathetic  Nerves. — The  sympathetic  fibers  which  in- 
fluence the  .salivary  secretion  emerge  from  the  spinal  cord  mainly 
through  the  second,  third,  and  fourth  thoracic  nerves.     After  passing 


DIGESTION.  163 

into  the  sympathetic  chain  they  ascend  to  the  superior  cervical  ganglion, 
with  the  cells  of  which  they  become  connected  through  the  interme- 
diation of  fine  terminal  branches.  From  this  point  non-medullated 
nerve-fibers  follow  the  branches  of  the  external  carotid  artery  to  the 
different  glands.  There  is  no  evidence  that  these  fibers  have  any 
connection,  anatomic  or  physiologic,  with  local  ganglia  at  or  near 
the  submaxillary,  sublingual  or  parotid  glands.  If  the  sympathetic 
nerve  in  the  neck,  especially  in  the  dog,  be  divided  and  the  peripheral 
end  stimulated  with  the  induced  electric  current,  there  is  at  once  a 
contraction  of  the  smaller  blood-vessels  of  the  submaxillary  and  sub- 
lingual glands  and  a  diminution  of  the  blood-supply,  a  result  showing 
the  presence  of  vaso-constrictor  fibers.  Nevertheless  both  the  sub- 
maxillary and  sublingual  glands  pour  out  a  saliva  which  is  different 
from  that  poured  out  when  the  chorda  tympani  is  stimulated.  The 
quantity  is  less,  it  is  more  viscid,  richer  in  organic  matter,  of  a  higher 
specific  gravity,  and  more  active  in  the  transformation  of  starch  into 
sugar. 

Stimulation  of  the  sympathetic  fibers  passing  to  the  parotid  gland 
is  followed  by  contraction  of  the  vessels  and  an  abolition  of  the  secre- 
tion; but  at  the  same  time  there  is  an  increased  activity  of  the  secretor 
cells,  for  subsequent  stimulation  of  the  auriculo-temporal  nerve  not 
only  causes  an  increase  in  the  amount  of  water  and  inorganic  salts, 
but  an  increase  also  in  the  amount  of  organic  matter  far  beyond  thai 
produced  when  the  auriculo-temporal  has  alone  been  stimulated. 
Histologic  examination  shows  that  the  small  ducts  of  the  gland  are 
filled  with  thick  organic  matter  after  stimulation  of  the  cervical  sym 
pathetic. 

The  foregoing  facts  led  Heidenhain  to  the  conclusion  that  there 
are  two  physiologically  distinct  efferent  nerve-fibers  distributed  to  the 
glands,  viz.,  trophic  nerves,  derived  from  the  sympathetic  which 
stimulate  the  cells  to  the  production  of  organic  constituents;  and 
secretor  nerves,  derived  from  the  cranial  nerves,  which  stimulate  the 
cells  to  the  production  of  water  and  inorganic  salts.  This  view  has 
however,  been  controverted  by  Langley  who  regards  the  secretor 
fibers  to  the  glands  as  essentially  the  same,  and  considers  the  differ- 
ences in  the  character  of  the  secretion  to  be  dependent  on  differences 
in  the  quantity  of  the  blood-supply  induced  by  the  simultaneous 
stimulation  of  the  vaso-motor  nerves. 


DEGLUTITION. 

Deglutition  is  that  part  of  the  digestive  process  which  is  concerned 
in  the  transference  of  the  food  from  the  mouth  through  the  pharynx 
and  esophagus  into  the  stomach.  This  is  an  extremely  complex  act 
and  involves  the  action  of  a  large  number  of  structures,  all  of  which 
are  made  to  act  in  proper  sequence  under  the  coordinating  influence 
of  the  nervous  system.     The  deglutitory  canai  consists  of  the  mouth. 


164 


TEXT-BOOK  OF  PHYSIOLOGY. 


pharynx,  and  esophagus,  each  of  which  presents  certain  anatomic 
features  on  which  its  physiologic  action  depends. 

The  cavity  of  the  mouth  communicates  posteriorly  with  the  pharynx 
by  a  narrow  orifice,  the  isthmus  of  the  fauces.  This  orifice  is  bounded 
above  by  the  soft  palate,  laterally  by  the  anterior  and  posterior  half 
arches,  and  below  by  the  tongue. 

The  pharynx  is  an  oval-shaped  cavity  extending  from  the  base  of 


Fig.  72. — Vertical  Section  of  the  Nasal  Fossa  and  Mouth,  i.  Left  nares. 
2.  Lateral  cartilage  of  the  nose.  3.  Portion  of  the  internal  alar  cartilage  forming 
the  skeleton  of  the  lower  part.  4.  Superior  meatus.  5.  Middle  meatus.  6.  Inferior 
meatus.  7.  Sphenoidal  sinuses.  8.  External  boundary  of  the  posterior  nares.  q. 
Internal  elliptical  opening  of  the  Eustachian  tube.  10.  Soft  palate,  n.  Vestibule 
of  the  mouth.  12.  Vault  of  palate.  13.  Genioglossus  muscle.  14.  Geniohyoid 
muscle.  15.  Cut  margin  of  the  mylohyoid  muscle.  16.  Anterior  pillar  of  the  palate 
(anterior  half-arch),  presenting  a  triangular  figure  with  the  base  inferiorly,  covering 
partly  the  tonsil.  17.  Posterior  pillar  (posterior  half-arch)  of  the  palate.  18.  Tonsil. 
19.  Follicular  (mucous)  glands  at  the  base  of  the  tongue.  20.  Cavity  of  the  larynx. 
21.  Ventricle  of  the  larynx.  22.  Epiglottis.  23.  Cut  os  hyoides,  24.  Cut  thyroid 
cartilage.  25.  Thyrohyoid  membrane.  26.  Section  of  posterior  portion  of  the  cricoid 
cartilage.  27.  Section  of  the  anterior  portion  of  the  same  cartilage.  28.  Crico-thyroid 
membrane. — (Sappey.) 


the  skull  to  the  lower  border  of  the  cricoid  cartilage,  a  distance  of 
about  12  centimeters.  (See  Fig.  72.)  Its  walls  are  formed  mainly  by 
three  pairs  of  muscles — the  superior,  middle,  and  inferior  constrictors 
—each  consisting  of  red,  striated  muscle-fibers,  and  hence  capable 
of  rapid  and-  energetic  contractions.  Superiorly  the  pharynx  is 
attached  to  and  supported  by  the  basilar  process  of  the  occipital 
bone;  inferiorly  it  becomes  continuous  with  the  esophagus.  The 
anterior  wall  of  the  pharynx  is  imperfect  and  presents  openings  which 


DIGESTION.  165 

communicate  with  the  nasal  chambers,  the  mouth,  and  the  larynx. 
The  lateral  wall  on  either  side  presents  the  opening  of  the  Eustachian 
tube  which  leads  directly  into  the  cavity  of  the  middle  ear.  The 
interior  of  the  pharynx  is  lined  by  mucous  membrane.  The  pharynx 
is  partially  separated  from  the  mouth  by  the  velum  pendulum  palati, 
a  muscular  structure  attached  above  to  the  hard  palate;  its  lower 
edge  or  border  is  directed  downward  and  backward  and  presents  in 
the  middle  line  a  conical  process,  the  uvula.  On  either  side  the  palate 
presents  two  curved  arches,  the  anterior  and  posterior,  formed  re- 
spectively by  the  palato-glossei  and  palato-pharyngei  muscles.  The 
laryngeal  orifice  or  glottis  is  placed  just  beneath  the  base  of  the  tongue. 
It  is  triangular  in  shape,  wide  in  front,  narrow  behind,  and  directed 
downward  and  backward.  It  is  bounded  above  by  a  thin  plate  of 
cartilage,  the  epiglottis,  placed  just  behind  the  tongue  and  so  arranged 
that  it  can  easily  be  depressed  and  elevated. 

The  esophagus,  the  continuation  of  the-  deglutitory  canal,  extends 
downward  from  the  lower  border  of  the  cricoid  cartilage  for  a  dis- 
tance of  from  22  to  25  centimeters,  to  a  point  opposite  the  ninth 
thoracic  vertebra,  where  it  expands  into  the  stomach.  Its  walls  are 
composed  of  an  internal  or  mucous  and  an  external  or  muscular  coat, 
united  by  areolar  tissue.  The  muscular  coat  consists  of  an  external 
layer  of  longitudinal  fibers  arranged  in  three  bands  and  of  an  internal 
layer  composed  of  fibers  arranged  circularly  in  the  upper  part  and 
obliquely  in  the  lower  part  of  the  esophagus.  In  the  upper  third 
the  fibers  are  striated;  in  the  middle  third  they  are  a  mixture  of  both 
striated  and  non-striated;  in  the  lower  third  they  are  entirely  non- 
striated. 

The  muscle  fibers  surrounding  the  esophago-gastric  orifice  are 
arranged  in  the  form  of  and  play  the  part  of  a  sphincter  muscle,  and 
for  this  reason  may  be  termed  the  sphincter  cardial  muscle.  By  its 
action  it  prevents  a  return  under  normal  conditions  of  food  into  the 
esophagus. 

The  deglutitive  act  may  be  for  convenience  divided  into  three 
stages,  viz. : 

1.  The  passage  of  the  food  from  the  mouth  into  the  pharynx. 

2.  The  passage  of  the  food  through  the  pharynx  into  the  esophagus. 

3.  The  passage  of  the  food  through  the  esophagus  into  the  stomach. 

In  the  first  stage  the  bolus  of  food  is  placed  on  the  superior  surface 
of  the  tongue.  The  mouth  is  then  closed  and  respiration  is  momen- 
tarily suspended.  The  tip  of  the  tongue  is  placed  against  the  pos- 
terior surfaces  of  the  teeth.  The  tongue,  because  of  its  intrinsic 
musculature,  then  arches  from  before  backward  against  the  roof  of 
the  mouth  and  pushes  the  bolus  of  food  through  the  isthmus  of  the 
fauces  into  the  pharynx.  This  completes  the  first  stage.  It  is  a 
voluntary  effort  and  accomplished  partly  by  the  tongue,  though,  as 
shown  by  Meltzer,  mainly  by  the  mylohyoid  muscles. 

The  second  and  third  stages,  or  the  passage  of  the  food  through 


166  TEXT-BOOK  OF  PHYSIOLOGY. 

the  pharynx  and  esophagus  into  the  stomach,  have  been  attributed 
until  quite  recently  entirely  to  peristaltic  movements  of  their  muscu- 
lature.* It  has  been  stated  that  with  the  passage  of  the  food  through 
the  isthmus  of  the  fauces  the  posterior  wall  of  the  pharynx  advances 
and  seizes  the  food,  and  in  consequence  of  a  rapid  peristaltic  move- 
ment running  through  its  constrictor  muscles  from  above  downward 
is  transferred  to  the  esophagus;  that  with  the  entrance  of  the  food 
into  the  esophagus  a  similar  peristalsis,  varying  in  rapidity  in  different 
sections  in  consequence  of  a  change  in  the  character  of  its  muscula- 
ture, gradually  transfers  the  food  into  the  stomach.  There  can  be 
but  slight  doubt  that  by  this  method  the  bolus  of  food,  especially  if 
it  is  of  firm  consistence  and  of  a  size  sufficient  to  distend  the  esoph- 
agus, is  transferred  into  the  stomach,  but  that  it  is  the  exceptional 
rather  than  the  usual  method  has  been  demonstrated  by  Kronecker, 
Falk,  and  Meltzer. 

In  1880  the  first  of  these  experimenters  made  the  observation  that 
the  sensation  in  the  stomach  following  the  swallowing  of  a  mouthful 
of  cold  water  occurred  too  quickly  to  be  explained  by  the  prevalent 
belief  that  its  transference  was  caused  by  ordinary  peristalsis,  the  rate 
of  progression  of  which  was  known  to  be  slow.  Falk  then  discovered 
the  fact,  by  introducing  through  the  mouth  into  the  pharynx  a  tube 
connected  externally  with  a  water  manometer,  that  during  the  act  of 
swallowing  there  is  a  sudden  rise  of  pressure  equal  to  about  twenty 
centimeters  of  water. 

These  experiments  demonstrated  that  at  the  beginning  of  degluti- 
tion there  is  a  sudden  rise  of  pressure,  the  result  of  a  quickly  acting 
force  resident  in  the  mouth  or  pharynx,  in  consequence  of  which  the 
food  is  rapidly  thrown  down  into  the  stomach,  peristalsis  playing  no 
part  in  the  process.  The  proof,  however,  of  these  statements  was 
furnished  by  Meltzer.  This  observer  introduced  into  the  pharynx 
and  esophagus  rubber  tubes,  the  ends  of  which  were  provided  with 
thin-walled  rubber  balloons  which  could  be  distended  with  air.  The 
outer  ends  of  the  tubes  were  connected  with  Marey's  recording  tam- 
bours. Any  compression  of  the  balloon  would  be  followed  by  the 
passage  of  the  air  into  the  tambour  and  an  elevation  of  the  lever. 
With  one  balloon  in  the  pharynx  and  the  other  in  the  esophagus  at 
varying  depths,  and  the  recording  levers  of  the  tambours  applied 
against  the  surface  of  a  revolving  cylinder,  it  became  possible,  with 
the  addition  of  a  chronogram,  to  obtain  a  graphic  representation  of 
the  time  relations  of  simultaneous  and  successive  compressions  of 
the  two  balloons. 

It  was  found  as  the  result  of  many  experiments  that  no  matter 

*  Peristalsis  may  be  defined  as  a  progressive  wave-like  movement  which  passes  over 
different  portions  of  the  walls  of  the  alimentary  canal.  Its  effect  physiologically  is  the 
propulsion  of  its  solid  and  semisolid  contents.  It  is  characterized  by  a  contraction  of 
the  muscle-fibers  behind  the  object  and  an  inhibition  or  relaxation  of  the  muscle-fibers  in 
front  of  it.     (Bayliss  and  Starling.) 


DIGESTION. 


167 


how  deep  the  position  of  the  esophageal  balloon,  it  was  compressed 
simultaneously  with  the  pharyngeal  balloon,  as  shown  by  the  rise  of 
the  levers  on  swallowing  a  mouthful  of  water.  The  interval  of  time 
between  the  rise  of  the  two  levers  did  not  amount  to  more  than  the 
tenth  of  a  second.  The  inference  was  that  the  water  was  projected 
or  shot  down  the  pharynx  and  esophagus  in  this  period  of  time,  and 
in  its  passage  compressed  both  balloons  practically  at  the  same  in- 
stant. The  same  was  found  to  be  true  when  small  masses  of  more 
consistent  food  were  swallowed. 

The  curves  of  the  entire  deglutitive  act  recorded  by  the  two  levers 
are,  however,  different  in  form.  (See  Fig.  73.)  The  pharyngeal  curve, 
1,  presents  two  crests,  the  first,  A,  being  due  to  the  compression  caused 
by  the  passage  of  the  bolus,  the  second,  B,  due  to  the  compression 


Fig.  73. — Tracing  of  the  Act  of  Deglutition,  i.  A  indicates  the  compression  of 
the  elastic  bag  caused  by  the  bolus  projected  by  the  contraction  of  the  mylohyoid  muscles. 
B.  Contraction  of  the  pharynx.  2.  Line  marking  seconds.  3.  Tracing  of  the  bag 
in  the.  esophagus  12  cm.  from  the  teeth.  C.  Compression  of  the  bag  by  the  bolus  corre- 
sponding to  A.  D.  Compression  by  the  residues  of  the  bolus  carried  on  by  the  contraction 
of  the  pharynx,  B.     E.  Contraction  of  the  esophagus. — (Landois  and  Stirling.) 


exerted  by  the  contraction  of  the  pharyngeal  muscles.  The  interval 
of  time  between  these  two  crests  amounts  to  not  more  than  0.3  second. 
In  the  esophageal  curve,  3,  the  elevation,  C,  corresponds  to  the  eleva- 
tion, A,  and  is  likewise  due  to  the  compression  exerted  by  the  bolus. 
The  interval  of  time  between  the  beginning  of  the  first  and  second 
curves  was  not  more  than  0.1  second,  regardless  of  the  depth  to  which 
the  esophageal  balloon  was  plunged.  At  a  later  period  a  second  rise 
of  the  lever  was  recorded;  the  time  of  its  appearance,  height,  duration, 
etc.,  were  found  to  increase  with  the  depth  of  the  balloon. 

These  facts  demonstrate  that  deglutition  consists  of  two  phases: 
(1)  a  rapid  rise  of  pressure  in  the  pharynx,  as  a  result  of  which  the 
bolus  is  suddenly  shot  down  to  the  lower  end  of  the  esophagus;  (2) 
a  peristaltic  contraction  of  the  musculature  of  the  canal,  which,  acting 
as  a  supplementary  force,  carries  onward  any  particles  of  food  in  the 
canal  and  forces  the  bolus  through  the  closed  sphincter  cardicr  at  the 
end  of  the  esophagus. 


1 68  TEXT-BOOK  OF  PHYSIOLOGY. 

The  immediate  cause  of  the  sudden  rise  of  pressure  was  shown  by 
Meltzer  to  be  the  contraction  of  the  mylohyoid  muscles.  When 
the  nerves  going  to  these  muscles  were  divided  in  a  dog,  deglutition 
was  practically  abolished.  These  muscles  are  probably  assisted  in 
their  action  by  the  contraction  of  the  hyoglossus  muscles  as  well  as 
the  tongue  itself. 

It  was  also  demonstrated  in  these  experiments  that  the  contrac- 
tion of  the  esophagus  did  not  partake  of  the  character  of  ordinary 
peristalsis.  It  was  found  that  the  esophagus  contracted  in  three 
distinct  segments,  corresponding  in  all  probability  to  the  difference  in 
the  character  of  their  muscular  fibers.  The  first  segment,  about  six 
centimeters  in  length,  was  found  to  begin  to  contract  about  1.2  seconds 
after  the  beginning  of  the  first  curve  and  lasting  2  seconds;  the  second 
segment,  about  twelve  centimeters  in  length,  beginning  to  contract 
about  1.8  seconds  or  3  seconds  after  the  beginning  of  the  first  section, 
and  lasting  for  from  5  to  7  seconds;  the  third  segment,  six  centimeters 
in  length,  contracting  from  6  to  7  seconds.  The  beginning  and  the 
end  of  the  contraction  for  each  segment  occurred  simultaneously 
throughout  its  entire  extent.  If,  however,  a  series  of  deglutitory  acts 
follow  each  other  in  quick  succession,  there  is  an  inhibition  of  the 
peristaltic  contractions  until  after  the  final  swallow. 

An  examination  of  the  action  of  the  esophagus  during  degluti- 
tion, made  by  Cannon  and  Moser  with  x-rays  and  the  fluoroscope, 
disclosed  the  fact  that  the  method  of  food  transmission  varied  in 
different  animals.  In  the  cat  and  dog  the  transmission  was  effected 
by  peristalsis  alone.  The  time  required  for  the  food  to  reach  the 
stomach  varied  in  the  cat  from  nine  to  twelve  seconds  and  in  the 
dog  from  four  to  five  seconds.  The  descent  of  the  bolus  was  more 
rapid  in  the  upper  than  in  the  lower  part  of  the  esophagus.  In  man, 
liquids  descended  rapidly,  at  the  rate  of  several  feet  a  second,  in  con- 
sequence of  the  rapid  and  energetic  contraction  of  the  mylohyoid  mus- 
cles. A  peristaltic  contraction,  passing  over  the  entire  esophagus, 
was  necessary  to  the  passage  of  solid  and  semisolid  food  through  it, 

Closure  of  the  Posterior  Nares  and  Larynx. — Notwithstand- 
ing the  rise  of  pressure  in  the  pharynx  during  the  act  of  swallowing, 
it  is  seldom  under  normal  circumstances  that  any  portion  of  the 
bolus  ever  finds  its  way  either  into  the  larynx  or  nasal  chambers, 
for  the  reason  that  the  openings  of  these  cavities  are  fully  closed  by 
appropriate  means. 

At  the  moment  the  food  passes  into  the  pharynx  the  posterior 
nasal  openings  are  closed  against  the  entrance  of  the  food  by  a  septum 
formed  by  the  pendulous  veil  of  the  palate  and  the  posterior  half 
arches.  The  palate  is  drawn  upward  and  backward  until  it  meets 
the  posterior  wall  of  the  pharynx,  and  at  the  same  time  is  made  tense, 
by  the  action  of  the  levator  palati  and  tensor  palati  muscles  respec- 
tively (Fig.  74).  This  septum  is  completed  by  the  advance  toward 
the  middle  line  of  the  posterior  half  arches  caused  by  the  contraction 


DIGESTION. 


169 


of  the  muscles  which  compose  them — the  palato-pharyngei.  When 
these  structures  are  impaired  in  their  functional  activity,  as  in  diph- 
theritic paralysis  and  ulcerations,  there  is  not  infrequently  a  regurgita- 
tion of  food,  especially  liquids,  into  the  nose. 

The  larynx  is  equally  protected  against  the  entrance  of  food  during 
deglutition  under  normal  circumstances.  That  this  accident  occasion- 
ally happens,  giving  rise  to  severe  spasmodic  coughing,  and  even  in 
extreme  cases  to  suffocation,  is  abundantly  shown  by  the  records 
of  clinical  medicine.  Usually  it  does  not  occur,  for  the  following  rea- 
sons: Just  preceding  and  during  the  act  of  deglutition  there  is  a  com- 
plete suspension  of  the  act  of 
inspiration  by  which  particles  of 
food  might  otherwise  be  drawn 
into  the  larynx;  at  the  same  time 
the  larynx  is  always  drawn  well 
up  under  the  base  of  the  tongue 
and  its  entrance  closed  by  the 
downward  and  backward  move- 
ment of  the  epiglottis. 

The  action  here  attributed 
to  the  epiglottis  has  been  denied 
by  Stuart  and  McCormick. 
These  observers  had  the  oppor- 
tunity of  looking  into  a  naso- 
pharynx which  had  been  laid 
open  by  a  surgical  operation  for 
the  removal  of  a  morbid  growth. 
In  this  patient,  the  epiglottis,  at 
the  time  of  deglutition,  was 
always  more  or  less  erect  and 
closely  applied  to  the  base  of  the 
tongue.  So  complete  was  this 
that  the  food  passed  over  its 
posterior  or  inferior  surface  for  a  certain  distance, 
it  ever  observed  to  fold  backward  like  a  lid. 

Because  of  the  possibility  that  this  position  of  the  epiglottis  was 
due  to  pathologic  causes,  Kanthack  and  Anderson  instituted  a  new 
series  of  experiments  with  a  view  of  determining  the  action  of  the 
epiglottis.  As  a  result  of  many  experiments  on  animals  and  of  ob- 
servations on  themselves,  these  observers  reaffirm  the  generally 
accepted  view,  that  under  normal  conditions,  the  entrance  of  the 
larynx  is  always  closed  by  the  epiglottis  after  the  manner  of  a  lid. 

In  addition  to  the  downward  and  backward  movement  of  the  epi- 
glottis and  the  ascent  of  the  larynx  under  the  base  of  the  tongue,  it  is 
also  certain  from  the  observations  of  Meltzer  that  the  larynx  is  protected 
from  the  entrance  of  food,  in  the  rabbit  at  least,  by  the  closure  of  the 
glottis  itself.     This  experimenter  noticed,  while  observing  the  interior 


TrcocJiea/ 


Fig.  74. — Diagram  Showing  the  Man- 
ner of  Closure  of  the  Posterior  Nares 
and  Larynx  during  Deglutition. —  (Lan- 
dois  and  Stirling.) 


In  no  instance  was 


i7o  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  larynx,  both  from  above,  through  an  opening  in  the  hyothyreoid 
membrane,  and  from  below,  through  an  opening  in  the  trachea  that 
when  an  act  of  deglutition  was  excited  by  touching  the  soft  palate  with 
a  sound,  there  was  simultaneously  with  the  contraction  of  the  mylo- 
hyoid muscles,  a  firm  closure  of  the  glottis.  This  was  accomplished 
by  an  approximation  of  the  true  vocal  bands,  a  close  approximation  and 
a  downward  and  forward  movement  of  the  arytenoid  cartilages,  until 
they  almost  touched  the  anterior  wall  of  the  thyreoid  cartilage.  This 
movement  preceded  the  ascent  of  the  larynx.  When  the  larynx  was 
separated  from  all  surrounding  structures  with  the  exception  of  the 
laryngeal  nerves,  a  touch  of  the  palate  excited  the  same  phenomena. 
Under  such  circumstances  the  closure  of  the  glottis  must  have  been 
due  to  the  contraction  of  its  own  intrinsic  muscles  and  in  consequence 
of  a  reflex  action  through  the  inferior  laryngeal  nerves. 

The  Nerve  Mechanism  of  Deglutition. — Deglutition  is  almost 
exclusively  a  reflex  act  throughout  its  entire  extent,  and  requires  for 
its  inauguration  merely  a  stimulus  to  some  portion  of  the  mucous 
membrane  of  the  deglutitory  canal.  The  first  stage  is  primarily 
voluntary,  but  from  inattention  to  the  process  may  become  secon- 
darily reflex.  The  origin  and  course  of  the  afferent  nerves,  stimu- 
lation of  which  excite  reflexly  the  movements  of  the  pharynx  and 
esophagus,  however,  are  practically  unknown.  In  the  rabbit  deg- 
lutition can  be  excited  by  stimulating  the  anterior  central  part  of 
the  soft  palate;  in  man  it  has  not  yet  been  possible  to  locate  an  area 
stimulation  of  which  will  give  rise  to  a  reflex  deglutitory  act.  Though 
electric  stimulation  of  the  superior  laryngeal  nerve  will  cause  reflex 
deglutitory  movements,  it  is  obvious  that  the  terminals  of  this  nerve 
can  not  be  the  source  of  the  natural  afferent  impulses.  Stimulation 
of  the  glossopharyngeal  nerve  causes  an  inhibition  of  the  movements. 

The  center  from  which  emanate  nerve  impulses  which  excite  the 
various  muscles  to  action  has  been  located  experimentally  in  the 
medulla  oblongata  just  above  the  alse  cinereae.  The  efferent  nerves 
comprise  branches  of  the  facial,  hypoglossal,  motor  filaments  of  the 
third  division  of  the  fifth  nerve,  motor  filaments  of  the  glossopharyn- 
geal and  vagus  derived  in  all  probability  directly  from  the  medulla 
oblongata.  Inasmuch  as  the  different  mechanisms  of  this  reflex,  act 
not  only  in  a  coordinate  but  sequential  manner,  it  would  appear  as  if 
the  deglutition  center  sent  out,  in  response  to  the  nerve  impulses  com- 
ing from  a  single  peripheral  area,' a  series  of  nerve  impulses  successively 
to  succeeding  portions  of  the  canal,  through  the  groups  of  nerve-cells 
corresponding  to  the  origins  of  the  efferent  nerves.  That  this  orderly 
and  progressive  peristalsis  usually  observed  is  due  to  a  sequence  of 
changes  in  the  central  nerve  system  is  shown  by  the  fact,  that  if  the 
esophagus  is  divided  or  a  ring  of  it  excised,  the  extremity  in  connection 
with  the  stomach  will  exhibit  a  well-marked  peristalsis  after  a  short 
interval,  when  an  act  of  deglutition  is  excited  in  the  customary  manner. 
The  efferent  nerve  fibers,  which  stimulate  the  esophageal  muscles  to 


DIGESTION.  171 

action  are  contained  in  the  trunk  of  the  vagi  nerves  for  after  their 
division  the  peristalsis  is  abolished. 

In  addition  to  this  primary  reflex  mechanism,  the  esophagus  appears 
to  possess  a  secondary  reflex  mechanism  consisting  of  a  series  of  reflex 
arcs,  whose  afferent  and  efferent  paths  are  found  in  the  trunk  of  the 
vagus  and  both  connected  with  successive  portions  of  the  esophagus. 
The  first  mechanism  is  temporarily  suspended  during  deep  anesthesia 
while  the  second  persists.     (Meltzer.) 

Though  the  peristalsis  of  the  esophagus  is  excited  by  nerve  impulses 
coming  through  the  vagus  nerves  and  is  abolished  by  their  division, 
Cannon  has  shown  by  means  of  the  Rontgen  rays  that  this  effect 
for  the  lower  portion  of  the  esophagus,  at  least  in  the  cat  and  monkey, 
is  of  a  temporary  duration  only,  lasting  from  one  to  several  days,  after 
which  a  peristalsis  again  develops  and  of  sufficient  vigor  to  force  food 
through  the  cardiac  orifice  into  the  stomach.  The  muscle  coat  of  this 
portion  of  the  esophagus  is  composed  of  non-striated  muscle-fibers,  is 
supplied  with  a  myenteric  nerve  plexus  and  resembles  lower  portions 
of  the  alimentary  canal.  It  is  capable  of  developing  a  peristalsis, 
solely  in  response  to  the  pressure  of  food  within  and  independent  of 
extrinsic  nerves. 

GASTRIC  DIGESTION. 

After  the  food  has  passed  through  the  esophagus  it  is  received  by 
the  stomach,  where  it  is  retained  for  a  variable  length  of  time,  during 
which  important  changes  are  induced  in  its  physical  and  chemic  com- 
position. The  disintegration  of  the  food  inaugurated  by  mastication 
and  insalivation  is  still  further  carried  on  in  the  stomach  by  the  sol- 
vent action  of  the  acid  fluid  there  present,  until  the  entire  mass  is 
reduced  to  a  liquid  or  semiliquid  condition. 

The  stomach  is  dilated  and  highly  specialized  portion  of  the 
alimentary  canal  intervening  between  the  esophagus  and  small  intes- 
tine. When  moderately  distended  with  food,  it  is  somewhat  conical 
or  pyriform  in  shape  and  slightly  curved  on  itself.  It  is  situated 
obliquely  and  in  some  individuals  almost  vertically  in  the  upper  part 
of  the  abdominal  cavity,  extending  from  the  left  hypochondrium  to 
the  right  of  the  epigastrium.  The  dimensions  and  capacity  of  the 
stomach  undergo  considerable  periodic  variation  according  to  the 
extent  to  which  it  is  distended  by  food.  In  the  average  condition  it 
measures  in  its  long  diameter  from  25  to  35  centimeters,  in  its  vertical 
diameter  at  the  cardia  15  centimeters,  in  its  antero-posterior  diameter 
from  11  to  12  centimeters.  The  capacity  of  the  stomach  varies 
from  1500  to  1700  c.c.  In  the  empty  condition  its  walls  are  con- 
tracted and  partly  in  contact,  and  the  entire  organ  is  drawn  up 
into  the  upper  part  of  the  abdominal  cavity.  The  opening  through 
which  the  food  passes  into  the  stomach  is  known  as  the  esoph- 
ago-gastric    orilice    or   the   cardia.     The   opening   through  which    it 


172 


TEXT-BOOK  OF  PHYSIOLOGY. 


passes  into  the  intestine  is  known  as  the  pylorus,  the  pyloric  or  gas  tro- 
duodenal  orifice.  Between  these  two  orifices  the  stomach  along  its 
upper  border  presents  a  curve  and  along  its  lower  border  a  much  larger 
curve,  known  as  the  lesser  and  greater  curvatures  respectively.  The 
left  end  of  the  stomach  is  termed  the  fundus  or  cardiac  end;  the  right, 
the  pyloric  end.  Passing  from  the  fundus  toward  the  pylorus,  the 
stomach  gradually  narrows,  and  at  a  point  situated  about  5  cm.  from 
the  pyloric  opening  it  frequently  presents  a  constriction  which  divides 
the  general  cavity  into  two  portions:  viz.,  the  fundus  and  the  antrum 
of  the  pylorus. 

The  walls  of  the  stomach  are  formed  by  four  distinct  coats  united 


Fig.  75.  —  Fibers  Seen  with  the  Stomach  Everted.  1,1.  Esophagus.  2. 
Circular  fibers  at  the  esophageal  opening.  3,  3.  Circular  fibers  at  the  lesser  curvature. 
4,  4.  Circular  fibers  at  the  pylorus.  5,  5,  6,  7,  8.  Oblique  fibers.  0,  10.  Fibers  of 
this  layer  covering  the  greater  pouch.  11.  Portion  of  the  stomach  from  which  these 
fibers  have  been  removed  to  show  the  subjacent  circular  fibers. — (Sappey.) 


by  areolar  tissue  and  named,  from  without  inward,  as  the  serous, 
muscle-,  submucous,  and  mucous. 

The  external  or  serous  coat  is  thin  and  transparent  and  formed  by  a 
reduplication  of  the  general  peritoneal  membrane. 

The  middle  or  muscle-coat  consists  of  three  layers  of  non-striated 
muscle-fibers,  named  from  their  direction  the  longitudinal,  circular, 
and  oblique  (Fig.  75).  The  longitudinal  fibers  are  most  abundant 
along  the  lesser  curvature  and  are  a  continuation  of  those  of  the 
esophagus;  over  the  remainder  of  the  stomach  they  are  thinly  scat- 
tered, but  toward  the  pyloric  orifice  they  are  more  numerous  and 
form  a  tolerably  thick  layer  which  becomes  continuous  with  the 
fibers  of  the  small  intestine.     The  circular  fibers  form  a  complete 


DIGESTION. 


173 


Mucosa. 


layer  encircling  the  entire  organ,  with  the  exception,  perhaps,  of  a 
portion  of  the  fundus.  The  fibers  of  this  coat  cross  the  longitudinal 
fibers  at  right  angles.  At  the  lower  end  of  the  esophagus  and  sur- 
rounding the  cardia  the  circular  muscle  fibers  form  a  true  sphincter 
which  is  known  as  sphincter  cardia.  At  the  junction  of  the  fundus 
with  the  pyloric  antrum  the  circular  fibers  are  arranged  in  a  well-de- 
fined bundle  termed  the  sphincter  antri  pylorici.  In  the  pyloric  region 
the  circular  fibers  are  more  closely  arranged,  forming  thick  well- 
defined  rings.  At  the  pyloric  opening  the  circular  fibers  are  again 
crowded  together  and  form 
a  distinct  muscle  band — the  f  Epithelium. 

sphincter  pylori  —  which 
projects  for  some  distance 
into  the  interior  of  the 
stomach.  It  has  been 
stated  by  Riidinger  that 
the  inner  fibers  of  the  lon- 
gitudinal coat  become  con- 
nected with  this  circular 
band  and  constitute  a  dis- 
tinct muscle,  the  dilatator 
pylori.  The  oblique  fibers 
are  most  distinct  over  the 
cardiac  portion  of  the 
stomach,  but  extend  from 
left  to  right  as  far  as  the 
junction  of  the  middle  and 
last  thirds  of  the  stomach. 
They  are  continuations  of 
the  circular  fibers  of  the 
esophagus. 

The  submucous  coat  con- 
sists of  loose  areolar  tissue 
carrying  blood-vessels, 
nerves,  and  lymphatics.  It 
serves  to  unite  the  muscle 

to  the  mucous  coat.  Its  inner  surface  bears  a  thin  layer  of  muscular 
tissue,  the  muscularis  mucosa,  which  supports  the  mucous  membranes. 
The  internal  or  mucous  coat  is  loosely  attached  to  the  muscular 
coat.  In  the  empty  and  contracted  state  of  the  stomach  it  is  thrown 
into  longitudinal  folds,  or  ruga?,  which  are,  however,  obliterated  when 
the  organ  is  distended  with  food.  The  mucous  membrane  in  adult 
life  is  smooth  and  velvety  in  appearance,  gray  in  color,  and  covered 
with  a  layer  of  mucus.  Its  average  thickness  is  about  one  millimeter. 
The  surface  of  the  membrane  is  covered  with  a  layer  of  columnar 
epithelial  cells,  each  of  which  possesses  a  nucleus  and  nucleolus.  At 
the  pylorus  there  is  a  circular  involution  of  the  mucous  membrane 


Serosa 


Fig.  76. — Transverse  Section  of  the  Wall 
of  A  Human  Stomach.  X  15-  The  tunica  pro- 
pria contains  glands  standing  so  close  together  that 
its  tissue  is  visible  only  at  the  base  of  the  glands 
toward  the  muscularis  mucosa*. — (Stolir.) 


J74 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  is  known  as  the  pyloric  valve.  This  is  strengthened  by  fibrous 
tissue  and  embraced  by  the  sphincter  muscle  previously  described. 
Gastric  Glands. — The  surface  of  the  mucous  membrane  when 
examined  with  a  low  magnifying  power  presents  throughout  innumer- 
able depressions  polygonal  in  shape  and  separated  by  slightly  elevated 

ridges.  At  the  bottom  of  these 
spaces  are  to  be  seen  small  orifices, 
which  are  the  mouths  of  the  glands 
embedded  in  the  mucous  mem- 
brane. A  vertical  section  of  the 
gastric  walls  (Fig.  76)  shows  not 
only  the  position  and  appearance  of 
the  glands,  but  the  relation  of  the 
various  tissues  which  enter  into  the 
formation  of  these  walls.  An  ex- 
amination of  the  mucous  membrane 
in  different  regions  of  the  stomach 
reveals  two  distinct  types  of  glands, 
cardiac  or  fundic,  and  pyloric,  which 


d-i 


Fig.  77. — Peptic  Gland  from 
Stomach  of  Dog.  a.  Wide 
mouth  and  duct  which  receive 
the  terminal  divisions  of  the 
gland,  b,  c.  Neck  and  fundus  of 
the  tubes,  e.  Central  or  chief 
cells,  d.  Parietal  or  acid  cells. 
— (After  Pier  sol.) 


u 

v&*ri 

fe 

1 

L.v 

''~'&\ ' 

Cj 

.Lumen. 


Secretory 
capillaries. 


•-.';' 


Fig.  78. — Section  of  Fundus  Gland 
OF  Mouse.  Left  upper  half  drawn  after 
an  alcohol  preparation,  right  upper  half 
after  a  Golgi  preparation.  The  entire 
lower  portion  is  a  diagrammatic  combi- 
nation of  both  preparations. — (Stohr.) 


differ  not  only  in  histologic  structure,  but  also  in  function.     Both  types 
extend  through  the  entire  thickness  of  the  mucosa. 

The  cardiac  or  fundic  glands  are  formed  by  an  involution  of  the 
basement  membrane  of  the  mucosa  and  lined  by  epithelial  cells. 
Each  gland  may  be  said  to  consist  of  a  short  duct,  or  neck,  and  a  body, 
or  fundus  (Fig.  77).  The  latter  portion  is  wavy  or  tortuous  and  fre- 
quently subdivided  into  as  many  as  8  to  10  distinct  and  separate 
tubules.     The  duct  is  lined  by  columnar  epithelial  cells  similar  to 


DIGESTION. 


i75 


those  covering  the  surface  of  the  mucosa.  The  lumen  of  the  fundus 
is  bordered  by  epithelial  cells,  cuboid  in  shape,  and  consisting  of  a 
granular  protoplasm  containing  a  distinct  spherical  nucleus.  These  cells 
are  generally  spoken  of  as  the  chief  or  central  cells.  In  addition  to  the 
chief  cells,  the  fundus  contains  a  second  variety  of  cell,  which  is  of  a 
larger  size,  of  a  triangular  or  oval  shape,  and  consisting  of  a  finely 
granular  protoplasm.  From  their  situation  in  and  just  beneath  the 
gland  wall  they  have  been  termed  parietal  or  border  cells.  Each  parie- 
tal cell  appears  to  be  surrounded  and  penetrated  by  a  system  of  pas- 
sages which  open  into  the  lumen  of  the  gland  by  means  of  a  delicate 
cleft  or  canaliculus  (Fig.  78).  Glands  with  these  histologic  features 
are  most  abundant  in  the  middle  zone  of  the  stomach. 

The  pyloric  glands  are  also  formed  by 
an  involution  of  the  mucous  membrane 
and  lined  by  epithelial  cells  (Fig.  79). 
The  ducts  are  much  longer  than  the 
ducts  of  the  fundic  glands.  At  its  ex- 
tremity each  duct  becomes  branched, 
giving  rise  to  a  number,  from  2  to  16,  of 
short  tubes,  each  of  which  has  a  large 
lumen  and  communicates  with  the  duct 
by  a  narrow  short  neck.  The  ducts  are 
lined  throughout  by  columnar  epithelium. 
According  to  Mall,  the  total  number  of 
openings  on  the  surface  of  the  mucous 
membrane  of  the  dog's  stomach  is  some- 
what over  1,000,000,  and  the  total  num- 
ber of  blind  tubes  opposite  the  muscularis 
mucosa  exceeds  16,500,000.  According 
to  Sappey,  the  surface  of  the  mucous 
membrane  of  the  human  stomach  pre- 
sents over  5,000,000  orifices  of  gastric 
glands. 

Blood-vessels,  Nerves,  and  Lymphatics. — The  blood-vessels 
of  the  stomach  after  entering  the  mucosa  break  up  into  a  number  of 
branches  which  are  distributed  to  the  muscular  and  mucous  coats. 
The  branches  to  the  latter  soon  form  a  capillary  network  with  oblong 
meshes  which  not  only  surround  the  tubules  but  form  a  network  just 
beneath  the  surface  of  the  mucosa.  Veins  gradually  arise  from  the 
capillaries  which  empty  into  the  larger  veins  of  the  mucosa.  The 
glands  are  also  supported  by  processes  of  smooth  muscle-fibers  passing 
up  from  the  muscularis  mucosa. 

The  nerve-fibers  distributed  to  the  stomach  are  derived  from  the 
vagus  and  the  sympathetic  branches  of  the  solar  plexus.  After  pierc- 
ing the  serous  coat  the  fibers  form  or  unite  with  a  plexus  of  fibers  sit- 
uated between  the  circular  and  longitudinal  layers  of  the  muscle-coat. 
At  1  he  nodal  points  of  this  plexus  large  nerve-ganglion  cells  are  to  be 


Fig.  79. — Section  of  Pylo- 
ric Glands  from  Human  Stom- 
ach, a.  Mouth  of  gland  leading 
into  long,  wide  duct  (6):  into 
which  open  the  terminal  divisions. 
c.  Connective  tissue  of  the  mu- 
cosa.— (After  Piersol.) 


176  TEXT-BOOK  OF  PHYSIOLOGY. 

found,  the  whole  forming  the  mechanism  known  as  Auerbach's 
plexus.  A  similar  plexus  of  cells  and  fibers  in  more  or  less  intimate 
anatomic  connection  with  the  foregoing  is  found  between  the  muscle 
and  submucous  coats,  and  is  known  as  Meissner's  plexus.  From 
this  plexus  fine  nerve  filaments  are  distributed  to  muscle-fibers, 
blood-vessels,  and  glands.  In  the  latter  structure  terminal  arbori- 
zations have  been  detected  in  close  contact  with  the  secreting  cells 
themselves. 

The  lymphatics,  which  are  quite  numerous,  originate  in  the  meshes 
of  the  mucosa.  The  larger  trunks  enter  lymph-glands  lying  along  the 
greater  and  lesser  curvatures  of  the  stomach. 

Gastric  Fistulae. — The  general  process  of  digestion,  as  it  takes 
place  in  the  stomach,  has  been  studied  in  human  beings  and  animals 
with  a  fistula  in  the  walls  of  the  stomach  and  abdomen,  the  result 
either  of  accident  or  of  necessary  surgical  or  experimental  procedures. 

The  earliest  observations  on  gastric  digestion  were  made  by  Dr. 
Beaumont  on  Alexis  St.  Martin,  who,  as  the  result  of  a  gunshot 
wound,  was  left  with  a  permanent  fistulous  opening  into  the  fundus 
of  the  stomach.  This  opening  two  years  after  the  accident  was  about 
two  and  a  half  inches  in  circumference  and  usually  closed  from  within 
by  a  fold  of  mucous  membrane  which  prevented  the  escape  of  the  food. 
This  valve  could  be  readily  displaced  by  the  finger  and  the  interior 
of  the  stomach  exposed  to  view.  After  the  complete  recovery  of  St. 
Martin,  Dr.  Beaumont  during  the  years  between  1825  and  183 1  at 
intervals  made  numerous  experiments  on  the  nature  of  gastric  diges- 
tion. As  the  result  of  an  admirable  series  of  investigations  it  was 
established  that  the  digestion  of  the  food  is  largely  a  chemic  act,  due 
to  the  presence  of  an  acid  fluid  secreted  by  the  mucous  membrane; 
that  this  fluid  is  secreted  most  abundantly  after  the  introduction  of 
food  into  the  stomach;  that  different  articles  of  food  possess  varying 
degrees  of  digestibility;  that  the  duration  of  digestion  varies  according 
to  the  nature  of  the  food,  exercise,  mental  states,  etc.,  and  that  the 
process  is  aided  by  continuous  movements  of  the  muscular  walls. 

Since  Dr.  Beaumont's  time  the  establishing  of  a  gastric  fistula  in 
human  beings  has  been  necessitated  by  pathologic  conditions  of  the 
esophagus.  After  recovery  these  cases  offered  fair  facilities  for  the 
study  of  the  process  when  the  food  was  introduced  through  the  opening. 
Similar  fistulae  have  been  established  in  both  carnivorous  and  her- 
bivorous animals  with  a  view  of  studying  the  process  as  it  takes  place 
in  them.  The  results  obtained  in  these  instances  in  many  respects 
corroborate  those  obtained  by  Dr.  Beaumont,  though  many  new  facts, 
unobserved  by  him,  have  been  brought  to  light. 

Much  additional  information  as  to  the  mode  of  secretion  and  the 
characteristics  of  the  gastric  juice  has  been  obtained,  since  the  intro- 
duction of  two  new  procedures  by  Pawlow.  The  first  consists  in 
establishing  a  gastric  fistula  and  subsequently  dividing  the  esophagus 
in  the  neck,  and  then  so  adjusting  the  divided  ends  that  they  heal 


DIGESTION. 


177 


separately  into  an  angle  of  the  skin  incision.  The  second  procedure 
consists  in  forming  a  diverticulum  or  pouch  out  of  the  cardiac  end  of 
the  stomach  which  opens  on  the  surface  of  the  abdomen  but  is  sepa- 
rated from  the  rest  of  the  stomach  by  a  thin  septum  formed  of  two 
layers  of  mucous  membrane.  (Fig.  80.)  The  serous  and  muscle-coats 
of  this  pouch  are  in  direct  continuity  with  the  large  stomach  and  all 
possess  the  same  vascular  and  nerve  connections.  Because  of  this  fact 
this  miniature  stomach,  about  one-tenth  the  size  of  the  natural  stomach, 
exhibits  the  same  phenomena,  so  far  as  the  secretion  of  the  gastric 
juice  is  concerned,  that  the  large 
stomach  does.  The  phenomena  which 
are  observed  in  it  may  be  taken  as  an 
indication  as  to  the  phenomena  which 
are  taking  place  in  the  natural  stomach. 

By  the  first  procedure  it  is  possible 
to  feed  an  animal  with  different  kinds 
of  food  and  to  observe  the  effects  of 
psychic  states  on  the  secretion  of  gas- 
tric juice.  As  the  swallowed  food  is 
discharged  from  the  lower  end  of  the 
divided  esophagus  the  appetite  con- 
tinues, and  hence  the  animal  will  eat 
for  several  hours.  By  the  second 
procedure  it  is  possible  to  collect  gas- 
tric juice  from  the  miniature  stomach 
and  to  study  the  effects  on  its  quantity 
and  quality  produced  by  psychic 
states,  mastication,  different  articles  of 
food,  and  by  the  process  of  digestion 
itself  as  it  goes  on  in  the  large  stomach. 
In  both  instances  the  juice  is  obtained 
free  from  admixture  with  saliva  or  food. 

Gastric  Juice. — The  gastric  juice  obtained  from  the  human 
stomach  free  from  mucus  and  other  impurities  is  a  clear,  colorless 
fluid  with  a  constant  acid  reaction,  a  slightly  saline  and  acid  taste, 
and  a  specific  gravity  varying  from  1.002  to  1.005.  The  juice  ob- 
tained from  the  dog's  stomach  possesses  essentially  the  same  char- 
acteristics, though  its  acidity  as  well  as  its  specific  gravity  are  slightly 
greater.  When  kept  from  atmospheric  influences,  it  resists  putre- 
factive change  for  a  long  period  of*  time,  undergoes  no  apparent 
change  in  composition,  and  loses  none  of  its  digestive  power.  It 
will  also  prevent  and  even  arrest  putrefactive  change  in  organic 
matter.  The  chemic  composition  of  the  gastric  juice  has  never  been 
satisfactorily  determined,  owing  to  the  fact  that  the  secretion  as 
obtained  from  fistulous  openings  has  not  been  absolutely  normal. 
The  following  analyses  represent  the  composition  of  a  sample  obtained 
by  Schmidt  from  the  stomach  of  a  woman  who  had  a  fistula,  but  who 


Fig.  So. — Diagram  Showing 
the  Relation  of  the  Natural 
Stomach  to  the  Miniature 
Stomach  or  Pouch  made  Ac- 
cording to  the  Procedure  De- 
vised by  Pawlow.  V.  The  nat- 
ural stomach.  5.  The  minature 
stomach,  e,  e.  The  septum  formed 
by  the  mucous  membrane.  A,  A. 
The  abdominal  walls. 


178  TEXT-BOOK  OF  PHYSIOLOGY. 

was  nevertheless  in  good  health;  also  the  composition  of  the  juice  from 
a  dog: 

COMPOSITION  OF  GASTRIC  JUICE. 

Human.  Dog. 

Water, 994-4°  973-°6 

Organic  matter, 3.19  I7-13 

Hydrochloric  acid, 0.20  ?  3.34 

Calcium  chlorid, 0.06  0.26 

Sodium  chlorid, 1.46  2.50 

Potassium  chlorid, 0.55  1.12 

Calcium    phosphate]  1.73 

Magnesium     "           [ 0.12  0.23 

Ferric                "  0.08 

Ammonium  chlorid, 0.47 

The  organic  matter  present  in  gastric  juice  is  a  mixture  of  mucin 
and  a  proteid,  products  of  the  metabolic  activity  of  the  epithelial 
cells  on  the  surface  of  the  mucous  membrane  and  of  the  chief  or 
central  cells  of  the  gastric  glands  respectively.  Associated  with  the 
proteid  material  are  two  ferment  or  enzyme  bodies,  termed  pepsin 
and  rennin.  As  is  the  case  with  other  enzymes,  their  true  chemic 
nature  is  practically  unknown. 

Pepsin,  though  present  in  gastric  juice,  is  not  present  as  such  in 
the  chief  cells  of  the  glands,  but  is  derived  from  a  zymogen,  propepsin 
or  pepsinogen,  when  the  latter  is  treated  with  hydrochloric  acid. 
This  antecedent  compound  is  related  to  the  granules  observed  in  and 
produced  by  the  cell  protoplasm  during  the  period  of  rest.  Though 
pepsin  is  largely  produced  by  the  central  cells  of  the  fundic  glands, 
it  is  also  produced,  though  in  less  amount,  by  the  cells  of  the  pyloric 
glands.  Pepsin  is  the  chief  proteolytic  agent  of  the  gastric  juice  and 
exerts  its  influence  most  energetically  in  the  presence  of  hydrochloric 
acid  and  at  a  temperature  of  about  400  C.  Other  acids —  e.  g.,  phos- 
phoric, nitric,  lactic,  etc. — are  also  capable  of  exciting  it  to  activity, 
though  with  less  intensity. 

Rennin  or  pexin  is  present  in  the  gastric  juice  not  only  of  man  and 
all  the  mammalia,  but  also  of  birds  and  even  fish.  In  its  origin  from  a 
zymogen  substance,  in  its  relation  to  an  acid  medium  and  an  optimum 
temperature,  it  bears  a  close  resemblance  to  pepsin.  Its  specific 
action  is  the  coagulation  of  milk,  a  condition  due  to  the  cleavage  of 
caseinogen  into  a  solid  flaky  body,  casein,  and  a  soluble,  albumin. 

Hydrochloric  acid  is  the  agent  which  gives  to  the  gastric  juice  its 
normal  acidity.  Though  the  juice  frequently  contains  lactic,  acetic, 
and  even  phosphoric  acids,  it  is  generally  believed  that  they  are  the 
result  of  fermentation  changes  occurring  in  the  food,  the  result  of 
bacterial  action.  The  percentage  of  hydrochloric  acid  has  been  the 
subject  of  much  discussion.  The  analysis  of  human  gastric  juice 
made  by  Schmidt  shows  a  percentage  of  0.02,  while  that  of  the  dog 
is  0.34.  it  i.  probable,  however,  that  the  low  percentage  of  HC1  in 
human  gastric  juice  was  due  to  the  admixture  with  saliva.     At  present 


DIGESTION.  i79 

it  is  believed  from  analyses  made  for  clinical  purposes  that  the  acid  is 
present  to  the  extent  of  at  least  0.2  per  cent.  This  degree  of  acidity  is 
not  constant  during  the  entire  process  of  digestion.  In  the  earlier  as 
well  as  in  the  later  stages  it  is  much  less. 

The  immediate  origin  of  the  hydrochloric  acid  is  difficult  of  ex- 
planation. That  it  is  derived,  however,  primarily  from  the  chlorids 
of  the  food  and  secondarily  from  the  blood-plasma  has  been  established 
by  direct  experiment.  If  all  the  chlorids  be  removed  from  the  food 
and  all  chlorids  be  withdrawn  from  the  animal  tissue  by  the  adminis- 
tration of  various  diuretics — e.  g.,  potassium  nitrate — there  will  be 
a  total  disappearance  of  hydrochloric  acid  from  the  stomach.  On 
the  addition  of  sodium  or  potassium  chlorids  to  the  food,  there  is  at 
once  a  reappearance  of  the  acid. 

As  to  the  nature  of  the  process  by  which  the  acid  is  formed,  noth- 
ing definite  is  known.  Various  theories  of  a  chemic  and  physical 
character  have  been  offered,  all  of  which  are  more  or  less  unsatis- 
factory. As  no  hydrochloric  acid  is  found  either  in  the  blood  or 
lymph,  the  most  plausible  view  as  to  its  origin  is  that  which  regards 
it  as  one  of  the  products  of  the  metabolism  of  the  gland-cells,  and  more 
particularly  of  the  parietal  or  border  cells,  and  which  for  this  reason 
have  been  termed  acid-producing  or  oxyntic  cells.  From  the  chlorids 
furnished  by  the  blood  the  chlorin  is  derived,  which,  uniting  with 
hydrogen,  forms  the  HC1.  The  base  set  free  returns  to  the  blood, 
which  in  part  accounts  for  its  increased  alkalinity  during  digestion 
as  well  as  the  diminished  acidity  of  the  urine.  The  acid  thus  formed 
passes  through  the  canaliculi,  which  penetrate  and  surround  the  cells, 
into  the  lumen  of  the  gland. 

Hydrochloric  acid  exerts  its  influence  in  a  variety  of  ways.  It  is  the 
main  agent  in  the  derivation  of  pepsin  and  rennin  or  pexin  from  their 
antecedent  zymogen  compounds,  pepsinogen  and  pexinogen  (Warren) ; 
it  imparts  activity  to  these  ferments;  it  prevents  and  even  arrests  fer- 
mentative and  putrefactive  changes  in  the  food  by  destroying  micro- 
organisms; it  softens  connective  tissue,  it  dissolves  proteids  and  acid- 
ifies the  proteids,  thus  making  possible  the  subsequent  action  of  pepsin. 

The  inorganic  salts  of  the  gastric  juice  are  probably  only  inci- 
dental and  play  no  part  in  the  digestive  process. 

Mode  of  Secretion. — The  observations  of  Dr.  Beaumont  and 
the  experiments  of  many  physiologists  have  made  it  certain  that  the 
secretion  of  the  gastric  juice  is  intermittent  and  not  continuous,  that 
it  is  only  on  the  introduction  and  digestion  of  the  food  that  the  normal 
amount  is  poured  out.  During  the  intervals  of  digestive  activity  the 
stomach  is  practically  free  from  all  traces  of  the  juice.  The  mucous 
membrane  is  pale  and  covered  with  a  layer  of  mucus  having  an  alka- 
line or  neutral  reaction.  The  introduction,  however,  of  small  por- 
tions of  food  or  irritation  with  a  glass  rod  causes  a  change  in  the  ap- 
pearance of  the  mucous  membrane.  At  the  points  of  irritation  the 
membrane  becomes  red  and  vascular  and  in  a  few  minutes  small 


1S0  TEXT-BOOK  OF  PHYSIOLOGY. 

drops  of  a  secretion  make  their  appearance;  these  coalesce  and  run 
down  the  sides  of  the  stomach. 

The  statements  of  Beaumont  and  many  subsequent  investigators 
that  the  secretion  thus  obtained  is  gastric  juice  have  been  apparently- 
disproved  by  Pawlow,  who  asserts  that  it  is  only  an  alkaline  mucous 
the  function  of  which  is  protective  in  character.  Mechanic  stimula- 
tion is  incapable  of  exciting  the  secretion. 

The  primary  stimulus  to  gastric  secretion,  according  to  Pawlow, 
is  a  psychic  state  induced,  on  the  one  hand,  by  the  sight  or  the  odor  of 
food  especially  if  the  animal  is  hungry  and  the  food  appetizing;  and 
on  the  other  hand  by  the  mastication  of  food  which  is  agreeable  to  the 
animal.  Thus  when  a  dog  was  tempted  by  the  sight  of  food,  the 
secretion  made  its  appearance  at  the  end  of  six  minutes  and  during 
the  time  of  the  experiment,  which  lasted  for  an  hour  and  a  half,  80 
cubic  centimeters  of  the  juice  were  obtained.  This  is  known  as 
psychic  or  appetite  juice.  The  character  of  a  psychic  state,  however, 
greatly  influences  the  amount  of  the  juice  secreted.  Agreeable  emo- 
tions increase,  depressing  emotions  inhibit  it.  Again  when  a  dog 
with  a  divided  esophagus  and  a  gastric  fistula  was  subjected  to  sham 
feeding,  mastication  continued  for  five  or  six  hours  during  which 
time  700  cubic  centimeters  of  juice  were  obtained  from  the  stomach. 
Similar  results  have  been  obtained  in  human  beings  with  an  occluded 
esophagus  and  a  gastric  fistula.  It  is  evident  from  these  facts  that  the 
secretion  of  gastric  juice  is  favorably  influenced  by  the  sight  and  odor 
of  appetizing  food,  by  exhilarating  emotional  states  and  thorough 
mastication. 

Though  the  secretion  of  the  gastric  juice  can  be  initiated  by  these 
means,  the  amount  secreted  is  but  small  compared  with  the  quantity 
secreted  after  digestion  has  begun.  Then  it  is  that  the  blood-vessels 
dilate  to  their  full  capacity  and  furnish  for  several  hours  the  requisite 
materials  for  the  production  of  the  juice  on  a  relatively  large  scale. 
That  some  factor  is  active  and  keeping  up  the  secretion  in  the  large 
stomach,  is  apparent  from  the  increase  in  the  quantity  and  the  change 
in  the  quality  of  the  juice  secreted  by  the  miniature  stomach.  This 
secondary  stimulus  to  the  gastric  secretion  is  in  all  probability  chemic 
in  character  and  developed  in  the  stomach  or  in  its  walls  during  diges- 
tive activity,  inasmuch  as  the  secretion  takes  place  independent  of  nerve 
influences  and  after  division  of  all  afferent  and  efferent  nerves  that 
pass  from  and  to  the  stomach. 

On  the  assumption  that  this  factor  might  be  developed  in  the  walls 
of  the  stomach  itself,  Edkins  conducted  a  series  of  experiments,  the  re- 
sults of  which  lead  to  the  inference  that  there  is  developed  in  the  pyloric 
mucous  membrane,  by  the  action  of  the  first  products  of  digestive 
activity  a  (hemic  agent  which,  absorbed  by  the  blood,  is  carried  to 
the  glands  throughout  the  stomach  and  which  stimulates  their  cells  in  a 
specific  manner.  For  this  reason  it  has  been  called  the  gastric  hor- 
mone or  the  gastric  secretin.     Whatever  the  agent  or  the  mechanism 


DIGESTION.  181 

may  be,  there  is  not  only  an  increase  in  the  quantity  but  a  change  in  the 
quality  of  the  juice  in  accordance  with  the  character  of  the  food;  in 
other  words,  there  is  an  adaptation  of  the  juice  to  the  kind  of  food  to 
be  digested.  Thus  the  protein  of  bread  causes  a  secretion  of  five  times 
more  pepsin  than  the  same  amount  of  the  protein  of  milk,  while  the 
protein  of  meat  causes  secretion  of  25  percent,  more  pepsin  than  milk. 
Meat  extract  and  bouillon  have  a  very  stimulating  effect  on  the  quan- 
tity of  juice  produced,  while  alkalies  have  an  inhibitor  effect. 

Histologic  Changes  in  the  Gastric  Cells  during  Secretion. — 
During  the  periods  of  rest  and  secretor  activity  the  cells  of  the  gas- 

.....h 


>pA  9 a  * ■*99»ze^*f?>\ 


,pr 


^fef®»  S@®^ 


*■"■', 

Fig.  81. — Sections  of  Deep  Ends  of  Fundus  Glands  of  the  Cat  in  Different 
Secretive  Phases.  X  1000. — (Benstey.)  A.  From  a  fasting  stomach.  The  chief  cells 
are  filled  with  large  zymogen  granules;  nuclei  near  the  outer  ends  of  cells.  Gentian- 
violet  preparation,  b  b  b.  Border  cells.  B.  Six  hours  after  an  abundant  meal  of  raw 
flesh.  The  chief  cells  exhibit  two  zones,  the  inner  occupied  by  large  zymogen  granules, 
the  outer  by  a  deeply  staining,  obscurely  fibrillar  element,  prozymogen;  the  nuclei  lie  at  the 
junction  of  the  two  zones,  bbb.  Border  cells,  pr.  Prozymogen.  c.  Mucin-secreting 
cell,  similar  to  those  found  in  the  neck  of  the  gland.  Gentian-violet  preparation. — 
(Hcmmeter  after  Bensley.) 

trie  glands  undergo  changes  in  histologic  structure  which  are  believed 
to  be  connected  with  the  production  of  the  enzymes,  pepsin  and  rennin, 
and  the  acid.  In  the  resting  period  the  protoplasm  of  the  chief  or  central 
cells  of  the  fundus  glands  becomes  crowded  with  large  and  well- 
defined  granules,  which  during  the  period  of  secretory  activity  largely 
disappear,  so  much  so,  that  only  the  luminal  border  of  the  cell  is 
occupied  by  them,  the  outer  border  being  clear  and  hyaline  in  appear- 
ance. The  parietal  cells  during  rest  are  large  and  finely  granular, 
but  after  secretion  they  are  smaller  in  size  though  still  granular.  (See 
Fig.  81,  A  and  B.) 

The  cells  of  the  pyloric  glands,  though  containing  granules,  do  not 
show  any  marked  difference  between  the  resting  and  active  condition. 
According  to  some  observers,  they  contain  pepsinogen;  according  to 
others,  mucin.     The  epithelial  cells  lining  the  ducts  of  the  pylorus 


i82  TEXT-BOOK  OF  PHYSIOLOGY. 

and  fundus  glands,  if  not  identical  with  the  epithelial  cells  on  the  sur- 
face of  the  mucous  membrane,  pass  by  transitional  forms  into  them. 
Among  these  cells  are  found  many  goblet  cells  which  secrete  a  portion 
of  the  mucus  found  in  the  stomach  and  gastric  juice.  In  the  period 
of  rest  the  protoplasm  of  the  epithelial  cells  absorbs  and  assimilates 
from  the  surrounding  lymph-spaces  material  which  eventually  makes 
its  reappearance  as  a  product  of  metabolism  in  the  form  of  granules 
and  hydrochloric  acid.  With  the  onset  of  digestive  activity  there  is  a 
dilatation  of  the  blood-vessels,  an  increase  in  the  blood-supply,  a 
stimulation  through  the  nerve-supply  of  the  cells,  and  an  output  of  a 
fluid  to  which  the  name  gastric  juice  is  given. 

Influence  of  the  Nerve  System. — The  primary  secretion  of  gas- 
tric juice  is  a  reflex  act  and  under  the  control  and  influence  of  the 
central  nerve  system.  Experimental  investigations  render  it  probable 
that  the  central  mechanism  is  located  in  the  medulla  oblongata  and 
that  the  efferent  path  lies  in  the  trunk  of  the  vagus  nerve.  Though 
this  nerve  has  been  the  subject  of  much  experimentation,  the  results 
which  have  been  obtained  have  not  been  uniform.  The  investigations 
of  Pawlow  seem  to  be  the  most  reliable.  He  found  that  after  division 
of  the  nerve,  secretion  was  arrested,  and  that  stimulation  of  the  per- 
ipheral ends  with  induced  electric  currents  at  the  rate  of  one  or  two 
per  second,  after  a  latent  period  of  several  minutes'  duration  caused  a 
flow  of  gastric  juice.  This  center  is  excited  to  activity  by  nerve  im- 
pulses descending  from  the  cerebrum  as  shown  by  the  effects  which 
follow  the  development  of  psychic  states  caused  by  the  sight  or  the 
odor  of  food,  as  well  as  by  the  mastication  of  agreeable  food;  for  with 
their  development  there  is  a  dilatation  of  blood-vessels  and  a  copious 
discharge  of  the  juice  in  a  few  minutes,  showing  the  cooperation  of 
both  vaso-motor  and  secretor  nerves.  It  is  uncertain  if  the  medullary 
center  can  be  influenced  by  nerve  impulses  reflected  from  the  stomach. 

The  Physiologic  Action  of  Gastric  Juice. — In  the  study  of  the 
physiology  of  gastric  digestion  as  it  takes  place  under  normal  con- 
ditions it  is  important  to  bear  in  mind  that  the  foods  introduced  into 
the  stomach  are  heterogeneous  compounds  consisting  of  both  nutritive 
and  non-nutritive  materials,  and  that  before  the  former  can  be  digested 
and  utilized  for  nutritive  purposes  they  must  be  freed  from  their 
combinations  with  the  latter.  This  is  accomplished  by  the  solvent 
action  of  the  gastric  juice,  which  in  virtue  of  the  chemic  activity  of  its 
constituents  cm  proteids,  gradually  disintegrates  the  food  and  reduces 
it  to  the  liquid  or  semiliquid  condition. 

The  nature  of  this  change  and  the  respective  influence  which  the 
acid  and  pepsin  exert  can  be  studied  with  almost  any  form  of  proteid. 
The  most  suitable  form,  however,  is  coagulated  fibrin  obtained  from 
blood  by  whipping  and  thoroughly  freed  from  blood  by  washing 
under  a  stream  of  water.  The  chemic  features  of  proteins,  as  well  as 
the  typical  forms  contained  in  the  different  articles  of  food,  have 
been  considered  in  connection  with  the  chemic  composition  of  the  body 


DIGESTION.  183 

and  the  composition  of  foods  (see  pages  16  and  139).  For  purposes  of 
experimentation  artificial  gastric  juice  may  be  employed.  This  is  as 
effective  as  the  normal  secretion  and  in  no  essential  respect  differs 
from  it.  A  glycerin  extract  of  the  mucous  membrane  acidulated 
with  0.2  per  cent,  hydrochloric  acid  is  probably  the  best. 

If  the  small  pieces  of  fibrin  be  suspended  in  clear  gastric  juice  and 
kept  at  a  temperature  of  1040  F.  (400  C.)  for  an  hour  or  two,  they  will 
be  dissolved  and  will  entirely  disappear,  giving  rise  to  a  slightly  opales- 
cent mixture.  In  the  early  stages  of  the  process  the  fibrin  becomes 
swollen  and  transparent  and  partly  dissolved.  If  at  this  time  the 
solution  be  carefully  neutralized,  the  dissolved  portion  can  be  regained 
in  the  form  of  acid-albumin  or  syntonin — a  fact  which  indicates  that 
the  first  effect  of  the  gastric  juice  is  the  acidification  of  the  proteids. 
This  having  been  accomplished,  the  pepsin  becomes  operative,  and 
in  a  varying  length  of  time  transforms  the  acid-albumin  into  a  new 
form  of  proteid,  termed  peptone.  This  form  of  proteid  differs  from 
all  other  forms  of  proteid  in  being  soluble  in  both  acids  and  alkalies 
and  non-coagulable  by  heat.  In  the  transformation  of  acid-albumin 
into  peptone  it  is  possible  to  isolate  by  the  addition  of  magnesium 
sulphate  and  ammonium  sulphate  intermediate  bodies  to  which  the 
term  albumoses  or  proteoses  has  been  given,  and  which  differ  somewhat 
in  their  solubility.  The  proteoses  are  termed,  from  the  order  in  which 
they  make  their  appearance,  primary  and  secondary.  The  primary 
proteoses  are  precipitated  by  magnesium  sulphate,  the  secondary  by 
ammonium  sulphate.  As  some  of  the  primary  proteoses  are  soluble 
in  water  while  others  require  in  addition  sodium  chlorid  for  their  solu- 
tion, they  have  been  divided  into  two  groups — viz. :  proto-  and  hetero- 
albumoses.  The  secondary  proteoses  or  deutero-albumoses  are  solu- 
ble in  water.  Though  in  the  subjoined  scheme  two  forms  of  deutero- 
albumose  are  represented  and  two  forms  of  peptone  developed  out  of 
them,  the  results  of  chemic  investigation  would  indicate  that  there  is 
but  one  form  of  deutero-albumose  and  hence  but  one  form  of  peptone. 
This  supposed  change  produced  by  gastric  juice  is  represented  by  the 
following  scheme: 

Albumin 
Acid-albumin 

Proto-albumose  =  (p  -jeose  )  =  Hetero-albumose 

Deutero-albumose(SpC°f dary)  =  Deutero-albumose 
V  Proteoses' 

Peptone         (Ampho-peptones)  Peptone. 

From  the  fact  that  when  peptones  are  subjected  to  the  prolonged 
action   of   pancreatic   juice   there   arise   compounds   such   as   leucin, 


i84  TEXT-BOOK  OF  PHYSIOLOGY. 

tyrosin,  aspartic  acid,  arginin,  etc.,  it  was  believed  that  two  kind 
of  peptones  were  formed  out  of  a  simple  protein  one  of  which  suc- 
cumbed to  the  destructive  action  of  pancreatic  juice,  while  the  other 
resisted  it;  for  this  reason  the  latter  was  termed  anti-  and  the  former 
hemi- peptone.  The  two  were  included  under  the  term  ampho- pep- 
tone. It  is  generally  admitted  now,  however,  that  the  body  termed 
anti-peptone  is  not  a  peptone  at  all,  but  a  compound  termed  carnic 
acid  and  which  is  also  separable  into  leucin,  tyrosin,  etc.  Hemi- 
peptone  has  never  been  isolated.  The  probabilities  are,  therefore, 
that  but  one  form  of  peptone  is  developed  from  any  given  simple  pro- 
tein. 

Nearly  all  forms  of  protein  are  in  a  similar  manner  transformed 
into  peptones  by  gastric  juice.  Beyond  this  stage,  however,  there 
does  not  seem  to  be  any  further  change,  peptones  apparently  being 
the  final  products  of  gastric  digestion.  The  intimate  nature  of  this 
change  is  practically  unknown,  but  there  are  reasons  for  thinking 
that  it  is  a  process  of  hydration,  attended  by  cleavage,  with  increasing 
solubility  of  the  resulting  products. 

Characters  of  Peptones. — The  peptones  resulting  from  the  diges- 
tion of  different  proteins,  though  resembling  each  other  in  many  re- 
spects, yet  possess  different  chemic  characteristics,  as  shown  by  their 
reaction  to  various  chemic  reagents.  Though  having  some  resem- 
blance to  the  proteids  from  which  they  are  derived,  they  are  to  be 
distinguished  from  them  by  the  following  general  characteristics: 
i.  They  are  not  coagulable  either  by  heat  or  by  nitric  acid. 

2.  They  are   soluble   in  water,  either   hot  or  cold,  and  in  acid  and 

alkaline  solutions. 

3.  They   are    diffusible,    passing    through   animal    membranes   with 

great  rapidity.  It  has  been  demonstrated  that  peptones  diffuse 
about  twelve  times  as  rapidly  as  the  proteids  from  which  they 
are  derived. 
Neither  on  fat  nor  starch  has  gastric  juice  any  appreciable  effect, 
and  when  these  substances  are  introduced  into  the  stomach  they 
pass  into  the  intestine  unchanged.  It  has  been  stated,  however,  that 
the  gastric  mucosa  produces  a  fat-splitting  enzyme,  but  that  its  action 
is  prevented  by  the  presence  of  the  hydrochloric  acid.  Notwithstanding 
the  fact  that  dilute  solutions  of  hydrochloric  acid  (0.3  per  cent.)  will 
promptly  invert  cane-sugar  into  dextrose  and  levulose,  and  that  gastric 
juice  will  accomplish  the  same  result  in  test-tubes,  there  is  no  strong 
evidence  for  the  belief  that  the  inversion  of  cane-sugar  takes  place  to 
any  marked  extent  in  the  stomach  under  normal  conditions.  The 
starch,  however,  which  has  been  subjected  to  the  action  of  the  saliva, 
still  continues  to  be  converted  into  maltose  during  the  first  fifteen  to 
thirty  minutes  or  even  longer.  Even  though  gastric  juice  is  being 
secreted  and  though  hydrochloric  acid  solutions  with  a  strength  of  0.3 
per  cent,  will  arrest  the  action  of  ptyalin,  starch  digestion  continues  for 
the  reason  that  the  acid  combines  with  the  proteids  and  is  thus  rendered 


DIGESTION.  185 

inoperative  and  for  the  further  reason  that  the  food  is  largely  retained 
in  the  extreme  cardiac  end  of  the  stomach  where  the  gastric  juice  is  not 
abundant.  After  the  above-mentioned  period,  free  acid  makes  its 
appearance  when  salivary  digestion  ceases. 

Action  of  Gastric  Juice  on  Foods. — The  action  of  gastric  juice 
on  proteids  affords  a  key  to  its  influence  in  the  reduction  of  foods  to 
the  liquid  or  semiliquid  condition.  It  is  evident  that  it  will  be  most 
active  in  the  digestion  of  food  consisting  largely  of  proteid  materials, 
such  as  meat,  eggs,  milk,  etc.  Meat  is  disintegrated  first  by  the  con- 
version of  the  proteids  of  the  connective  tissue,  which  have  been  more 
or  less  gelatinized  by  cooking,  into  peptones.  The  sarcolemma  of 
the  muscle-fibers  which  have  been  thus  separated  is  in  a  similar  man- 
ner attacked  and  converted  into  peptones.  The  true  muscle  or  sar- 
coid substance,  consisting  largely  of  myosin,  undergoes  a  correspond- 
ing change.  If  the  quantity  of  meat  be  not  too  large  and  the  gastric 
juice  be  secreted  in  proper  amount,  it  is  possible  that  all  the  meat  will 
be  digested  in  the  stomach.  It  is  quite  probable,  however,  that  this 
is  not  the  case  and  that  a  portion  of  the  semidigested  meat  passes  into 
the  intestine,  where  its  final  solution  is  effected. 

The  white  of  egg,  especially  when  slightly  boiled,  is  much  more 
readily  digested  than  when  raw  or  firmly  coagulated  by  prolonged 
boiling.  In  either  condition,  however,  the  connective  tissue  is  dis- 
solved and  peptonized,  after  which  the  native  albumin  undergoes  the 
same  change.  The  yolk  of  the  egg  consists  largely  of  fat  held  in  sus- 
pension by  a  proteid  substance,  vitellin,  which  is  also  capable  of  trans- 
formation into  peptone. 

Adipose  tissue  is  similarly  reduced.  The  proteids  of  the  con- 
nective tissue  and  of  the  fat  vesicles  are  dissolved  and  peptonized 
and  the  fat-drops  set  free. 

Milk  undergoes  a  peculiar  change  in  composition  before  its  proteid 
constituents  can  be  transformed  into  peptones.  The  caseinogen 
in  the  presence  of  calcium  salts  is  always  in  the  soluble  state.  When 
acted  on  by  the  gastric  juice,  the  caseinogen  undergoes  coagulation 
which  consists  in  the  formation  of  a  solid  compound,  casein,  and  a 
soluble  albumin.  This  change  is  due  to  the  presence  and  activity 
of  the  ferment,  rennin.  The  necessity  for  this  change,  however,  is 
not  apparent.  The  coagulated  casein  presents  itself  in  the  form  of  a 
flocculent  curd,  which  is  finer  in  human  than  in  cow's  milk,  and  hence 
more  easily  digestible.  The  casein  is  acidified  by  the  hydrochloric 
acid  and  then  converted  by  the  pepsin  into  peptone. 

Vegetables,  though  consisting  of  a  woody  or  cellulose  framework, 
undergo  a  partial  disintegration  in  the  stomach.  When  boiled  and 
physically  disintegrated  by  the  teeth,  the  gastric  juice  is  enabled  to 
penetrate  the  framework  and  dissolve  and  peptonize  the  various 
proteid  constituents.  As  a  general  rule,  the  vegetable  proteids  are 
more  difficult  of  digestion  than  the  animal  proteids. 

Duration  of  Gastric  Digestion.— The  length  of  time  the  food 


iS6 


TEXT-BOOK  OF  PHYSIOLOGY. 


remains  in  the  stomach  and  the  relative  digestibility  of  different 
articles  of  food  were  carefully  studied  by  Dr.  Beaumont  on  St.  Martin, 
and  though  the  results  obtained  by  him  may  not  be  absolutely  correct, 
viewed  in  the  light  of  recent  knowledge  of  the  digestive  process,  yet 
in  the  main  they  have  been  corroborated  in  various  ways.  As  a 
result  of  many  observations  Dr.  Beaumont  came  to  the  conclusion 
that  the  average  length  of  time  an  ordinary  meal  consisting  of  meat, 
bread,  potatoes,  etc.,  remained  in  the  stomach  undergoing  digestion 
was  about  three  and  a  half  hours,  the  duration  of  the  process,  how- 
ever, being  increased  when  an  excessive  quantity  of  food  was  taken  or 
the  quantity  and  quality  of  the  gastric  juice  impaired  by  abnormal 
conditions  of  the  system.  As  soon  as  the  food  is  liquefied  by  the 
gastric  juice  that  portion  not  absorbed  by  the  gastric  vessels  passes 
into  the  intestines,  this  continuing  for  two  to  three  hours  until  the 
stomach  is  completely  emptied.  The  relative  digestibility  of  the  differ- 
ent foods  was  also  made  the  subject  of  many  experiments  by  Dr. 
Beaumont.  After  repeating  and  verifying  his  observations  made 
under  varying  conditions,  he  summed  up  his  results  in  a  table,  of 
which  the  following  is  an  abstract,  in  which  the  mode  of  preparation 
and  the  time  required  for  the  digestion  of  different  foods  are  exhibited : 


TABLE  SHOWING  DIGESTIBILITY  QF  VARIOUS  ARTICLES  OF  FOOD 

Hours       Minutes. 

Eggs,  whipped, i  20 

"      soft  boiled, 3 

"      hard  boiled, 3  30 

Oysters,  raw, 2  55 

"       stewed,   3  30 

Lamb,  broiled, 2  30 

Veal,  broiled, 4 

Pork,  roasted, 5 

Beefsteak,  broiled, 3 

Turkey,  roasted, 2 

Chicken,  boiled, 4 

"         fricasseed, 2  45 

Duck,  roasted, 4 

Soup,  barley,  boiled, 1  30 

bean,         "      3 

"       chicken,    "       3 

"       mutton,    "        3  30 

Liver,  beef,  broiled, 2 

Sausage, 3  20 

Green  corn,  boiled, 3  45 

Beans,  "        2  30 

Potatoes,  roasted, 2  30 

boiler], 3  30 

Cabbage,       "       4  3° 

Turnips,         "       3  30 

Beets,  "       3  45 

Parsnips,       "       2  30 


Movements  of  the  Stomach. — During  the  period  of  gastric 
digestion  the  muscle  walls  of  the  stomach  become  the  seat  of  a  series 
of  movements,  peristaltic  in  character,  which  not  only  incorporate 


DIGESTION. 


187 


the  gastric  juice  with  the  food,  but  also  serve  to  eject  the  liquefied 

portions  of  the  food  into  the  small  intestine. 

The  movements  of  the  human  stomach  as  described  by  Beaumont, 

as  well  as  the  movements  of  the  dog's  stomach  as  stated  by  different 

observers,  are  not  in  agreement  in 
all  respects,  and  are,  moreover, 
open  to  question  for  the  reason 
that  they  were  not  observed  under 
strictly  physiologic  conditions.  The 
more  recent  investigations  of  Can- 
non have  thrown  new  light  on  this 
subject.  By  means  of  the  Rontgen 
rays  he  has  been  enabled  to  study 
the  movements  in  the  living  animal 
and  under  normal  conditions.  The 
animal  (the  cat)  was  fed  with  bread 
and  milk,  to  which  was  added  sub- 
nitrate    of    bismuth.       This    sub- 


Left 


Fig.  82. — Shadow  Sketches 
of  the  Outlines  of  the 
Stomach  of  a  cat  Immedi- 
ately after  a  Meal  (ii.o), 
and  at  Various  Intervals 
Afterward  (at  12.0,  at  2.0, 
3.30,  4.30). — (IF.  B.  Cannon.) 


Post 


Fig.  S3. — The  cardiac  portion  is  all  that 
part  to  the  left,  as  the  stomach  lies  in  the 
body,  of  WX.  The  cardia  is  at  C.  The 
pylorus  is  at  P,  and  the  pyloric  portion 
is  the  part  between  P  and  WX.  This 
has  two  divisions:  the  antrum,  between 
P  and  YZ,  and  the  pre-antral  part,  between 
WX  and  YZ.  The  lesser  curvature  is  on  the 
top  of  the  outline  between  C  and  P,  and  the 
greater  curvature  between  the  same  points 
along  the  lower  border. — (Amer.  Jour,  oj 
Physiology,  Cannon.') 


stance,  being  opaque,  rendered  the  movements  of  the  stomach  walls 
visible  on  the  fluorescent  screen.  With  paper  placed  over  the  screen 
it  was  possible  to  sketch  the  changes  in  shape  that  the  stomach  under- 
goes at  different  periods  of  the  digestive  act.  Some  of  these  changes 
are  represented  in  Fig.  82.  The  anatomic  features  of  the  cat  stomach 
of  interest  in  this  connection  are  represented  in  Fig.  83. 

These  investigations  show  that  different  portions  of  the  stomach 
walls  exhibit  different  forms  of  activity,  which  for  convenience  of 
description  are  separately  described  by  Cannon  as  follows : 

1.   The  Movements   oj  the  Pyloric  Part. — Within  five  minutes  after 


188  TEXT-BOOK  OF  PHYSIOLOGY. 

a  cat  has  finished  a  meal  of  bread  there  is  visible  near  the  duodenal 
end  of  the  antrum  a  slight  annular  contraction  which  moves  peristal- 
tically  to  the  pylorus;  this  is  followed  by  several  waves  recurring  at 
regular  intervals.  Two  or  three  minutes  after  the  first  movement 
is  seen,  very  slight  constrictions  appear  near  the  middle  of  the  stomach, 
and,  pressing  deeper  into  the  greater  curvature,  course  slowly  toward 
the  pyloric  end.  As  new  regions  enter  into  constriction,  the  fibers 
just  previously  contracted  become  relaxed,  so  that  there  is  a  true  moving 
wave,  with  a  trough  between  two  crests.  When  a  wave  swings  round 
the  bend  in  the  pyloric  part,  the  indentation  made  by  it  deepens; 
and  as  digestion  goes  on  the  antrum  elongates  and  the  constrictions 
running  over  it  grow  stronger,  but,  until  the  stomach  is  nearly  empty, 
they  do  not  entirely  divide  the  cavity.  After  the  antrum  has  length- 
ened, a  wave  takes  about  thirty-six  seconds  to  move  from  the  middle 
of  the  stomach  to  the  pylorus.  At  all  periods  of  digestion  the  waves 
recur  at  intervals  of  almost  exactly  ten  seconds.  It  results  from  this 
rhythm  that  when  one  wave  is  just  beginning  several  others  are  already 
running  in  order  before  it.  Between  the  rings  of  constriction  the 
stomach  is  bulged  out,  as  shown  in  the  various  outlines  in  Fig.  82. 

Movements  of  the  Pyloric  Sphincter.- — During  the  first  ten  or  fifteen 
minutes  after  the  first  constriction  of  the  antrum  the  pylorus  is  tightly 
closed.  After  this  period  it  opens  at  irregular  intervals  to  permit 
the  passage  of  liquefied  food  which  is  ejected  by  peristaltic  waves  for 
a  distance  of  two  or  three  centimeters  into  the  duodenum.  The  fre- 
quency with  which  the  pylorus  opens  depends  apparently  on  the  degree 
to  which  the  food  is  softened.  When  the  food  is  hard,  the  pylorus 
closes  more  tightly  and  remains  closed  a  longer  period  than  when  it  is 
soft. 

The  Activity  of  the  Cardiac  Portion. — As  digestion  proceeds,  the 
pre-antral  part  of  the  stomach  elongates  and  assumes  the  shape  of  a 
tube,  which  becomes  the  seat  also  of  peristaltic  constriction  waves. 
As  a  result,  some  of  the  food  is  gradually  forced  into  the  antrum  to 
succeed  that  which  has  been  prepared  and  ejected  into  the  duodenum. 
As  the  pre-antral  tube  is  emptied  of  its  contents  the  longitudinal  and 
circular  fibers  of  the  fundus  steadily  contract  and  gradually  force  its 
contents  into  the  tubular  portion.  This  continues  until  the  fundus  is 
completely  emptied.  The  changes  in  shape  which  the  cardiac  portion 
undergoes  during  digestion  are  represented  in  Fig.  82.  The  fundus 
acts  as  a  reservoir  for  the  food  and  forces  out  its  contents  a  little  at  a 
time  as  the  antral  mechanism  is  ready  to  receive  them.  Since  peristal- 
tic movements  are  absent  from  the  cardiac  portion  the  food  is  not 
mixed  with  gastric  juice,  and  therefore  salivary  digestion  can  continue 
for  a  considerable  period.  There  is  no  evidence  of  a  circulation  of 
food  in  the  stomach  as  usually  described.  On  the  contrary,  the  move- 
ment through  the  pre-antral  tube  and  antrum  is  in  general  a  progres- 
sive though  an  oscillating  one.  As  the  constriction  waves  rapidly 
pass  over  the  food  it  is  advanced  toward  the  pyloric  opening,  but  as 


DIGESTION.  189 

this  is  closed  the  food  is  forced  backward  through  the  advancing 
constricted  ring  for  a  variable  distance. 

The  effect  of  the  constriction  waves  is  to  mix  the  food  with  the 
gastric  juice,  triturate  and  soften  it.  So  soon  as  this  is  effected,  the 
pylorus  relaxes,  when  the  advancing  constriction  wave  expels  it  into 
the  intestine.  With  its  expulsion  room  is  afforded  for  an  additional 
quantity  of  food,  and  hence  there  is  a  general  advance  of  the  food  mass 
toward  the  pylorus. 

Though  these  observations  were  made  on  the  cat,  evidence  is 
accumulating  which  goes  to  show  that  in  human  beings  the  walls 
of  the  stomach  exhibit  constriction  waves  which  are  similar  in  all 
respects  to  those  above  described. 

The  Nerve  Mechanism  of  the  Stomach. — In  preceding  para- 
graphs it  was  stated  that  during  the  period  of  gastric  digestion  the 
food  is  retained  in  the  stomach  because  of  the  closure  of  the  cardia 
(the  esophago-gastric  orifice)  and  of  the  pylorus  (the  gastro-duode- 
nal  orifice)  both  orifices  being  tightly  closed  by  the  tonic  contraction 
of  sphincter  muscles;  that  both  sphincters  relax  from  time  to  time, 
the  one  to  permit  the  entrance  of  food  into  the  stomach  for  further 
digestion,  the  other  to  permit  the  exit  of  food  into  the  intestine  after 
its  more  or  less  complete  digestion,  after  which  in  both  instances  the 
sphincters  again  contract  and  close  the  orifices;  that  the  pyloric  or 
antral  muscles  are  vigorously  active  throughout  the  digestive  period, 
triturating  the  food,  mixing  it  with  gastric  juice,  and  finally  driving  it 
through  the  temporarily  open  pylorus  into  the  intestine. 

These  separate  but  related  groups  of  muscle-fibers,  by  reason  of 
their  endowments,  and  possibly  by  virtue  of  the  presence  of  local 
nerve  mechanisms,  exhibit  activities  which  are  independent  of  the 
central  nerve  system.  Thus  the  isolated  stomach  of  the  dog  and  of 
other  animals  as  well,  if  kept  warm  and  moist,  will  exhibit  rhythmic 
movements  for  a  period  of  time  varying  from  an  hour  to  an  hour  and 
a  half.  Though  nerve-cells  and  nerve-fibers  (Auerbach's  plexus) 
are  present  in  the  walls  of  the  stomach  between  the  layers  of  muscle- 
fibers,  it  is  not  believed  that  they  are  the  immediate  sources  of  the 
stimulus  to  the  contraction,  though  they  may  act  as  a  coordinating 
mechanism.  The  stimulus  in  all  probability  develops  in  the  muscle- 
fiber  itself  and  is  therefore  myogenic  in  origin. 

The  sphincter  car  dire  muscle  surrounding  the  esophago-gastric 
orifice  is  always  under  normal  conditions,  tonically  contracted  and  the 
orifice  closed.  This  contraction  is  partly  due  to  inherent  causes  as 
shown  by  the  fact  that  it  persists  for  from  24  hours  to  several  days 
after  division  of  all  nerves  distributed  to  it.  The  contraction  may  be 
so  pronounced  as  to  offer  considerable  resistance  to  the  passage  not 
only  of  food  but  even  to  the  introduction  of  a  sound  into  the  stomach. 
(Cannon.)  That  the  normal  contraction  is  under  the  influence  of  the 
central  nerve  system  is  shown  by  the  effects  which  follow  stimulation 
of  the  peripheral  end  of  the  divided  vagus.     If  it  is  stimulated  with 


i9o  TEXT-BOOK  OF  PHYSIOLOGY. 

weak  induced  currents,  the  contraction  is  somewhat  inhibited  and  the 
orifice  enlarged ;  if  it  is  stimulated  with  strong  currents  the  contraction 
is  markedly  increased.  Apparently  there  are  in  the  vagus  two  sets 
of  efferent  nerve-fibers,  one  of  which  augments,  while  the  other  inhibits 
the  contraction,  and  corresponding  to  the  nerves  there  must  be  in  the 
medulla  oblongata  two  centers  from  which  they  arise,  an  augmentor 
and  an  inhibitor. 

Observation  has  shown  that  at  the  beginning  of  each  act  of  degluti-. 
tion,  there  is  an  inhibition  of  the  sphincter  muscle,  and  if  the  acts  fol- 
low each  other  in  quick  succession,  the  inhibition  and  relaxation  are 
increased.  With  the  passage  of  the  food  into  the  stomach  the  tonic  con- 
traction again  supervenes.  These  effects  also  follow  stimulation  of  the 
glossopharyngeal  nerve  which  evidently  carries  nerve  impulses  to 
these  centers.  Whether  the  sphincter  inhibition  is  the  result  of  an 
inhibition  of  the  center  which  maintains  the  tonus,  or  a  stimulation 
of  the  inhibitor  center,  is  uncertain. 

The  degree  of  activity  also  of  both  the  pyloric  sphincter  and  the  antral 
muscles  is  modified  by  the  central  nerve  system  either  in  the  way  of 
inhibition  or  augmentation  and  in  response  to  gastric  stimulation. 
The  nerves  more  especially  concerned  in  the  maintenance  and  regula- 
tion of  the  gastric  contractions  are  the  vagi  and  the  splanchnics. 
The  afferent  fibers  through  which  nerve  impulses  pass  to  the  nerve 
centers  are  in  all  probability  contained  in  the  trunk  of  the  vagus  nerve; 
the  efferent  fibers  through  which  nerve  impulses  from  the  centers  reach 
the  stomach,  are  contained  partly  in  the  trunk  of  the  vagus  and  partly 
in  the  trunk  of  the  splanchnic  nerve. 

If  the  vagus  nerves  are  divided  in  the  neck,  there  is  a  loss  of  muscle 
tonus  though  the  contractions  do  not  wholly  disappear.  Stimulation 
of  the  peripheral  end  of  one  divided  vagus  is  followed  by  an  augmen- 
tation in  the  vigor  of  the  contraction  of  the  antral  muscles,  an  increase 
in  the  tone  of  the  fundus  muscles,  as  well  as  an  increase  in  the  con- 
traction of  the  sphincter  pylori  and  sphincter  cardiae.  Though  this 
is  the  usual  result  there  may  be  a  primary  relaxation  or  inhibition  of 
short  duration  of  one  or  all  of  these  structures  before  the  augmen- 
tation occurs.  May  states  that  this  was  always  the  case  in  his  experi- 
ments. A  similar  inhibition  may  be  brought  about  reflexly  by  stimu- 
lation of  the  central  end  of  a  divided  vagus.  This  result  will  not  be 
produced  if  the  opposite  vagus  has  previously  been  divided.  The 
vagi,  therefore,  contain  both  inhibitor  and  augmentor  nerve-fibers 
for  the  gastric  musculature. 

Stimulation  of  the  peripheral  end  of  a  divided  splanchnic  is  followed 
by  an  inhibition  of  the  peristalsis  and  a  loss  of  tone.  Morat,  however, 
has  observed  a  primary  opposite  effect.  From  these  facts  it  would 
appear  that  the  gastric  muscles  receive  both  inhibitor  and  augmentor 
fibers  from  two  different  sources. 

The  excitatory  cause  for  the  activity  of  this  mechanism  is  doubtless 
connected  with  the  chemic  and  mechanic  stimulation  by  the  gastric 


DIGESTION.  191 

contents.  The  relaxation  or  inhibition  of  the  sphincter  pylori  appears 
to  be  caused  by  the  presence  of  free  acid  at  the  pylorus;  its  contrac- 
tion, by  the  presence  of  acids  in  the  duodenum.  With  their  neutral- 
ization in  the  duodenum,  their  influence  on  the  sphincter  muscle 
is  weakened  after  which  it  again  becomes  susceptible  to  the  inhibitor 
influence  of  the  acid  within  the  stomach.  It  is  probably  for  this 
reason  that  carbohydrates  which  do  not  absorb  the  acid  are  discharged 
from  the  stomach  early;  that  the  proteids  which  postpone  the  appear- 
ance of  free  acid  are  retained  longer  and  that  fats  which  check  the 
secretion  of  gastric  juice  are  discharged  slowly  (Cannon).  It  should  be 
emphasized,  however,  that  the  relaxation  and  contraction  of  the  pyloric 
sphincter,  due  to  the  action  of  free  acid  on  the  gastric  and  duodenal 
sides,  respectively,  can  take  place  independently  of  the  nerve  system. 

INTESTINAL  DIGESTION. 

The  physical  and  chemic  changes  which  the  alimentary  principles 
undergo  in  the  small  intestine,  and  which  collectively  constitute  in- 
testinal digestion,  are  probably  more  important  and  complex  than 
those  taking  place  in  the  stomach,  for  the  food  is,  in  this  situation, 
subjected  to  the  solvent  action  of  the  pancreatic  and  intestinal  juices, 
as  well  as  to  the  action  of  the  bile,  each  of  which  exerts  a  transforming 
influence  on  one  or  more  substances  and  still  further  prepares  them 
for  absorption  into  the  blood. 

To  rightly  appreciate  the  physiologic  actions  of  the  digestive 
juices  poured  into  the  intestine,  the  nature  of  the  partially  digested 
food  as  it  comes  from  the  stomach  must  be  kept  in  mind.  This 
consists  of  water,  inorganic  salts,  acidified  proteids,  albumoses,  pep- 
tones, starch,  maltose,  liquefied  fat,  saccharose,  lactose,  dextrose, 
cellulose,  and  the  indigestible  portion  of  meats,  cereals,  and  fruits. 
Collectively  they  are  known  as  chyme.  As  this  acidified  mass  passes 
through  the  duodenum  its  contained  acids  excite  a  reflex  secretion 
and  discharge  of  the  intestinal  fluids:  e.  g.}  pancreatic  juice,  bile,  and 
intestinal  juice.  Inasmuch  as  these  fluids  are  alkaline  in  reaction 
they  exert  a  neutralizing  and  precipitating  influence  on  various  con- 
stituents of  the  chyme.  As  soon  as  this  has  taken  place,  gastric 
digestion  ceases  and  those  chemic  changes  are  inaugurated  which 
eventuate  in  the  transformation  of  all  the  remaining  undigested 
nutritive  materials  into  absorbable  and  assimilable  compounds  which 
collectively  constitute  intestinal  digestion. 

THE  SMALL  INTESTINE. 

The  small  intestine,  in  which  this  stage  of  digestion  takes  place, 
is  a  convoluted  tube,  measuring  about  seven  meters  in  length  and  3.5 
cm.  in  diameter,  and  extending  from  the  pyloric  orifice  of  the  stomach 
to  the  beginning  of  the  large  intestine. 

The  intestine  consists  of  four  coats:  viz.,  serous,  muscular,  sub- 
mucous, and  mucous. 


i92  TEXT-BOOK  OF  PHYSIOLOGY. 

The  serous  coat  is  the  most  external  and  is  formed  by  a  reflection 
of  the  general  peritoneal  membrane.  It  is,  however,  wanting  in  the 
duodenal  portion. 

The  muscle  coat,  situated  just  beneath  the  former,  surrounds 
the  entire  intestine.  It  is  composed  of  non-striated  fibers  which  are 
more  abundant  and  better  developed  in  the  upper  than  in  the  lower 
portions  of  the  intestine.  The  muscle  coat  consists  of  two  layers 
of  fibers:  (i)  an  external  or  longitudinal,  and  (2)  an  internal  or  circular 
layer.  The  longitudinal  fibers  are  most  marked  at  that  border  of  the 
intestine  free  from  peritoneal  attachment,  though  they  form  a  thin 
layer  all  over  the  intestine.  The  circular  fibers  are  much  more  numer- 
ous, and  completely  encircle  the  intestine  throughout  its  entire  extent. 
It  has  been  demonstrated  that  at  the  junction  of  the  ileum  and  colon, 
and  surrounding  the  orifice,  the  ileo-colic,  common  to  both,  the  muscle- 
fibers  are  arranged  in  the  form  of,  and  play  the  part  of,  a  sphincter 
muscle,  which  has  been  termed  the  ileo-colic  sphincter.  It  is  usually 
in  a  state  of  tonic  contraction  and  regulates  the  passage  of  materials 
from  the  small  into  the  large  intestine,  and  possibly  also  in  the  reverse 
direction  under  special  circumstances. 

The  submucous  coat  consists  of  areolar  tissue  and  serves  to  unite 
the  muscular  with  the  mucous  coat.  A  thin  layer  of  muscle-fibers, 
the  muscularis  mucosa,  is  placed  on  its  inner  surface. 

The  mucous  coat  is  soft  and  velvety  in  appearance  and  covered 
by  a  single  layer  of  columnar  epithelium.  Its  entire  surface  is  covered 
with  small  conical  projections  termed  villi.  Throughout  its  entire 
extent,  with  the  exception  of  the  lower  portion  of  the  ileum  and  the 
duodenum,  the  mucous  membrane  presents  a  series  of  transverse 
folds — the  valvules  conniventes,  or  valves  of  Kirkring.  These  folds 
vary  from  one-fourth  to  half  an  inch  in  width  and  extend  one-half 
to  two-thirds  of  the  distance  around  the  interior  of  the  bowel.  Each 
valve  consists  of  two  layers  of  the  mucous  membrane  permanently 
united  by  fibrous  tissue.  It  is  believed  that  the  valves  retard  to  some 
extent  the  passage  of  the  food  through  the  intestine  and  present  a 
greater  surface  for  absorption. 

Blood-vessels,  Nerves,  and  Lymphatics. — The  blood-vessels, 
of  the  small  intestine,  which  are  very  numerous,  are  derived  mainly 
from  the  superior  mesenteric  artery.  After  penetrating  the  intestinal 
walls  the  smaller  vessels  ramify  in  the  submucous  coat  and  send 
branches  to  the  muscle  and  mucous  coats,  supplying  all  their  struc- 
tures with  blood.  After  circulating  through  the  capillary  vessels  the 
blood  is  returned  by  small  veins  which  subsequently  unite  to  form 
the  superior  mesenteric  vein,  which,  uniting  with  the  splenic  and  gas- 
tric veins,  forms  the  portal  vein.  The  nerves  are  derived  from  the 
lower  part  of  the  solar  plexus.  The  branches  follow  the  blood-vessels 
and  become  associated  with  two  plexuses,  one  (Auerbach's)  lying 
between  the  muscle  coats,  the  other  (Meissner's)  lying  in  the  sub- 
mucous coat.     The  lymphatics,  which  originate  in  the  mucous   and 


DIGESTION.  193 

muscle  coats,  are  very  abundant.  They  unite  to  form  those  vessels 
seen  in  the  mesentery  and  empty  into  the  thoracic  duct. 

Intestinal  Glands. — -The  gland  apparatus  of  the  intestine  by 
which  the  intestinal  juice  is  secreted  consists  of  the  duodenal  (Brun- 
ner's)  and  the  intestinal  (Lieberkiihn's)  glands. 

The  duodenal  glands  are  situated  beneath  the  mucous  membrane 
and  open  by  a  short  wide  duct  on  its  free  surface.  They  are  racemose 
glands  lined  by  nucleated  epithelium.  The  secretion  of  these  glands 
is  clear,  slightly  viscid,  and  alkaline.  Its  chemic  composition  and 
function  are  unknown.  % 

The  intestinal  glands  or  follicles  are  distributed  throughout  the 
entire  mucous  membrane  in  enormous  numbers.  They  are  formed 
mainly  by  an  inversion  of  the  mucous  membrane  and  hence  open  on 
its  free  surface.  Each  tubule  consists  of  a  thin  basement  membrane 
lined  by  a  layer  of  spheric  epithelial  cells,  some  of  which  undergo 
distention  by  mucin  and  become  converted  into  mucous  or  goblet 
cells.  The  epithelial  secreting  cells  consist  of  granular  protoplasm 
containing  a  well-defined  nucleus.  The  intestinal  follicles  constitute 
the  apparatus  which  secretes  the  chief  portion  of  the  intestinal  juice. 

Intestinal  Juice.— Owing  to  its  admixture  with  other  secretions 
and  to  the  profound  disturbance  of  the  digestive  function,  caused  by 
the  establishment  of  intestinal  fistula?,  this  fluid  has  rarely  been  ob- 
tained in  a  state  of  purity  or  in  quantities  sufficient  for  accurate  analyses 
or  for  experimental  purposes.  Its  physiologic  properties  and  functions 
are  therefore  imperfectly  known.  Various  attempts  have  been  made 
by  physiologists,  by  the  employment  of  different  methods,  to  obtain 
this  secretion.  The  method  usually  employed  is  that  of  Thiry  and 
Vella.  This  consists  in  dividing  the  intestine  at  two  places,  about 
eight  or  ten  inches  apart,  restoring  the  continuity  of  the  intestine, 
and  then  uniting  the  two  ends  of  the  resected  portion  to  the  edges  of 
two  openings  in  the  abdominal  walls.  The  resected  portion,  being 
supplied  with  blood-vessels  and  nerves,  maintains  its  nutrition  and 
secretes  a  more  or  less  normal  juice. 

When  obtained  from  a  dog  under  these  circumstances  the  intestinal 
juice  is  watery  in  consistence,  slightly  opalescent,  light  yellow  in  color, 
alkaline  in  reaction,  with  a  specific  gravity  of  1.010.  Chemic  analysis 
reveals  the  presence  of  proteids,  mucin,  and  sodium  carbonate. 

The  intestinal  juice  obtained  by  Tubbey  and  Manning  from  a 
small  portion  of  the  human  intestine  (ileum)  was  opalescent,  occa- 
sionally brownish  in  color,  alkaline,  and  had  a  specific  gravity  of  1.006. 
On  the  addition  of  hydrochloric  acid,  carbonic  acid  was  given  off, 
showing  the  presence  of  carbonates.  It  contained  proteids  and 
mucins. 

PANCREAS. 

The  pancreas   is   a  long   flattened   gland,   situated  deep   in  the 
abdominal  cavity,  lying  just  behind  the  stomach.     It  measures  from 
13 


194 


TEXT-BOOK  OF  PHYSIOLOGY. 


six  to  eight  inches  in  length,  two  and  a  half  in  breadth,  and  one  in 
thickness.  It  is  usually  divided  into  a  head,  body,  and  tail.  The 
head  is  directed  to  the  right  side  and  is  embraced  by  the  curved  portion 


Pancreatic  ducts 


Common  bile-duct- 


Fig.  84. — Pancreas  .and  Duodenum  Removed  from  the  Body  and  Seen  from 
Behind.     The  Gland  is  Cut  to  Show  the  Ducts. — (Landois  and  Stirling.) 


of  the  duodenum;  the  tail  is  directed  to  the  left  side  and  extends  as 
far  as  the  spleen  (Fig.  84.)  The  pancreas  communicates  with  the 
intestine  by  means  of  a  duct.  This  duct  commences  at  the  tail  and 
runs  transversely  through  the  body  of  the  gland.     As  it  approaches 

the  head  of  the  gland  it 
gradually  increases  in 
size  until  it  measures 
about  one-tenth  of  an 
inch  in  diameter.  It 
then  curves  downward 
and  forward  and  opens 
into  the  duodenum.  In 
its  course  through  the 
gland  it  receives 
branches  which  enter  it 
nearly  at  right  angles. 
The  pancreas  is  richly 
supplied  with  blood- 
vessels and  nerves,  the 
latter  coming  from  the 
solar  plexus. 

Histologic  Struc- 
ture.— In  its  structure 
the  pancreas  resembles 
the  salivary  glands.  It  consists  of  a  connective-tissue  framework 
which  divides  the  gland  tissue  into  lobules.  Each  lobule  is  com- 
posed of  a  number  of  acini  or  alveoli,  more  or  less  elongated  or  tubular 
in  shape.     Each  acinus  gives  origin  to  a  small  duct  which,  uniting 


Fig.  85. 


Fig.  86. 


One  Saccule  of  the  Pancreas  of  the  Rabbit 
in  Different  States  of  Activity.  Fig.  85. — After 
a  period  of  rest,  in  which  case  the  outlines  of  the  cells 
are  indistinct  and  the  inner  zone — i.  e.,  the  part  of  the 
cells  (a)  next  the  lumen  (c) — is  broad  and  filled  with 
fine  granules.  Fig.  86. — After  the  gland  has  poured 
out  its  secretion,  when  the  cell  outlines  (d)  are  clearer, 
the  granular  zone  (a)  is  smaller,  and  the  clear  outer 
zone  is  wider. — (Yeo's  "Text-book  of  Physiology," 
after  Kiihne  and  Lea.) 


DIGESTION. 


i95 


with  adjoining  ducts,  forms  the  lobular  duct,  which  becomes  tributary 
to  the  main  duct.  The  acinus  is  lined  by  a  layer  of  cylindric  epithelial 
cells  characterized  by  a  difference  in  structure  between  the  central 
and  peripheral  ends  (Fig.  85).  The  central  end,  that  bordering  the 
lumen  of  the  acinus,  is  dark  in  appearance  and  filled  with  dark  granules, 
while  the  peripheral  end  is  clear  and  homogeneous.  The  relative 
depth  of  these  two  zones  varies  according  to  the  functional  activity 
of  the  gland.  During  the  intervals  of  digestion  the  granular  layer 
is  very  deep  and  occupies  almost  the  entire  cell;  after  active  digestion 
the  granular  layer  is  very  narrow,  while  the  clear  zone  is  largely  in- 
creased in  depth.  The  blood-vessels  of  the  pancreas  are  arranged 
around  the  acini  in  a  manner  similar  to  that  observed  in  the  salivary 


Fig.  87. — Section  of  Human 
Pancreas,  including  Several  Acini 
and  Two  Ducts.  The  Cells  Pre- 
sent a  Central  Granular  and  a 
Peripheral  Clear  Zone. — (Piersol.) 


Fig.  88. — Section  of  Human  Pan- 
creas showing,  a,  a,  Islands  of 
Langerhans,  and  b,  the  Usual  Acini. — 
(Piersol): 


glands.  The  ultimate  terminations  of  the  nerves  in  the  epithelium 
are  probably  by  means  of  the  usual  end-tufts. 

The  Islands  of  Langerhans. — Throughout  the  body  of  the  pan- 
creas and  especially  in  the  outer  extremity  there  are  found  between 
and  among  the  acini  collections  of  globular  cells  arranged  in  the 
form  of  rods  or  columns,  separated  from  the  acini  and  from  one  another 
by  layers  of  connective  tissue  in  which  ramify  large  tortuous  capillary 
blood-vessels.  These  columnar  bodies,  seen  in  cross-section  in  Fig. 
88,  have  been  named,  after  their  discoverer,  the  islands  of  Langerhans. 

Embryologic  investigations  have  shown  that  these  cells  are  out- 
growths from  the  primitive  acini,  to  which  they  remain  attached  for 
some  time  by  means  of  a  foot-stalk.  This  subsequently  becomes 
constricted  by  the  connective  tissue  and  the  cells  become  completely 
detached.  The  cells  then  assume  the  columnar  arrangement,  after 
which  vascularization  takes  place. 

From  the  fact  that  complete  extirpation  of  the  pancreas  as  well  as 
its  various  diseases  is  followed  by  serious  disturbances  of  the  carbohy- 


196  TEXT-BOOK  OF  PHYSIOLOGY. 

drate  metabolism  it  has  been  suggested  that  the  islands  of  Langerhans 
have  a  function  separate  and  distinct  from  that  of  the  glandular  portion 
of  the  pancreas ;  that  they  secrete  a  specific  material  which  partakes  of 
the  nature  of  an  internal  secretion  which  is  absorbed  by  the  blood 
circulating  around  them  and  carried  to  different  tissues.  The  effect 
on  the  metabolism  of  the  body  which  follows  extirpation  of  the  pan- 
creas will  be  referred  to  in  a  subsequent  chapter. 

Pancreatic  Juice. — The  pancreatic  juice  may  be  obtained  by 
introducing  a  silver  cannula,  through  an  opening  in  the  abdominal 
wall,  into  the  duct,  and  securing  it  by  a  ligature.  In  a  short  time 
the  juice  flows  from  the  distal  end  of  the  cannula,  when  it  can  be 
collected.  According  to  Bernard,  normal  juice  can  only  be  obtained 
during  the  first  twenty-four  hours.  The  juice  obtained  from  a  tem- 
porary fistula  is  clear,  slightly  opalescent,  viscid,  of  a  decidedly  alkaline 
reaction,  and  has  a  specific  gravity  in  the  dog  of  1.040.  When  cooled 
to  o°  C,  it  assumes  a  gelatinous  consistence.  At  ioo°  C.  it  com- 
pletely coagulates.  When  obtained  from  a  permanent  fistula,  the  juice  is 
watery  and  the  solid  constituents  are  very  much  diminished  in  amount. 

The  chemic  composition  of  the  pancreatic  juice  of  the  dog  as  deter- 
mined by  Schmidt  is  as  follows:  water,  900.76;  organic  matter,  90.44; 
inorganic  salts,  8.80.  Of  the  inorganic  salts,  sodium  carbonate  is 
probably  the  most  essential,  as  it  is  this  salt  which  gives  to  the  juice  its 
alkaline  reaction. 

Mode  of  Secretion. — The  secretion  of  the  juice  is,  in  the  rabbit 
and  dog  at  least,  almost  continuous  during  a  period  of  twenty-four 
hours  after  a  single  average  meal,  though  the  rate  of  flow  varies  con- 
siderably during  this  period.  As  soon  as  food  enters  the  stomach 
'the  flow  of  the  pancreatic  juice  begins  and  steadily  increases  in  amount 
until  about  the  third  hour,  when  it  reaches  its  maximum;  after  this 
period  the  flow  diminishes  until  the  sixth  hour,  when  it  again  increases 
for  about  an  hour.  It  then  gradually  diminishes  until  it  ceases  entirely. 
During  the  period  of  secretory  activity  the  gland  becomes  red  and 
vascular  from  a  dilatation  of  the  blood-vessels. . 

The  discharge  of  the  juice  associated  with  the  introduction  of 
food  into  the  stomach  is  brought  about  in  all  probability  through 
the  agency  of  the  nerve  system,  though  the  exact  mechanism  is  im- 
perfectly understood.  It  is  probable  that  impressions  made  on  the 
terminal  filaments  of  the  pneumogastric  nerve  ascend  to  the  medulla, 
whence  impulses  pass  outward  through  vaso-motor  and  secretor 
nerves  to  the  blood-vessels  and  secreting  cells  of  the  glands.  Stim- 
ulation of  the  peripheral  end  of  the  divided  vagus  gives  rise  to  increased 
secretion.  Inasmuch  as  various  agents,  such  as  mineral  and  organic 
acids,  placed  on  the  duodenal  mucous  membrane  excite  the  flow, 
it  is  quite  probable  that  the  passage  of  the  acid  contents  of  the  stomach 
through  the  duodenum  also  acts  as  a  powerful  stimulus  to  the  dis- 
charge of  the  juice.  But  as  the  secretion  and  discharge  of  the  juice 
is  excited  by  the  same  conditions  after  the  division  of  all  related  nerves, 


DIGESTION.  197 

other  explanations  were  sought  for.  Bayliss  and  Starling  made  the 
discovery  that  the  secretory  activity  of  the  pancreas  is  initiated  and 
maintained  by  the  action  of  a  specific  substance  to  which  they  have 
given  the  term  secretin.  This  substance  is  developed  in  the  duodenal 
glands  out  of  a  precursor,  prosecretin,  in  consequence  of  the  action 
of  the  acids  in  the  chyme,  after  which  it  is  carried  by  the  blood-stream 
to  the  pancreas.  An  extract  of  the  duodenal  mucous  membrane  made 
with  hydrochloric  acid  0.4  per  cent,  and  presumably  containing 
secretin,  when  injected  into  the  blood  will  evoke  a  profuse  discharge 
of  pancreatic  juice.  Hydrochloric  acid  alone  will  not  have  this  effect. 
The  total  amount  of  pancreatic  juice  secreted  in  twenty-four  hours 
has  been  only  approximately  determined;  the  estimates  based  upon 
the  amount  obtained  from  dogs  vary  from  175  to  800  grams. 

Histologic  Changes  in  the  Cells  during  Secretory  Activity. — 
Reference  has  already  been  made  to  the  fact  that  the  cells  lining  the 
acini  consist  of  two  zones:  an  outer  one,  clear  and  homogeneous; 
and  an  inner  one,  dark  and  granular.  The  position  of  the  nucleus 
of  the  cell  varies,  being  at  one  time  in  the  outer,  at  another  time  in  the 
inner,  zone.  If  the  pancreas  be  examined  microscopically  during 
the  intervals  of  digestion,  it  will  be  observed  that  the  inner  zone  is 
broad,  highly  granular,  occupying  nearly  the  entire  cell,  while  the 
outer  zone  is  narrow  and  clear.  If,  however,  the  gland  be  examined 
shortly  after  a  period  of  active  secretion,  the  reverse  conditions  will 
be  observed;  that  is,  the  inner  zone  will  be  narrow,  containing  rela- 
tively few  granules,  while  the  outer  zone  will  be  clear  and  wide.  This 
change  in  the  cell  has  been  witnessed  in  the  pancreas  of  the  living 
animal — rabbit— by  Kiihne  and  Lea.  They  observed  that  as  soon 
as  digestion  set  in,  the  granules  of  the  broad  inner  zone  began  to  pass 
toward  the  lumen  of  the  acinus  and  to  gradually  disappear  as  the 
secretion  was  poured  out,  while  the  outer  zone  increased  in  width 
until  almost  the  entire  cell  became  clear  and  homogeneous.  (See 
Fig.  86.)  After  secretion  ceased  the  granules  again  made  their 
appearance,  the  result,  in  all  probability,  of  metabolic  activity. 

Physiologic  Action  of  Pancreatic  Juice. — Experimental  in- 
vestigations have  demonstrated  the  fact  that  pancreatic  juice  is  the 
most  complex  in  its  physiologic  action  of  all  the  digestive  fluids.  In 
virtue  of  its  contained  enzymes,  pancreatic  juice  acts: 

1.  On  starch.  When  normal  pancreatic  juice  or  a  glycerin 
extract  of  the  gland  is  added  to  a  solution  of  hydrated  starch,  the 
latter  is  speedily  transformed  into  maltose,  passing  through  the  inter- 
mediate stage  of  dextrin.  The  process  is  in  all  respects  similar  to  that 
observed  in  the  digestion  of  starch  by  saliva.  Pancreatic  juice,  how- 
ever, is  more  energetic  in  this  respect  than  saliva.  The  enzyme  which 
effects  this  change  is  termed  amylopsin.  When  the  starch  which 
escapes  salivary  digestion  passes  into  the  small  intestine  and  mingles 
with  pancreatic  juice,  it  is  very  promptly  converted  into  maltose  by 
the  action  or  in  the  presence  of  this  enzyme. 


i9S  TEXT-BOOK  OF  PHYSIOLOGY. 

2.  On  protein.  When  protein  compounds  are  subjected  to  the  ac- 
tion of  pancreatic  juice,  they  are  transformed  into  peptones  which  do 
not  differ  in  essential  respects  from  those  formed  by  gastric  juice.  The 
intermediate  stages,  however,  are  believed  to  be  somewhat  different. 
The  enzyme  which  effects  this  change  is  termed  trypsin. 

When  fibrin,  for  example,  is  added  to  trypsin  in  a  solution  rendered 
alkaline  by  sodium  carbonate,  it  does  not  swell  up  and  become  trans- 
lucent, as  it  does  when  treated  with  hydrochloric  acid  and  pepsin. 
On  the  contrary,  it  becomes  corroded  on  the  surface,  fragile,  and  in  a 
short  time  undergoes  solution.  The  first  product  is  a  compound 
termed  alkali-albumin.  After  solution  has  taken  place,  various 
chemic  changes  are  initiated  which  eventuate  in  the  production  of 
peptone  and  certain  nitrogenized  bodies,  leucin,  tyrosin,  aspartic  acid, 
etc.  The  intermediate  stages  in  this  process  have  not  been  satis- 
factorily determined.  At  no  time  during  artificial  pancreatic  digestion 
is  there  any  evidence  of  the  presence  of  the  primary  proteoses  (proto- 
albumose  and  hetero-albumose).  The  secondary  proteoses  (deutero- 
albumose)  are  usually  present.  It  will  be -recalled  that  when  the 
peptone  of  peptic  digestion  is  subjected  to  the  action  of  trypsin  a  por- 
tion of  it  is  decomposed  into  leucin  and  tyrosin,  while  another  portion 
presumably  is  not  so  decomposed,  for  which  reason  the  latter  was 
called  anti-  and  the  former,  /zemi-peptone.  It  is  now  believed  that 
anti-peptone  is  not  a  peptone  at  all,  but  a  compound  termed  carnic  acid, 
which  can  be  decomposed  into  simpler  nitrogen-holding  bodies  such 
as  leucin,  tyrosin,  arginin,  etc. 

The  action  of  trypsin  on  proteins  in  an  alkaline  medium  may  be 
illustrated  by  the  following  scheme: 

Protein 

I 
Alkali-albumin 

I 
Deutero-proteose  or  deutero-albumose 

Peptone 


Leucin         Tyrosin       Aspartic  acid       Arginin       Ammonia 

When  the  proteins  which  have  escaped  digestion  in  the  stomach 
pass  into  the  small  intestine  and  mingle  with  the  pancreatic  juice, 
they  are  doubtless  digested  in  the  course  of  the  intestinal  canal,  passing 
through  the  stages  just  described.  As  leucin  and  tyrosin  are  found 
in  the  intestine  during  digestion,  it  is  probable  that  a  portion  of  the 
peptone  undergoes  decomposition  into  these  bodies;  but  as  to  the 
extent  to  which  this  takes  place  or  in  how  far  it  is  a  necessary  process 
under  normal  conditions  is  yet  a  subject  of  investigation.  It  is  certain 
that  it  takes  place  when  there  is  an  excess  of  protein  food  or  when  for 
any  reason  digestion  is  prolonged  or  absorption  is  delayed. 

While  the  view  that  the  final  stage  in  the  digestion  of  proteins  is 


DIGESTION.  i99 

the  formation  of  peptones,  which  in  due  time  are  absorbed  and  syn- 
thetized  into  blood  albumin,  is  generally  accepted,  there  is  some 
evidence  that  it  is  not  wholly  true,  and  that  the  final  stage  may  be 
the  formation  of  the  nitrogen-holding  compounds  above  mentioned; 
in  other  words,  that  the  cleavage  of  the  proteins  is  far  more  com- 
plete than  has  heretofore  been  assumed.  Indeed  it  has  been  asserted 
that  they  are  reduced,  if  not  to  their  ultimate  constituents,  the  amino- 
and  diamino-acids,  at  least  to  one  or  more  of  the  different  polypeptid 
stages.  Ever  since  the  discovery  by  Cohnheim  of  the  existence  in  the 
intestinal  juice  of  a  substance  termed  by  him  erepsin,  which  is  capable 
of  splitting  proteoses  and  peptones  into  simple  nitrogen-holding  com- 
pounds, there  has  been  slowly  developing  the  idea  that  normally 
during  intestinal  digestion  the  proteoses  and  peptones  are  reduced  by 
this  agent  to  leucin,  tyrosin,  histidin,  arginin,  aspartic  acid,  etc.,  which 
in  turn  are  absorbed  and  synthetized  to  blood  or  tissue  albumin. 
The  discovery  by  Vernon  of  erepsin  in  pancreatic  juice  lends  further 
support  to  this  view.  Until  more  convincing  evidence  is  furnished, 
however,  it  may  be  assumed  that  peptone  represents  the  final  stage  in 
the  digestion  of  proteins. 

3.  On  fat.  If  pancreatic  juice  be  added  to  a  perfectly  neutral 
fat — olein,  palmitin,  or  stearin — and  kept  at  a  temperature  of  about 
ioo°  F.  (380  C),  it  will  at  the  end  of  an  hour  or  two  be  partially  de- 
composed into  glycerin  and  the  particular  fatty  acid  indicated  by 
the  name  of  the  fat  used — e.  g.,  oleic,  palmitic,  stearic.  The  oil  will 
then  exhibit  an  acid  reaction.  The  reaction  is  represented  in  the 
following  formula: 

C3H;(ClSH3302)3     +     3H20     =     (Cl8H3402\3     +     C3H=(HO)3 
Triolein.  Water.  Oleic  Acid.  Glycerin. 

If  to  this  acidified  oil  there  be  added  an  alkali,  e.  g.,  potassium  or 
sodium  carbonate,  the  latter  will  at  once  combine  with  the  fatty  acid 
to  form  a  salt  known  as  a  soap.  The  reaction  is  expressed  in  the 
following  equation: 

Sodium  Carbonate.        Oleic  Acid.  Sodium  Oleate.  Carbonic  Acid. 

Na2C03       +       ClSH3402     =     Na2OCl8H3202       +       H2C03 

Coincident  with  the  formation  of  the  soap  the  remaining  neutral  oil 
undergoes  diviion  into  drops  of  microscopic  size,  which  float  in  the 
soap  solution,  forming  what  has  been  termed  an  emulsion,  which  is 
white  and  creamy  in  appearance.  The  action  of  the  pancreatic  juice 
may  then  be  said  to  consist  in  the  cleavage  of  the  neutral  fats  into 
fatty  acids  and  glycerin,  after  which  the  formation  of  the  soap  and 
the  division  of  the  fat  takes  place  spontaneously.  The  enzyme  which 
produces  the  cleavage  of  the  neutral  fats  has  been  termed  steapsin 
or  lipase.  The  extent  to  which  the  cleavage  of  the  fat  takes  place  in 
the  intestine  has  not  been  definitely  determined.  There  are  some 
who  think  the  amount  is  relatively  small,  while  others  consider  that 
it  i-;  large,  practically  all  of  the  fat  undergoing  this  decomposition, 
with  the  formation  of  soap  and  glycerin  prior  to  their  absorption. 


200  TEXT-BOOK  OF  PHYSIOLOGY. 

According  to  Pawlow,  the  relative  amounts  of  the  pancreatic 
enzymes  produced  are  conditioned  by  the  character  and  amounts  of 
the  food  principles  consumed.  Thus,  if  chyme  contains  an  excess 
of  either  starch,  proteid,  or  fat,  there  is  a  corresponding  increase  in 
the  amount  of  either  amylopsin,  trypsin,  or  steapsin  produced.  The 
pancreas  apparently  adapts  its  activities  to  the  character  of  the  food. 
Though  it  is  probable  that  each  enzyme  is  a  derivative  of  a  special 
zymogen,  it  is  only  positively  known  that  this  is  the  case  with  trypsin. 
This  enzyme  is  a  derivative  of  the  zymogen,  trypsinogen,  the  pro- 
duction of  which  is  thought  to  be  the  special  function  of  secretin. 
The  pancreatic  juice  at  the  moment  of  its  discharge  into  the  intestine 
does  not  contain  trypsin  but  trypsinogen.  The  transformation  of 
the  latter  into  the  former  is  accomplished,  according  to  Pawlow,  by 
a  special  activating  ferment  secreted  by  the  epithelium  of  the  small 
intestine  and  termed  enter okinase. 

The  rapidity  with  which  pancreatic  juice  in  the  presence  of  bile 
and  hydrochloric  acid  (under  conditions  such  as  are  present  in  the 
duodenum)  can  develop  sufficient  fatty  acid  to  form  an  emulsion  was 
determined  by  Rachford  to  be  two  minutes.  The  activity  of  steapsin 
is  thus  shown  to  be  very  great. 

Physiologic  Action  of  the  Intestinal  Juice.— The  part  played 
by  the  intestinal  juice  in  the  digestive  process  is  yet  a  subject  of  dis- 
cussion, as  the  results  obtained  by  different  observers  are  in  some 
respects  contradictory,  due  to  the  fact  that  animals,  including  human 
beings,  have  been  the  subjects  of  experimentation.  Notwithstanding 
the  actions  of  saliva,  gastric  and  pancreatic  juice,  there  yet  remain 
in  the  food  saccharose,  maltose,  and  lactose,  three  forms  of  sugar  which 
are  believed  by  most  observers  to  be  non-assimilable  and  therefore 
require  some  change  before  they  can  be  absorbed  and  assimilated. 
An  extract  of  the  intestinal  mucous  membrane  or  the  intestinal  juice 
of  a  dog,  added  to  a  solution  of  saccharose,  will  in  a  very  short  time 
convert  it  into  dextrose  and  levulose,  which  together  constitute  invert 
sugar.  The  enzyme  by  which  this  inversion  is  produced,  though 
nothing  definite  is  known  as  to  its  nature,  has  been  termed  invertin 
or  invertase.  Tubbey  and  Manning  state  that  the  human  intestinal 
juice  as  obtained  by  them  has  the  same  action.  In  the  case  of  intestinal 
fistulae  reported  by  Busch,  which  were  supposed  to  be  located  in  the 
upper  third  of  the  intestine,  it  was  found  that  when  saccharose  was 
introduced  into  the  lower  opening,  it  was  not  inverted  but  appeared 
in  the  feces  unchanged. 

Maltose  is  also  rapidly  transformed  into  dextrose.  Lactose 
appears  to  be  unaffected  by  the  pure  juice.  As  it  is  non-assimilable 
it  has  been  supposed  to  undergo  conversion  into  dextrose  and  galac- 
tose while  passing  through  the  epithelial  cells  of  the  intestinal  mu- 
cosa. In  either  case  the  transformation  is  brought  about  by  two 
ferments  known  respectively  as  mallase  and  lactase. 

Intestinal  juice  also  has  a  slight  diastatic  action  on  starch. 


DIGESTION.  201 

THE  LIVER. 

The  liver  is  a  highly  vascular  conglomerate  gland  situated  in  the 
right  hypochondriac  region  and  connected  with  the  intestine  by  a  duct. 

Inasmuch  as  the  liver  performs  several  functions  related  to  both 
secretion  and  excretion,  a  consideration  of  its  structure  and  its  vari- 
ous functions  will  be  deferred  to  a  subsequent  chapter.  In  this  con- 
nection the  bile,  its  physical  properties  and  chemic  composition  in 
relation  to  the  digestive  process,  will  only  be  considered. 

The  bile  is  a  product  of  the  secretory  activity  of  the  liver  cells. 
As  it  is  poured  into  the  intestine  in  man  and  most  mammals  at  a  point 


"1  29 


Fig.  89. — Gall-bladder,  Hepatic,  Cystic,  and  Common  Ducts,  i,  2,  3.  Duode- 
num 4,  4,  5,  6,  7,  7,  8.  Pancreas  and  pancreatic  ducts.  9,  10,  11,  12,  13.  Liver.  14. 
Gall-bladder.  15.  Hepatic  duct.  16.  Cystic  duct.  17.  Common  duct.  18.  Portal 
vein.  19.  Branch  from  the  celiac  axis.  20.  Hepatic  artery.  21.  Coronary  artery 
of  the  stomach.  22.  Cardiac  portion  of  the  stomach.  23.  Splenic  artery.  24. 
Spleen.  25.  Left  kidney.  26.  Right  kidney.  27.  Superior  mesenteric  artery  and  vein. 
28.   Inferior  vena  cava. — (Sappey.) 

corresponding  to  the  orifice  of  the  pancreatic  duct,  and  most  abundantly 
at  the  time  the  food  is  passing  through  the  duodenum,  it  is  usually 
regarded  as  a  digestive  fluid  possessing  an  influence  favorable  if  not 
necessary  to  the  completion  of  the  general  digestive  process. 

Anatomic  Relations  of  the  Biliary  Passages. — After  its  forma- 
tion by  the  liver  cells  the  bile  is  conveyed  from  the  liver  by  the  bile 
capillaries,  which  uniting  finally  from  the  main  hepatic  duct.  This 
duct  emerges  from  the  liver  at  the  transverse  fissure.  At  a  distance 
of  about  5  centimeters  it  is  joined  by  the  cystic  duct,  the  distal  ex- 
tremity of  which  expands  into  a  pear-shaped  reservoir,  the  gall-bladder 
in  which  the  bile  is  temporarily  stored  (Fig.  89).  The  duct  formed 
by  the  union  of  the  hepatic  and  cystic  ducts,  the  common  bile-duct, 


202  TEXT-BOOK  OF  PHYSIOLOGY. 

passes  downward  and  forward  for  a  distance  of  about  7  centimeters, 
pierces  the  walls  of  the  intestine  and  passes  obliquely  through  its  coats 
for  about  a  centimeter  and  opens  on  the  surface  of  a  papilla  in  con- 
junction with  the  pancreatic  duct.  The  walls  of  the  biliary  passages 
are  composed  of  a  mucous  membrane  internally,  a  fibrous  and  mus- 
cular coat  externally.  The  termination  of  the  common  bile-duct  is 
provided  with  a  distinct  band  of  circularly  disposed  muscle-fibers, 
which  when  in  action  completely  close  the  orifice  and  prevent  the  dis- 
charge of  bile.  It  may  therefore  be  regarded  as  a  true  sphincter 
muscle.  Small  racemose  glands  are  embedded  in  the  mucous  mem- 
brane of  the  main  ducts. 

Physical  Properties  and  Chemic  Composition  of  Bile. — The 
bile  obtained  directly  from  the  liver  through  a  fistulous  opening  in 
the  hepatic  duct  is  always  thin  and  watery,  while  that  obtained  from 
the  gall-bladder  is  more  or  less  viscid  from  admixture  with  mucin, 
the  degree  of  this  viscidity  depending  on  the  length  of  time  it  remains 
in  this  reservoir.  The  specific  gravity  of  human  bile  varies  within 
normal  limits  from  1.010  to  1.020.  The  reaction  is  invariably  alka- 
line in  the  human  subject  when  first  discharged  from  the  liver,  but 
may  become  neutral  in  the  gall-bladder.  The  alkalinity  depends  on  the 
presence  of  sodium  carbonate  and  sodium  phosphate.  When  fresh, 
it  is  inodorous;  but  it  readily  undergoes  putrefactive  changes,  and 
soon  becomes  offensive.  Its  taste  is  decidedly  bitter.  When  shaken 
with  water,  it  becomes  frothy — a  condition  which  lasts  for  some  time 
and  which  is  due  to  the  presence  of  mucin.  In  ox  bile  the  mucin  is 
replaced  by  a  nucleo-proteid. 

The  color  of  bile  obtained  from  the  hepatic  duct  is  variable,  usually 
a  shade  between  a  greenish-yellow  and  a  brownish-red.  In  different 
animals  the  color  varies.  In  the  herbivorous  animals  it  is  usually 
green;  in  the  carnivorous  animals  it  is  orange  or  brown.  In  man  it  is 
green  or  a  golden  yellow.  The  colors  are  due  to.  the  presence  of  pig- 
ments. Microscopic  examination  does  not  show  the  presence  of  struc- 
tural elements. 

Human  bile  obtained  from  an  accidental  biliary  fistula  was  shown 
by  Jacobson  to  contain  the  following  ingredients,  viz.: 

COMPOSITION  OF  HUMAN  BILE. 

Water, 977-4© 

Sodium  glycocholate, 9.94 

Sodium  taurocholate, a  trace 

Cholesterin, 0.54 

Free  fat,  o.  10 

,    Sodium  palmitate  and  stearate, 1.36 

Lecithin, 0.04 

Other  organic  matters, 2.26 

Sodium  chlorid, 5.45 

Potassium  chlorid, 0.28 

Sodium  phosphate. 1.33 

Calcium  phosphate, 0.37 

Sodium  1  arbonate, 0.93 

1000.00 


DIGESTION.  203 

In  this  analysis  the  solid  ingredients  constitute  22.5  parts  per  1000, 
of  which  two-thirds  are  organic  and  one-third  inorganic.  The  amount 
of  solid  varies  according  to  the  animal  from  which  the  bile  is  obtained. 
Sodium  Glycocholate  and  Taurocholate. — Of  the  various  in- 
gredients of  the  bile  none  are  more  important  than  these  two  salts, 
usually  known  as  the  bile  salts.  The  sodium  glycocholate  is  found 
most  abundantly  in  the  bile  of  herbivora,  the  sodium  taurocholate 
in  the  bile  of  the  carnivora.  These  salts  are  compounds  of  sodium 
and  glycocholic  and  taurocholic  acids.  When  separated  from  the 
sodium,  the  acids  will  crystallize  in  the  form 
of  fine  acicular  needles.  Under  the  influ- 
ence of  hydrating  agents,  such  as  dilute 
acids  and  alkalies,  both  acids  will  undergo 
cleavage  into  their  respective  components — 
e.  g.,  glycocoll  and  cholalic  acid,  taurine 
and  cholalic  acid.  Glycocoll  and  taurine 
are  crystallizable  nitrogenized  compounds 

known    chemicallv    as    amido-acetic    and         FlG-  9°- — Chol  ester  in 
•  j     •     ,1  •  -j  1  rpi  Crystals.  —  (Landois   and 

amido-isotnionic   acids  respectively.       I  he     Stirling.) 

bile  salts  are  produced  in  the  liver  by  a 

true  act  of  secretion,  as  they  are  not  found  in  any  of  the  tissues  and 

fluids  of  the  body.      After  being  discharged  into  the  intestine  they 

undergo  chemic  changes,  after  which  they  can  no  longer  be  recognized. 

In  all  probability  they  are  reabsorbed  into  the  blood  and  play  some 

ulterior  part  in  the  nutrition  of  the  body. 

Cholesterin. — Cholesterin  is  a  constant  ingredient  of  bile,  though 
it  is  not  confined  to  this  fluid,  as  its  presence  has  been  determined  in 
the  crystalline  lens,  blood-corpuscles,  nerve-tissue,  and  various  patho- 
logic fluids.  It  is  an  organic  non-nitrogenized  substance  resembling 
the  fats  in  some  particulars,  but  differing  from  them  in  not  being  ca- 
pable of  saponification  with  alkalies.  It  presents  itself  in  the  form 
of  thin  transparent  rectangular  crystals,  insoluble  in  water  but  soluble 
in  ether  and  boiling  alcohol  (Fig.  90) .  It  is  held  in  solution  in  bile  by 
the  bile  salts.  If  they  are  deficient  in  amount,  the  cholesterin  may 
pass  out  of  solution,  collect  around  some  foreign  matter,  and  form 
a  gall-stone.  Cholesterin  is  a  product  of  the  metabolism  largely  of 
nerve-tissue,  from  which  it  is  absorbed  by  the  blood,  carried  to  the 
liver,  and  excreted.  In  the  intestine  it  is  converted  into  stercorin 
and  discharged  from  the  body  in  the  feces. 

Bilirubin,  Biliverdin. — These  two  pigments  impart  to  the  bile  its 
red  and  green  colors  respectively.  Bilirubin  is  present  in  the  bile  of 
human  beings  and  the  carnivora,  biliverdin  in  the  bile  of  the  herbivora. 
As  the  former  pigment  readily  undergoes  oxidation  in  the  gall-bladder, 
giving  rise  to  the  latter  pigment,  almost  any  specimen  of  bile  may 
present  any  shade  of  color  between  red  and  green.  Bilirubin  is  re- 
garded as  a  derivative  of  hematin,one  of  the  cleavage  products  of  hem- 
oglobin, the  coloring-matter  of  the  blood.     In  the  liver  the  hematin 


2o4  TEXT-BOOK  OF  PHYSIOLOGY. 

combines  with  water,  loses  its  iron,  and  is  changed  to  bilirubin.  By 
continuous  oxidation  there  are  formed  biliverdin,  bilicyanin,  and 
choletelin.  After  their  discharge  into  the  intestine  the  bile  pigments 
are  finally  reduced  to  hydrobilirubin,  which  becomes  one  of  the  con- 
stituents of  the  feces.  An  oxidation  of  the  bilirubin  can  be  produced 
by  nitroso-nitric  acid.  If  this  agent  is  added  to  a  thin  layer  of  bile  on  a 
porcelain  surface,  a  series  of  colors  will  rapidly  succeed  one  another, 
commencing  with  green  and  passing  to  blue,  orange,  purple,  and  yellow. 
This  is  the  basis  of  the  well-known  test  for  bile  pigments  suggested  by 
Gmelin. 

Lecithin.— Lecithin  is  regarded,  because  of  its  physical  properties 
and  chemic  composition,  as  a  complex  fat.  When  pure  it  presents 
itself  generally  as  a  white  crystalline  powder,  though  very  frequently 
as  a  white  waxy  mass  which  is  soluble  in  ether  and  alcohol.  Its  chemic 
formula  is  C44H90NP09.  Lecithin  is  widely  distributed  throughout 
the  body,  being  found  in  blood,  lymph,  red  and  white  corpuscles,  nerve- 
tissue,  yolk  of  egg,  semen,  milk  and  bile.  It  is  readily  decomposed, 
yielding  with  various  reagents  gly co-phosphoric  acid,  a  fat  acid  (stearic), 
and  the  alkaloid,  cholin.  Lecithin  has  been  regarded  as  one  of  the 
decomposition  products  of  nerve-tissue,  removed  from  the  blood  by 
the  liver  and  thus  becoming  one  of  the  constituents  of  the  bile  in 
which  it  is  held  in  solution  by  the  bile  salts. 

The  Mode  of  Secretion  and  Discharge  of  Bile. — The  manner 
in  which  the  bile  flows  from  the  liver  into  the  main  hepatic  ducts, 
the  variations  in  the  rate  of  its  discharge  into  the  intestine,  as  well 
as  the  total  quantity  secreted  daily,  have  been  approximately  deter- 
mined by  fistulous  openings  either  in  the  hepatic  ducts  or  in  the  gall- 
bladder. Although  the  liver  presents  some  physiologic  peculiarities, 
there  is  no  reason  to  believe  that  the  conditions  of  secretion  therein  are 
different  from  those  in  any  other  secretory  organ,  or  that  any  other 
structure  than  the  cell  is  engaged  in  this  process.  As  shown  by  chemic 
analysis,  the  bile  consists  of  compounds,  some  of  which,  like  the  bile 
salts,  are  formed  in  the  liver  cells,  out  of  material  furnished  by  the  blood 
by  a  true  act  of  secretion,  while  others,  such  as  cholesterin  and  lecithin, 
principles  of  waste,  are  merely  excreted  from  the  blood  to  be  finally 
eliminated  from  the  body.  The  bile  is  thus  a  compound  of  both 
secretory  and  excretory  principles. 

The  flow  of  bile  from  the  liver  is  continuous  but  subject  to  con- 
siderable variation  during  the  twenty-four  hours.  The  introduction 
of  food  into  the  stomach  at  once  causes  a  slight  increase  in  the  flow, 
but  it  is  not  until  about  two  hours  later  that  the  amount  secreted  reaches 
its  maximum;  after  this  period  it  gradually  decreases  up  to  the  eighth 
hour,  but  never  entirely  ceases.  During  the  intervals  of  digestion 
though  a  small  quantity  passes  into  the  intestine,  the  main  portion  is 
diverted  into  the  gall-bladder,  because  of  the  closure  of  the  common 
bile-duct  by  the  sphincter  muscle  near  its  termination,  where  it  is  re- 
tained until  required  for  digestive  purposes.     When  acidulated  food 


DIGESTION.  205 

passes  over  the  surface  of  the  duodenum  there  is  excited,  through  re- 
flex action,  a  contraction  of  the  muscle  walls  of  the  gall-bladder  and 
ducts,  a  relaxation  of  the  sphincter,  and  a  gush  of  bile  into  the  intestine, 
the  discharge  continuing  intermittently  until  digestion  ceases  and  the 
intestine  is  emptied  of  its  contents. 

The  storage  and  the  discharge  of  bile,  brought  about  by  the  alter- 
nate contraction  and  relaxation  of  the  muscle  walls  of  the  gall-bladder 
and  of  the  sphincter  is  regulated  by  the  nerve  system.  As  the  result 
of  his  experiments  Doyon  concludes,  that  during  the  intervals  of  intes- 
tinal digestion  the  vagus  nerve  is  carrying  nerve  impulses  which  on 
the  one  hand  augment  the  contraction  of  the  sphincter  and  inhibit 
the  contraction  of  the  walls  of  the  gall-bladder,  thus  establishing  the 
conditions  for  the  storage  of  bile;  but  when  intestinal  digestion  is  in- 
augurated the  splanchnic  nerve  carries  nerve  impulses  which  inhibit 
the  sphincter  and  augment  the  contraction  of  the  walls  of  the  gall- 
bladder, thus  establishing  the  condition  for  the  discharge  of  the  bile. 

The  total  quantity  of  bile  secreted  daily  has  been  estimated  to  be 
from  500  to  800  grams. 

Physiologic  Action  of  Bile.— Notwithstanding  our  knowledge 
of  the  complex  composition  of  bile,  the  quantity  discharged  daily,  and 
the  time  and  place  of  its  discharge,  its  exact  relation  to  the  digestive 
process  has  not  been  fully  determined.  No  specific  action  can  be 
attributed  to  it.  It  has  but  a  slight,  if  any,  diastatic  action  on  starch. 
It  is  without  influence  on  proteins  or  on  fats  directly.  But  indirectly 
and  by  virtue  of  the  bile  salts  it  contains,  it  plays  an  important  part 
in  increasing  the  action  of  the  pancreatic  enzymes.  Thus  the  anxiolytic 
power  of  the  pancreatic  juice  is  almost  doubled  and  the  same  is  true 
for  its  proteolytic  power,  while  its  fat-splitting  power  is  tripled.  The 
bile  salts  also  dissolve  insoluble  soaps  which  may  be  formed  during 
digestion. 

Bile  thus  favors  the  digestion  of  fat.  If  it  be  excluded  from  the  in- 
testine there  is  found  in  the  feces  from  22  to  58  per  cent,  of  the  ingested 
fats.  At  the  same  time  the  chyle,  instead  of  presenting  the  usual  white 
creamy  appearance,  is  thin  and  slightly  yellow.  The  manner  in  which 
the  bile  promotes  fat  digestion  is  yet  a  subject  of  investigation.  If 
all  the  fat  is  converted  into  fatty  acid  and  glycerin,  with  the  formation 
of  soaps,  as  seems  probable,  the  action  of  the  bile  becomes  more  ap- 
parent from  the  fact,  already  stated,  that  it  dissolves  and  holds  in  so- 
lution the  soaps  so  formed  which  would  be  necessary  to  their  absorp- 
tion by  the  epithelial  cells.  As  an  aid  to  digestion  the  bile  has  been 
regarded  as  important,  for  the  reason  that  its  entrance  into  the  intes- 
tine is  attended  by  a  neutralization  and  precipitation  of  the  proteins 
which  have  not  been  fully  digested  and  are  yet  in  the  stage  of  acid- 
albumin.  In  this  way  gastric  digestion  is  arrested  and  the  foods  are 
prepared  for  intestinal  digestion. 

Though  bile  possesses  no  antiseptic  properties  outside  the  bodv, 
itself  undergoing  putrefactive  changes  very  rapidly,  it  has  been  believed 


2o6  TEXT-BOOK  OF  PHYSIOLOGY. 

that  in  the  intestine  it  in  some  way  prevents  or  retards  putrefactive 
changes  in  the  food.  There  can  be  no  doubt  that  if  the  bile  be  pre- 
vented from  entering  the  intestine  there  is  an  increase  in  the  formation 
of  gases  and  other  products  which  impart  to  the  feces  certain  char- 
acteristics which  are  indicative  of  putrefaction.  As  to  the  manner 
in  which  bile  retards  this  process  nothing  definite  can  be  stated. 

Bile  has  been  supposed  to  be  a  stimulant  to  the  peristaltic  move- 
ments of  the  intestine,  inasmuch  as  these  movements  diminish  when 
bile  is  diverted  from  the  intestine. 

Though  no  definite  nor  specific  action  on  any  of  the  different  classes 
of  food  principles  can  be  attributed  to  the  bile,  there  is  abundant  evi- 
dence to  show  that  its  presence  in  the  alimentary  canal  during  digestion 
is  essential  to  the  maintenance  of  the  nutrition  of  the  body.  That 
the  bile  as  a  whole,  or  at  least  part  of  its  constituents,  favorably  in- 
fluences digestion  and  general  nutrition  is  evident  from  the  phenomena 
which  follow  its  total  exclusion  from  the  intestine,  as  when  the  common 
bile-duct  is  ligated  and  a  fistula  of  the  gall-bladder  is  established. 
The  following  phenomena  were  observed  in  a  young  dog  so  prepared 
by  Professor  Flint.  During  the  first  five  days  succeeding  the  opera- 
tion the  abdomen  was  tumid  and  there  was  some  rumbling  in  the  bowels. 
Though  the  animal  ate  every  day,  the  discharge  of  fecal  matter  became 
infrequent,  the  matter  passed  being  grayish  in  color  and  highly  offen- 
sive. After  two  weeks  the  alvine  discharges  took  place  three  and  four 
times  daily.  For  four  days  the  weight  remained  normal;  afterward 
it  began  to  diminish,  and  from  this  time  the  animal  continued  to  lose 
strength  and  weight  until  its  death,  thirty-eight  days  after  the  opera- 
tion. Ten  days  after  the  operation  the  appetite,  which  had  been  very 
good,  increased,  but  did  not  become  ravenous  until  a  few  days  before 
death.  The  animal  usually  ate  from  a  pound  to  a  pound  and  a  half 
of  beef-heart  daily,  always  refusing  fat.  There  was  an  absence  at 
all  times  of  jaundice,  fetor  of  the  breath,  and  falling  of  the  hair.  Post- 
mortem examination  showed  that  the  bile-duct  was  obliterated,  and 
there  was  no  evidence  that  any  bile  could  have  passed  into  the  intes- 
tine. The  results  of  this  and  similar  cases  go  to  show  that  that  portion 
of  the  bile  which  is  secretory  in  character  is  essential  to  digestion  and 
the  nutrition  of  the  body- — that,  though  large  quantities  of  food  are  con- 
sumed, progressive  diminution  of  weight  takes  place  until  nearly  40 
per  cent,  of  the  body  is  consumed.  In  some  instances  the  breath  be- 
comes fetid  and  there  is  a  falling  of  the  hair,  showing  some  profound 
disturbance  of  the  general  nutritive  process. 

The  Movements  of  the  Intestine. — The  movements  of  the  in- 
testine have  been  studied  by  means  of  the  Rontgen  rays  by  Cannon. 
The  method  adopted  was  to  mix  with  the  food,  subnitrate  of  bismuth, 
which  being  opaque  rendered  the  movements  of  the  intestinal  con- 
tents and  thereby  the  movements  of  the  intestinal  walls  visible,  on  the 
fluorescent  screen.  These  investigations  revealed  the  presence  of 
two  forms  of  activity,  one  of  which  is  more  or  less  stationary  and  due 


DIGESTION. 


207 


to  rhythmic  contraction  of  circular  muscle-fibers,  the  other  progres- 
sive, passing  from  above  downward  and  due  to  the  contraction  of 
circular  and  longitudinal  muscle-fibers.  The  former  activity,  which 
is  by  far  the  more  common,  results  in  a  division  of  the  intestinal  con- 
tents into  small  segments  and  for  this  reason  was  termed  by  Cannon, 
rhythmic  segmentation;  the  latter  activity  is  the  well-known  peristaltic 
wave. 

After  bands   of  circular   muscle-fibers,   situated   at  variable   dis- 
tances one  from  another,  contract  and  divide  a  mass  of  food  into  seg- 


B 


/\/\/\/\/\ 


\/\/\/\/w 


D 


Fig.  qi. — The  Divisive  or  Segmenting  Movements  of  the  Small  Intestine. 
A,  surface  view  of  a  portion  of  the  intestine,  showing  six  constrictions  which  divide  the 
contents  into  five  segments,  as  shown  in  B;  as  these  constrictions  pass  away  new  ones 
come  in  between  them  and  divide  each  segment  of  the  contents  into  two,  the  adjoining 
halves  of  neighboring  segments  fusing  to  make  the  new  segments  shown  in  C.  Repetition 
of  this  process  results  in  the  condition  shown  in  D.  {Modified,  After  Hougli  and  Sedge 
wick,  "The  Human  Mechanism.") 


ments,  they  at  once  relax  and  are  followed  by  contraction  of  other 
bands  in  the  segments  of  the  intestines  overlying  the  segments  of  food. 
The  result  is  again  a  division  of  the  food  into  two  new  segments  (Fig. 
91).  The  lower  half  of  each  segment  then  unites  with  the  upper  half  of 
the  segment  below  to  commingle  with  it  and  expose  new  surfaces  of 
the  food  mass  to  contact  with  the  actively  absorbing  mucosa.  The 
continual  repetition  of  this  process  results  in  a  thorough  mixing  of  the 
food  with  the  digestive  juices.  From  the  manner  in  which  these  con- 
tractions make  their  appearance  it  would  seem  that  the  mere  presence 
of  a  segment  in  the  lumen  of  the  intestine  is  sufficient  to  excite  the 
overlying  fibers  to  activity. 

In   certain  regions  of  the  intestine  rhythmic  segmentation  may 
continue  for  half  to  three-quarters  of  an  hour  without  moving  the 


2oS  TEXT-BOOK  OF  PHYSIOLOGY. 

food  forward  to  any  marked  extent.  In  the  cat  the  segmentation 
may  proceed  at  the  rate  of  thirty  divisions  a  minute. 

Bayliss  and  Starling  state,  from  observations  made  on  the  exposed 
intestine  of  a  dog,  that  in  addition  to  the  usual  peristaltic  movement 
the  intestinal  coils  exhibit  a  swaying  or  pendulum  movement  accom- 
panied by  slight  waves  of  contraction  which  may  arise  apparently  at 
any  point  and  pass  down  the  intestine.  These  contractions  may 
occur  from  ten  to  twelve  times  a  minute  and  travel  at  a  rate  varying 
from  two  to  five  centimeters  a  second.  In  how  far  this  movement 
represents  the  normal  movement  as  it  takes  place  under  physiologic 
conditions  and  as  observed  by  Cannon,  remains  for  further  investiga- 
tors to  decide. 

After  the  food  has  been  prepared  by  the  process  above  described, 
it  is  then  slowly  carried  downward  by  what  is  known  as  the  vermicular 
or  peristaltic  wave.  This  wave  is  characterized  by  a  contraction  of 
the  circular  fibers  behind  a  bolus  and  a  relaxation  of  the  fibers  in  ad- 
vance of  it.  The  result  is  a  movement  forward  of  the  bolus,  and  as 
it  moves  it  is  followed  by  a  ring  of  constriction  and  preceded  by  a 
ring  of  relaxation  or  inhibition.  The  rate  of  movement  of  the  peris- 
taltic wave  is  extremely  slow. 

The  Nerve  Mechanism  of  the  Intestine. — The  causes  of  these 
two  forms  of  intestinal  activity,  rhythmic  segmentation  or  pendulum 
movement  and  peristalsis,  have  been  the  subject  of  much  investiga- 
tion. Because  of  the  presence  of  a  network  or  plexus  (Auerbach's 
and  Meissner's)  of  nerve-cells  and  nerve-fibers  in  the  walls  of  the  intes- 
tines and  in  close  relation  to  the  muscle  cells,  and  because  of  the  fact 
that  the  intestines  will  contract  for  some  time  after  removal  from  the 
body  of  the  animal,  it  has  been  difficult  to  decide  whether  the  contrac- 
tions are  of  myogenic  or  neurogenic  origin. 

As  the  rhythmic  contractions  continue,  though  the  peristaltic  are 
abolished  by  the  introduction  of  nicotin  into  the  blood,  an  agent  which 
temporarily  paralyses  peripheral  nerve-cells,  it  was  concluded  by 
Bayliss  and  Starling  that  the  rhythmic  contractions  are  of  myogenic 
origin  and  propagated  from  fiber  to  fiber  and  that  the  peristaltic  con- 
tractions are  reflex  in  character,  the  coordination  being  carried  out  by 
the  local  nerve  mechanisms  and  initiated  by  stimulation  of  the  intes- 
tine. Whether  this  is  the  case  or  not,  the  general  contractions  of  the 
intestine  are  augmented  and  inhibited  by  the  central  nerve  system 
through  the  vagus  and  splanchnic  nerves. 

Stimulation  of  the  vagus  is  followed  by  an  augmentation  of  the 
contraction,  though  not  infrequently  there  is  a  primary  inhibition  of 
short  duration.  Stimulation  of  the  splanchnic  is  followed  by  a  relax- 
ation or  inhibition  of  the  contraction,  though  according  to  some  observ- 
ers there  is  at  times  an  opposite  effect. 

The  Large  Intestine. — The  large  intestine  is  that  portion  of  the 
alimentary  canal  situated  between  the  termination  of  the  ileum  and 
the  anus.     It  varies  in  length  from  four  and  a  half  to  five  feet,  in  di- 


DIGESTION.  209 

ameter  from  one  and  a  half  to  two  and  a  half  inches.  It  is  divided 
into  the  cecum,  the  colon  (subdivided  into  an  ascending,  transverse, 
and  descending  portion,  including  the  sigmoid  flexure),  and  the  rectum. 

The  cecum  is  situated  in  the  right  iliac  fossa.  It  is  that  dilated 
portion  of  the  large  intestine  below  the  orifice  of  the  small  intestine. 
The  posterior  and  inner  wall  presents  a  small  opening  which  leads 
into  a  narrow  round  process  about  four  inches  in  length — the  vermi- 
form appendix.  The  opening  of  the  small  intestine  into  the  cecum  is 
narrow  and  elongated  and  bordered  by  two  folds  of  mucous  membrane 
strengthened  by  fibrous  and  muscle-tissue.  These  folds  constitute  the 
so-called  ileo-cecal  valve.  When  the  cecum  is  distended  the  margins 
of  these  folds  are  approximated  and  effectually  prevent  the  return  of 
material  into  the  small  intestine. 

The  closure  of  this  opening  is  now  attributed  to  a  sphincter  muscle 
— the  ileo-colic — the  action  of  which  is  regulated  by  the  nerve  system. 

The  colon  ascends  to  the  under  surface  of  the  liver,  where  it  bends 
at  a  right  angle,  crosses  the  abdominal  cavity  to  the  spleen,  bends 
again,  and  descends  to  the  left  iliac  fossa.  At  this  point  it  turns  upon 
itself  to  form  the  sigmoid  flexure.  The  rectum  is  a  dilated  pouch, 
situated  within  the  true  pelvis.  It  measures  from  15  to  18  centimeters 
in  length.  Within  an  inch  of  its  termination  at  the  anus  it  presents  a 
constriction  formed  by  a  circular  band  of  muscle-fibers  known  as  the 
internal  sphincter.  The  margin  of  the  anus  is  also  surrounded  by 
bands  of  muscle-fibers  known  collectively  as  the  external  sphincter. 

The  walls  of  the  large  intestine  consist  of  three  coats:  viz.,  serous, 
muscular,  and  mucous. 

The  serous  is  a  reflection  of  the  general  peritoneal  membrane. 

The  muscle  is  composed  of  both  longitudinal  and  circular  fibers. 
The  longitudinal  fibers  are  collected  into  three  narrow  bands  which 
are  situated  at  points  equidistant  from  one  another.  At  the  rectum 
they  spread  out  so  as  to  completely  surround  it.  As  the  longitudinal 
bands  are  shorter  than  the  intestine  itself,  its  surface  becomes  saccu- 
lated, each  sac  being  partially  separated  from  adjoining  sacs  by  nar- 
row constrictions.  The  circular  fibers  are  arranged  in  the  form  of  a 
thin  layer  over  the  entire  intestine.  Between  the  sacculi,  however, 
they  are  more  closely  arranged.  In  the  rectum  they  are  well  developed, 
and  at  a  point  an  inch  above  the  anus  they  form,  as  stated  above,  the 
internal  sphincter. 

The  mucous  membrane  of  the  large  intestine  possesses  neither  villi 
nor  valvulse  conniventes.  It  contains  a  large  number  of  tubules  consist- 
ing of  a  basement  membrane  lined  by  columnar  epithelium.  They 
resemble  the  follicles  of  Lieberkuhn.  The  secretion  of  these  glands 
is  thick  and  viscid  and  contains  a  large  quantity  of  mucin. 

Contents  of  the  Large  Intestine. — As  a  result  of  the  actions 

of  saliva,  of  gastric,  intestinal,  and  pancreatic  juice,  and  of  the  bile, 

the  food  is  disintegrated  and  liquefied.     The  nutritive  principles,  pro- 

teid,  starches,  sugars,  and  fats,  undergo  chemic  changes  and  are  trans- 

14 


210  TEXT-BOOK  OF  PHYSIOLOGY. 

formed  into  peptones,  dextrose,  soap  and  glycerin,  fatty  acids,  under 
which  forms  they  are  absorbed.  After  the  more  or  less  complete  di- 
gestion and  absorption  of  these  nutritive  substances  the  residue  of  the 
food,  comprising  the  indigestible  and  undigested  matter,  passes  out 
of  the  small  intestine  into  the  large  intestine  and  forms  a  portion  of  its 
contents.  This  residue  consists  of  the  hard  parts  of  the  cereals,  veget- 
able seeds,  cellulose,  etc.,  the  quantity  and  variety  of  which  depend  on 
the  nature  of  the  food.  These  substances,  passing  into  the  large  intes- 
tine along  with  the  excrementitious  matter  of  the  bile,  become  incor- 
porated with  the  mucous  secretions  and  assist  in  the  formation  of  the 
feces.  Under  the  influence  of  a  peristaltic  movement  similar  to  that 
witnessed  in  the  small  intestine,  all  this  excrementitious  matter,-  de- 
prived by  absorption  of  the  excess  of  its  contained  water  and  nutritive 
material,  is  gradually  carried  downward  to  the  sigmoid  flexure,  where 
it  accumulates  prior  to  its  extrusion  from  the  body. 

The  effects  of  the  peristaltic  waves  are  to  some  extent  interfered 
with  by  anti-peristaltic  waves  which  beginning  in  the  transverse  colon 
run  toward  and  to  the  cecum.  An  anti-peristaltic  wave  occurs  in  the 
cat  about  every  fifteen  minutes  and  lasts  for  about  five  minutes.  The 
intestinal  contents  are  thereby  driven  back  toward  the  cecum.  The 
effect  is  a  still  further  admixture  with  the  secretions  and  exposure  to 
the  absorbing  mucosa.  There  is  some  evidence  also  that  the  anti-per- 
istaltic wave  may  force  some  of  the  liquefied  contents  through  the  ileo- 
colic opening  into  the  small  intestine  because  of  the  relaxation  of  the 
sphincter  muscle. 

Intestinal  Fermentation.— Owing  to  the  favorable  conditions 
in  the  intestine  for  fermentative  and  putrefactive  processes — e.  g., 
heat,  moisture,  oxygen,  and  the  presence  of  various  microorganisms 
— the  food,  when  consumed  in  excessive  quantity  or  when  acted  on 
by  defective  secretions,  undergoes  a  series  of  decomposition  changes 
which  are  attended  by  the  production  of  gases  and  various  chemic 
compounds.  Dextrose  and  maltose  are  partially  reduced  to  lactic 
acid;  this  to  butyric  acid,  carbon  dioxid,  and  hydrogen.  Fats  are 
reduced  to  glycerol  and  fatty  acids,  the  glycerol,  according  to  the  or- 
ganisms present,  yields  succinic  acid,  carbon  dioxid,  and  hydrogen. 
The  proteids  under  the  prolonged  action  of  the  pancreatic  juice  are 
decomposed,  with  the  production  of  leucin  and  tyrosin.  These  crys- 
talline compounds  are  in  turn  reduced  to  simpler  forms.  The  former 
yields  valerianic  acid,  ammonia,  and  carbon  dioxid;  the  latter  gives 
rise  to  indol,  which  is  the  antecedent  of  indican,  found  in  the  urine. 
Skatol,  another  derivative  of  the  proteid  molecule,  due  to  bacterial 
action,  gives  the  characteristic  odor  to  the  feces. 

Feces. — The  feces  consist  of  water,  mucin,  the  indigestible  resi- 
due of  the  food,  decomposition  products,  and  inorganic  salts.  The 
consistency  of  the  fecal  matter  varies  from  fluid  to  semifluid,  depend- 
ing largely  on  the  length  of  time  it  remains  in  the  intestine  and  the 
extent  to  which  absorption  of  its  watery  portion  has  taken  place.     The 


DIGESTION.  211 

odor  is  due  to  the  presence  of  sulphuretted  hydrogen  and  skatol.  The 
color  is  due  partly  to  the  altered  coloring-matter  of  the  bile,  hydro- 
bilirubin  or  'stercobilin,  and  partly  to  the  character  of  the  food.  The 
total  quantity  discharged  daily  varies  from  four  to  six  ounces. 

Defecation. — Defecation  is  the  final  act  of  the  digestive  process 
and  consists  in  the  expulsion  of  the  indigestible  residue  of  the  food 
from  the  intestine.  This  act  usually  takes  place  in  the  human  being 
but  once  in  twenty-four  hours,  as  the  diet  contains  but  a  minimum 
quantity  of  indigestible  matter.  Previous  to  their  expulsion  the  feces 
which  have  accumulated  in  the  sigmoid  flexure  must  pass  downward 
into  the  rectum.  In  so  doing  they  develop  the  sensation  which  leads 
to  the  act  of  defecation.  The  descent  of  the  feces  is  accomplished  by 
the  peristaltic  contraction  of  the  intestinal  wall.  Coincident  with  the 
passage  of  the  feces  into  the  rectum  there  is  a  relaxation  of  the  sphinc- 
ter muscles  and  a  contraction  of  the  longitudinal  and  circular  mus- 
cular fibers,  in  consequence  of  which  the  feces  are  expelled.  These 
complex  muscular  actions  are  also  aided  by  the  voluntary  contrac- 
tions of  the  diaphragm  and  abdominal  muscles. 

Nerve  Mechanism  of  Defecation. — The  act  of  defecation  is 
primarily  reflex,  though  somewhat  influenced  by  voluntary  efforts. 
Under  normal  conditions  the  sphincter  muscles  governing  the  anal 
orifice  are  firmly  contracted,  thus  preventing  the  escape  of  gases  or 
semisolid  matter.  This  tonic  contraction  is  maintained  by  a  nerve- 
center  located  in  the  lumbar  region  of  the  spinal  cord.  The  circu- 
lar and  longitudinal  muscle-fibers  of  the  rectum  are  at  the  same  time 
in  a  relaxed  condition.  When  the  desire  to  evacuate  the  bowels  is  ex- 
perienced, the  impressions  made  by  the  feces  on  the  afferent  nerves  of 
the  rectal  mucous  membrane  develop  nerve  impulses,  which  trans- 
mitted to  the  rectal  center  and  to  the  brain,  influence  in  one  direction 
or  another,  their  activities.  If  the  act  of  defecation  is  to  take  place 
there  is  an  inhibition  of  the  contraction  of  the  sphincter  ani  muscles 
and  an  augmentation  of  the  contraction  of  the  rectal  muscles.  In 
their  expulsive  efforts,  these  latter  muscles  are  assisted  by  the  con- 
traction of  the  diaphragm,  abdominal  and  other  muscles.  After  the 
expulsion  of  the  feces  there  is  a  return  to  the  former  condition,  viz.,  a 
relaxation  of  the  rectal  muscles  and  a  contraction  of  the  sphincters. 

If  the  act  is  to  be  suppressed,  the  controlling  influence, of  the  rectal 
or  sphincter  center  is  strengthened  and  the  reflex  mechanism  for  a 
while  held  in  abeyance. 

The  exact  course  of  the  afferent  and  efferent  nerves  concerned 
in  this  reflex  is  yet  a  subject  of  investigation. 

The  nerve-supply  for  the  circular  and  longitudinal  muscles  of* 
the  lower  part  of  the  colon  and  rectum  varies  somewhat  in  different 
animals,  though  it  is  usually  derived  from  the  second,  third,  and  fourth 
lumbar  and  the  second  and  third  sacral  or  pelvic  nerves. 

The  lumbar  nerves  pass  into  and  through  the  sympathetic  chain 
and  thence  to  the  inferior  mesenteric  ganglion,  around  the  cells  of  which 


212  TEXT-BOOK  OF  PHYSIOLOGY. 

most  of  the  nerves  arborize.     From  this  ganglion  nerve-fibers  pass  to 
both  the  circular  and  longitudinal  fibers. 

The  sacral  or  pelvic  nerves  pass  to  the  hypogastric  plexus  and 
are  ultimately  connected  with  ganglion  cells,  which  in  turn  send  fibers 
to  both  the  longitudinal  and  circular  fibers.  The  explanation  of  the 
action  of  this  complex  mechanism  is  a  subject  of  discussion  because 
of  the  want  of  agreement  in  the  results  that  follow  stimulation  of  these 
nerves. 


CHAPTER  XL 
ABSORPTION. 

Absorption  is  the  process  by  which  nutritive  material  is  trans- 
ferred from  the  tissues,  from  the  serous  cavities,  and  from  the  mucous 
surfaces  of  the  body,  into  the  blood.  The  most  important  of  these 
surfaces,  especially  in  its  relation  to  the  formation  of  blood,  is  the 
mucous  surface  of  the  alimentary  canal,  for  it  is  from  this  organ  that 
the  new  materials  are  derived  which  maintain  the  quantity  and  quality 
of  the  blood.  The  absorption  of  material  from  the  interstices  of  the 
tissues  and  from  the  serous  cavities  may  be  regarded  as  an  act  of  re- 
sorption, or  a  return  to  the  blood  of  liquid  nutritive  material  which  has 
escaped  through  the  walls  of  the  capillary  blood-vessels  for  purposes 
of  nutrition,  and  which,  if  not  returned,  would  lead  to  an  accumulation 
and  the  development  of  edematous  conditions. 

The  anatomic  mechanisms  involved  in  the  absorptive  process 
are,  primarily,  the  tissue  or  lymph-spaces,  the  lymph-  and  blood-cap- 
illaries; secondarily,  the  lymph-vessels  and  the  veins. 

Tissue  or  Lymph-spaces;  Lymph-capillaries. — Everywhere 
throughout  the  body,  in  the  connective-tissue  system  and  in  the  inter- 
stices of  the  several  structures  of  which  an  organ  is  composed,  are 
found  spaces  or  clefts  of  irregular  shape  and  size,  determined  largely 
by  the  structure  of  the  organ  in  which  they  are  found,  which  have 
been  termed  tissue  or  lymph-spaces,  from  the  fact  that  they  contain 
a  clear  fluid,  the  lymph.  These  spaces  are  devoid  for  the  most  part  of 
any  endothelial  lining,  but  as  they  communicate  more  or  less  freely 
one  with  another,  there  is  a  circulation  of  lymph  through  them  and 
around  the  islets  of  tissue  (Fig.  93).  In  addition  to  the  connective- 
tissue  lymph-spaces,  different  observers  have  described  special  spaces 
or  clefts  in  organs  such  as  the  kidney,  liver,  spleen,  testicle,  and  in 
all  secreting  glands  between  their  basement  membrane  and  the  sur- 
rounding blood-vessels,  all  of  which  contain  a  greater  or  less  quan- 
tity of  lymph.  Within  the  brain,  spinal  cord,  bone,  and  other  tissues 
it  has  been  shown  that  the  smallest  blood-vessels  and  capillaries  are 
bounded  and  limited  by  a  cylindrical  sheath  containing  lymph,  which 
is  known  as  a  perivascular  lymph-space.  A  similar  sheath  surrounds 
the  smallest  nerve-bundles  and  fibers,  enclosing  a  perineural  lymph- 
space.  The  large  serous  cavities  of  the  body,  pleural,  peritoneal, 
pericardial,  etc.,  are  also  to  be  regarded  as  lymph-spaces.  The  sur- 
faces of  these  cavities,  however,  are  covered  with  a  layer  of  endo- 
thelial cells  with  sinuous  margins.  At  intervals  between  these  cells  are 
to  be  found  small  free  openings  which  have  received  the  name  of  stomata. 

213 


214 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  lymph-capillaries  in  which  the  lymph-vessels  proper  take 
their  origin  are  arranged  in  the  form  of  plexuses  of  quite  irregular 
shape.  In  most  situations  they  are  intimately  interwoven  with  the 
blood-vessels,  from  which  they  can  be  readily  distinguished  by  their 
larger  caliber  and  irregular  expansions.  The  wall  of  the  lymph-cap- 
illary is  formed  by  a  single  layer  of  endothelial  cells  with  characteristic 
sinuous  outlines.  These  capillaries  anastomose  very  freely  one  with 
another  and  communicate,  on  the  one  hand,  with  the  lymph-spaces 
and  on  the  other  with  the  lymph-vessels  proper.  As  the  shape,  size, 
etc.,  of  .both  lymph-spaces  and  capillaries  are  determined  largely  by 

the  nature  of  the  tissue  in 
which  they  are  found,  it  is 
not  always  possible  to 
separate  one  from  the 
other.  Their  function, 
however,  may  be  regarded 
as  similar:  viz.,  the  recep- 
tion and  collection  of  the 
lymph  which  has  transuded 
through  the  walls  of  the 
blood-vessels  and  its  trans- 
mission onward  into  the 
regular  lymph-vessels. 

The  blood-capillaries 
not  only  permit  of  a  transu- 
dation of  the  liquid  nutri- 
tive material  from  the  blood 
through  their  delicate  walls, 
but  are  also  engaged,  if  not 
in  the  reabsorption  of  a 
portion  of  this  transudate, 
at  least  in  the  absorption  of 
waste  products  resulting 
from  tissue  metabolism. 
Lymph-vessels.- — The  lymph- vessels  constitute  a  system  of  minute, 
delicate,  transparent  vessels  found  in  nearly  all  the  organs  and  tissues 
of  the  body,  and  take  their  origin  from  the  lymph-capillaries  and  spaces 
above  described  (Figs.  94  and  95.)  From  their  origin  they  gradually  con- 
verge toward  the  trunk  of  the  body,  and  finally  empty  into  the  thoracic 
duct.  In  their  course  they  anastomose  very  freely  with  adjoining  ves- 
sels. The  diameter  of  a  lymph-vessel  varies  from  1  to  2  mm.  After  the 
lymphatics  have  emerged  from  the  lymph-capillaries  they  acquire 
three  distinct  coats,  each  of  which  possesses  definite  histologic  features. 
The  internal  coal  is  composed  of  a  delicate  lamina  of  longitudinally 
disposed  elastic  fibers  covered  with  a  layer  of  flattened  nucleated 
endothelial  cells  with  wavy  outlines. 

The  middle  coat  consists  of  white  fibrous  tissue  arranged  longit- 


Fig.  93. — Origin  of  Lymphatics  from  the 
Central  Tendon  of  the  Diaphragm  Stained 
with  Nitrate  of  Silver.  5.  The  lymph-spaces 
and  lymph-canals  communicating  at  x  with  the 
lymphatics,  a.  Origin  of  the  lymphatics  by  the 
confluence  of  several  juice  canals.  B.  Capillary 
blood-vessel. — (Landois  and  Stirling.) 


ABSORPTION. 


215 


udinally  and  of  non-striated  muscle  and  elastic  fibers  arranged  trans- 
versely. 

The  external  coat  consists  of  practically  the  same  structures,  though 
the  muscle-fibers  are  longitudinally  disposed. 

The  lymphatics  are  provided  with  valves  which  are  so  numerous 
and  located  at  such  short  intervals  as  to  give  the  vessels  a  beaded  ap- 
pearance. These  valves  are  arranged  in  pairs  and  consist  of  two  semi- 
lunar folds  with  their  concavities  directed  toward  the  larger  vessels. 


Fig.  94. — Lymph-vessels   and  Lymph-glands  of  the  Head  and  Neck. — {From 
Gould's  Dictionary.) 

They  are  formed  by  a  reduplication  of  the  lining  membrane,  which  is 
strengthened  by  fibrous  tissue  derived  from  the  middle  coat. 

Lymph-glands.— In  their  course  toward  the  thoracic  duct  the 
lymph-vessels  pass  through  a  number  of  small  lenticular  bodies  termed 
lymph-glands.  These  are  exceedingly  abundant  in  some  situations, 
as  the  cervical,  axillary,  and  inguinal  regions,  and  the  abdominal 
cavity.  As  the  lymph-vessels  approach  a  gland  they  divide  into  a 
number  of  branches  before  entering  it,  known  as  the  afferent  vessels. 
From  the  opposite  side  of  the  gland  the  lymphatics  again  emerge  as 
efferent  vessels  to  unite  to  form  larger  trunks.  A  section  of  a  gland 
shows  that  it  consists  of  an  outer  dense  cortical  and  an  inner  soft  pulpy 


2l6 


TEXT-BOOK  OF  PHYSIOLOGY. 


medullary  portion.  Each  gland  is  covered  externally  by  a  dense  mem- 
brane of  fibrous  tissue  containing  in  its  meshes  non-striated  muscle- 
fibers.     From  the  inner  surface  of  this  membrane  there  pass  inward 

septa  of  connective  tissue  which,  as 
they  converge  toward  the  center  of  the 
gland,  divide  the  outer  zone  of  the 
gland  into  small  conical  compartments 
or  alveoli.  When  the  septa  reach  the 
medullary  portion,  they  subdivide  and 
form  bands  or  cords  which  interlace 
in  every  direction  and  constitute  a 
loose  meshwork  the  spaces  of  which 
communicate  with  one  another  and 
with  the  alveoli  (Fig.  96).  Within 
the  meshes  of  this  framework  the 
proper  gland  substance  is  contained. 
In  the  cortical  compartments  it  is 
moulded  into  pear-shaped  masses;  in 
the  medullary  meshwork  it  assumes 
the  form  of  rounded  cords  which  are 
connected  with  one  another.  In  both 
regions,  however,  it  is  separated  from 
the  septa  by  a  space  termed  a  lymph 
sinus,  through  which  the  lymph  flows 
as  it  passes  through  the  gland.  The 
lymph  sinus  is  crossed  by  a  network 
of  retiform  connective  tissue  which 
offers  considerable  resistance  to  the 
passage  of  the  lymph.  The  gland  sub- 
stance consists  also  of  a  framework  of 
retiform  connective  tissue  in  the 
meshes  of  which  large  numbers  of 
lymph-corpuscles  are  contained.  The 
gland  substance  is  separated  from  the 
lymph  sinus  by  a  dense  layer  of  a 
reticulum,  which,  however,  does  not 
prevent  lymph  and  even  corpuscles 
from  passing  through  it  into  the  lymph 
sinus. 

The  lymph-glands  are  abundantly 

supplied    with    blood-vessels.       The 

arteries  enter  the  gland  at  the  hilum, 

penetrate  into  the  medullary  substance,  and  terminate  in  a  fine  capillary 

plexus  which  is  supported  by  the  connective  tissue.     The  veins  arising 

from  this  plexus  leave  the  gland  also  at  the  hilum. 

The  lymph-vessels  which  enter  a  gland  first  ramify  in  the  invest- 
ing  membrane  and  then  open  directly  into  the  lymph  sinus.     The 


Fig.  95. — Lymph-vessels  of  the 
Arm. — (Deaver.) 


ABSORPTION. 


217 


vessels  which  leave  the  gland  are  also  in  communication  with  the 
sinus.  After  the  lymphatics  enter  the  gland  they  lose  their  external 
and  middle  coats,  retaining  only  the  internal  or  endothelial  coat,  which 
lines  the  inner  surface  of  the  lymph  sinus.  The  current  of  lymph, 
therefore,  is  from  the  afferent  vessels  through  the  lymph  sinus  into 
the  efferent  vessels.  In  addition  to  this  primary  current,  there 
is  a  secondary  current  flowing  from  the  capillary  blood-vessels  outward 
and  into  the  sinus,  which  carries  with  it  large  numbers  of  lymph-cor- 
puscles. It  is  quite  probable  that  the  movement  of  the  lymph  through 
this  complicated  system  of  passages  is  aided  by  the  contraction  of  the 
muscle-fibers  in  the  capsule  of  the  gland. 

The  lymph-corpuscles  or  lymphocytes  originate  for  the  most  part 


a.l. 


Fig.  96. — Diagrammatic  Section  of  a  Lymph-gland,  a.l.,  Afferent,  e.  I.,  efferent 
lymphatics.  C.  Cortical  substance.  M.  Reticular  cords  of  medulla,  l.s.  Lymph  sinus. 
c.  Capsule,  with  trabecular,  tr. — {Landois  and  Stirling.) 


in  the  gland  substance  of  the  cortical  alveoli.  In  this  situation  there 
are  groups  of  cells,  so-called  germ  centers,  which  divide  very  rapidly 
by  mitosis  and  give  rise  constantly  to  groups  of  young  cells  which 
soon  find  their  way  into  the  lymph  stream. 

The  Thoracic  Duct. — The  thoracic  duct  is  the  general  trunk  of 
the  lymph  system,  into  which  the  vessels  of  the  lower  extremities, 
of  the  abdominal  organs,  of  the  trunk,  of  the  left  arm,  and  of  the  left 
side  of  the  head  empty  their  contents.  It  is  about  fifty  centimeters 
in  length  and  four  millimeters  in  diameter.  It  extends  upward 
from  the  third  lumbar  vertebra  along  the  vertebral  column  to  the 
seventh  cervical  vertebra,  where  it  empties  into  the  venous  system 
at  the  function  of  the  internal  jugular  and  subclavian  veins  on  the  left 
side.  The  thoracic  duct  wall  has  the  same  general  structure  as  the 
wall  of  the  lymph- vessel:    viz.,  an  internal  or  endothelial;  a  middle 


2l8 


TEXT-BOOK  OF  PHYSIOLOGY. 

It  is  also  provided 


elastic  and   muscular;   an  external  or  fibrous, 
with  numerous  valves. 

The  lymph-vessels  of  the  right  side  of  the  head,  of  the  right  arm, 
and  a  portion  of  the  right  ride  of  the  trunk  terminate  in  the  right 


Fig.  97. — Diagram  Showing  the  Course  of  the  Main  Trunks  of  the  Absorbent 
System.  The  lymphatics  of  lower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC) 
at  the  receptaculum  chyli  (RC),  where  the  thoracic  duct  begins.  The  superficial  vessels 
are  shown  in  the  diagram  on  the  right  arm  and  leg  (S),  and  the  deeper  ones  on  the  arm 
to  the  left  (D).  The  glands  are  here  and  there  shown  in  groups.  The  small  right  duct 
opens  into  the  veins  on  the  right  side.  The  thoracic  duct  opens  into  the  union  of  the 
great  veins  of  the  left  side  of  the  neck  (T). — (Yeo's  "Text-book  0}  Physiology.") 

thoracic  duct,  which  is  about  25  to  30  mm.  in  length  and  which  emp- 
ties into  the  venous  system  at  the  junction  of  the  internal  jugular  and 
subclavian  veins  on  the  right  side.  The  general  arrangement  of  the 
lymphatic  system  is  diagrammatically  shown  in  Fig.  07. 

LYMPH. 

Lymph  is  the  clear  fluid  found  within  the  tissue  spaces  and  with- 
in  the  lymph-vessels.     Inasmuch  as  there  are  reasons  for  the  view 


ABSORPTION.  219 

that  lymph  varies  in  composition,  as  well  as  in  function,  in  these 
different  regions  it  will  be  found  conducive  to  clearness  to  designate 
the  lymph  found  in  the  tissue  spaces  as  intercellular  lymph,  and  that 
found  in  the  lymph-vessels  as  intravascular  lymph. 

The  Physical  Properties  of  Lymph. — Whether  obtained  from 
tissue  spaces  or  from  lymph-vessels,  the  lymph  presents  practically 
the  same  physical  properties.  The  lymph  obtained  from  the  tho- 
racic duct  during  the  intervals  of  digestion  or  from  one  of  the  large 
trunks  of  the  leg  is  a  clear,  colorless  or  slightly  opalescent  fluid  hav- 
ing an  alkaline  reaction  and  a  specific  gravity  of  1.020  to  1.040.  Ex- 
amined microscopically  it  is  seen  to  hold  in  suspension  a  large  number 
of  corpuscles  similar  to  those  seen  in  the  lymph-glands  and  to  the 
white  corpuscles  of  the  blood.  Their  number  has  been  estimated 
at  about  8200  per  cubic  millimeter,  though  this  count  will  vary  within 
wide  limits  according  as  the  lymph  examined  has  passed  through  a 
larger  or  smaller  number  of  glands.  The  lymph-corpuscle  consists 
of  a  small  quantity  of  protoplasm  in  which  is  embedded  a  distinct 
nucleus.  Some  of  these  lymphocytes  contain  distinct  granules, 
more  or  less  refractive,  which  impart  to  the  corpuscle  a  distinctly 
granular  appearance.  When  withdrawn  from  the  vessels  lymph 
undergoes  a  spontaneous  coagulation,  though  the  coagulum  is  never 
as  firm  as  that  observed  in  the  coagulation  of  the  blood.  The  cause 
of  the  coagulation  is  the  appearance  of  fibrin.  After  a  variable  length 
of  time  the  coagulum  separates  into  a  liquid  and  a  solid  portion,  the 
serum  and  the  clot. 

The  Chemic  Composition  of  Lymph. — Although  the  lymph 
obtained  from  the  tissue  spaces,  from  the  lymph-vessels,  as  well  as 
from  the  so-called  serous  cavities  has  the  same  general  chemic  char- 
acteristics, there  is  reason  for  the  view  that  it  varies  in  its  ultimate 
composition  according  as  it  is  derived  from  one  region  of  the  body 
or  from  another.  The  needs  of  any  individual  tissue  as  well  as  the 
character  of  its  metabolic  products  will  in  all  probability  not  only 
change  its  normal  composition,  but  also  the  relative  amounts  of  its 
normal  constituents. 

Chemic  analysis  has  shown  that  the  lymph  from  the  thoracic  duct 
contains  from  3.4  to  4.1  per  cent,  of  proteins  (serum-albumin,  fibrin- 
ogen), 0.046  to  0.13  per  cent,  of  substances  soluble  in  ether  (probably 
fat),  0.1  per  cent,  of  sugar,  and  from  0.8  to  0.9  per  cent,  of  inorganic 
salts,  of  which  sodium  chlorid  (0.55  per  cent.)  and  sodium  carbonate 
(0.24  per  cent.)  are  the  most  abundant  (Munk).  There  are  usually 
in  most  specimens  small  quantities  of  potassium,  calcium,  and  mag- 
nesium salts.  Fibrinogen  is  seldom  present  beyond  0.1  per  cent., 
which  will  account  for  the  feeble  and  slow  coagulation.  Lymph 
contains  both  free  oxygen  and  carbon  dioxid.  Of  the  former,  how- 
ever, there  is  but  a  small  percentage;  of  the  latter,  about  45  vols,  per 
cent.,  partially  in  the  free  state  and  partially  combined  with  sodium. 
Urea  is  also  present  in  very  small  amounts.     This  analysis  indicates 


220  TEXT-BOOK  OF  PHYSIOLOGY. 

that  lymph  resembles  blood-plasma  in  the  character  of  its  constitu- 
ents, though  their  relative  quantities  vary  considerably.  With  the 
exception  that  it  contains  no  red  corpuscles,  lymph  may  be  regarded 
as  a  diluted  blood. 

The  Production  of  Lymph. — Though  blood  is  the  common  res- 
ervoir of  nutritive  material,  the  latter  is  not  available  for  nutritive 
purposes  as  long  as  it  is  confined  within  the  blood-vessels.  The 
capillary  wall,  thin  as  it  is,  and  composed  of  but  a  single  layer  of 
endothelial  cells,  would  be  sufficient  to  prevent  its  utilization  by  the 
tissues,  if  it  were  not  permeable  to  the  liquid  portion  of  the  blood. 
As  this  is  the  case,  however,  it  is  found  that  as  the  blood  flows  through 
the  capillary  vessels  a  portion  of  the  blood-plasma  passes  through  the 
capillary  wall  and  is  received  into  the  tissue  spaces,  where  it  comes  into 
intimate  contact  with  the  tissue-cells. 

The  forces  concerned  in  the  passage  of  the  constituents  of  the 
blood-plasma  across  the  capillary  wall  have  been  the  subject  of  much 
investigation.  According  to  some  investigators,  diffusion,  osmosis 
and  filtration  are  sufficient  to  account  for  all  the  phenomena.  For 
a  consideration  of  the  phenomena  of  diffusion,  osmosis  and  filtration 
the  reader  is  referred  to  paragraphs  at  the  end  of  this  chapter.  It  is 
assumed  that  the  capillary  wall,  being  an  animal  membrane  is  freely 
permeable  to  water  and  crystalloid  bodies  generally;  less  so,  however, 
to  colloid  bodies,  such  as  the  proteids  of  the  blood-plasma;  moreover, 
it  is  further  assumed  that  the  physiologic  conditions  of  the  capillary 
walls  are  such  as  not  only  to  permit  of  the  passage  of  the  constituents 
of  the  blood  into  the  tissue  spaces,  but  also  the  passage  of  the  con- 
stituents of  the  intercellular  lymph  into  the  blood,  according  to  laws 
similar  at  least  to  those  determining  the  passage  of  substances  through 
animal  membranes  as  determined  experimentally.  The  force  giving 
rise  to  filtration  is  the  difference  of  pressure  between  that  exerted  by 
the  blood  within  the  capillary  vessels  and  that  exerted  by  the  fluid  in 
the  tissue  spaces;  hence  any  increase  or  decrease  of  this  difference  of 
pressure  is  attended  by  an  increase  or  decrease  in  the  production  of 
lymph.  Thus  compression  of  the  veins  of  a  part  which  interferes  with 
the  outflow  of  blood  from  the  capillaries,  or  a  dilatation  of  the  arterioles 
which  increases  the  inflow  of  blood  to  them  will  increase  the  capillary 
pressure  and  therefore  the  production  of  lymph.  The  reverse  con- 
ditions will,  of  course,  diminish  the  intracapillary  pressure  and 
lymph  production.  Hemorrhages  which  lower  the  general  blood- 
pressure  may  so  lower  the  capillary  pressure  as  not  only  to  stop  the 
flow  of  lymph  to  the  tissues,  but  may  give  rise  to  a  diffusion  current 
from  the  tissues  into  the  blood. 

The  quantitative  composition  of  the  lymph  compared  with  that 
of  the  blood  indicates  that  it  is  produced  by  diffusion,  osmosis,  and 
filtration.  In  the  lymph  the  concentration  of  the  inorganic  salts  is 
practically  the  same  as  in  the  blood;  the  concentration  of  the  pro- 
teids, however,  is  somewhat  less.     These  facts  are  in  accordance  with 


ABSORPTION.  221 

what  is  known  regarding  the  diffusibility  of  both  crystalloids  and  col- 
loids through  animal  membranes. 

According  to  other  investigators,  the  production  of  lymph  is  not 
so  much  due  to  intracapillary  pressure  as  it  is  to  the  specialized  ac- 
tivities of  the  endothelial  cells,  activities  which  indicate  that  lymph 
is  a  secretion  the  composition  of  which  varies  in  different  situations 
by  virtue  of  a  difference  in  the  molecular  structure  of  the  endothelial 
cells.  As  is  the  case  with  many  of  the  secreting  cells  of  the  body,  the 
injection  of  various  substances  into  the  blood  apparently  increases 
the  activity  of  the  endothelial  cells,  as  shown  by  an  increased  lymph 
production  without  any  appreciable  increase  of  intracapillary  pressure. 
Thus  it  has  been  shown  that  after  the  injection  into  the  blood  of 
sugar,  sodium  chlorid,  sodium  sulphate,  urea,  etc.,  there  is  an  increase 
in  the  flow  of  lymph  from  the  thoracic  duct.  The  lymph,  however, 
under  these  circumstances  is  richer  in  water  than  is  normally  the 
case.  As  the  blood  at  the  same  time  increases  its  percentage  of 
water,  it  is  assumed  that  the  water  is  extracted  from  the  tissues,  by 
reason  of  an  increased  percentage  of  salts  in  the  tissue  spaces  due  to 
increased  activity  of  the  endothelial  cells.  A  higher  percentage  of 
salts  in  the  lymph  than  in  the  blood  is  difficult  to  account  for  on  the 
diffusion-filtration  theory.  The  injection  of  peptones,  albumin,  the 
extract  of  the  muscles  of  the  leech,  crab,  mussel,  etc.,  is  also  followed 
by  an  increase  in  the  amount  of  lymph  discharged  from  the  thoracic 
duct;  but  in  this  instance  the  lymph  possesses  a  higher  degree  of  con- 
centration, being  richer  not  only  in  inorganic  but  also  in  organic  constit- 
uents. The  cause  of  this  increase  in  both  the  quantity  and  quality 
of  the  lymph  is  believed  to  be  an  increased  activity  in  the  secreting 
power  of  the  endothelial  cells.  The  more  recent  experiments  of 
Starling  indicate  that  in  addition  to  the  difference  of  pressure  be- 
tween the  blood  in  the  capillaries  and  the  lymph  in  the  tissue  spaces, 
a  new  factor  must  be  considered  and  that  is,  the  permeability  of  the 
capillary  wall.  This  he  finds  to  vary  considerably  in  different  parts 
of  the  vascular  apparatus,  being  greatest  in  the  capillaries  of  the  liver, 
less  in  the  capillaries  of  the  intestines  and  least  in  the  capillaries  of 
the  extremities.  It  also  varies  doubtless  in  all  other  situations.  The 
increase  in  the  production  of  lymph  by  the  injection  of  peptones, 
extract  of  muscles  of  the  leech,  the  crab,  etc.,  Starling  explains  by  the 
assumption  that  these  substances  alter  the  properties  of  the  capillary 
wall  and  thus  increase  its  permeability.  The  difference  of  pressure, 
therefore,  between  blood  and  lymph  taken  in  connection  with  the  de- 
gree of  permeability  of  the  capillary  wall  will  account  for  the  pro- 
duction of  lymph  in  all  regions  of  the  body.  It  is  possible  that  all 
these  facts  may  be  otherwise  interpreted;  the  subject  is  yet  a  matter 
of  investigation. 

The  Functions  of  Intercellular  Lymph. — The  origin  and 
composition  of  lymph,  its  situation  and  relation  to  the  tissue  cells 
indicate  that  its  function  is  to  provide  the  tissue  cells  with  those  nutri- 


222  TEXT-BOOK  OF  PHYSIOLOGY. 

tive  materials  necessary  to  their  growth,  repair,  and  functional  activ- 
ities, and  to  receive  from  the  tissue  cells  their  waste  products  prior 
to  their  removal  by  the  blood-  and  lymph-vessels. 

The  necessity  for  the  production  of  lymph  becomes  apparent 
when  the  chemic  changes  which  the  tissues  undergo  at  all  times  are 
considered.  Thus  whether  in  a  state  of  relative  rest  or  in  a  state  of 
activity,  disintegrative  changes  are  constantly  taking  place  and  al- 
ways in  direct  proportion  to  the  degree  and  continuance  of  the  activity. 
If  the  tissues  are  to  continue  in  the  performance  of  their  customary 
activities,  it  is  essential  that  repair  and  restoration  be  at  once  established. 
This  is  made  possible  by  the  presence  of  lymph,  and  by  the  power  which 
living  material  possesses  of  absorbing  from  the  lymph  the  necessary 
nutritive  materials,  of  assimilating  them  and  transforming  them  into 
material  like  unto  itself  and  endowing  them  with  their  own  physiologic 
properties. 

Coincidently  with  the  loss  of  nutritive  material,  the  lymph  receives 
the  waste  products  of  the  tissues  and  hence  changes  in  composition. 
Should  this  change  in  composition  continue  for  any  length  of  time, 
the  lymph  would  lose  its  restorative  character  and  become  destruc- 
tive to  tissue  vitality.  Therefore  it  is  essential  that  the  nutritive 
material  be  renewed  as  rapidly  as  consumed  and  the  waste  products 
be  carried  away  as  rapidly  as  produced.  Both  these  conditions 
are  fulfilled  by  the  blood-  and  lymph-vessels. 

The  Absorption  of  Intercellular  Lymph. — From  the  fact  that 
lymph  is  being  discharged  from  the  thoracic  duct  into  the  blood, 
more  or  less  continually,  it  is  evident  that  lymph  is  being  absorbed 
from  the  intercellular  spaces;  from  which  fact  it  may  be  inferred 
that  the  production  of  lymph  is  a  continuous  process  and  that 
it  is  passing  through  the  capillaries  in  amounts  greater  than  is 
necessary  for  the  immediate  needs  of  the  tissues.  Should  this  excess 
accumulate  there  would  soon  arise  the  condition  of  edema  and  an  inter- 
ference with  the  functional  activities  of  the  tissues.  Therefore  so  soon 
as  the  accumulation  attains  a  certain  volume  it  is  absorbed  in  large 
measure  by  the  lymph-capillaries  and  transmitted  to  the  lymph- 
vessels  and  thoracic  duct.  Because  of  the  general  belief  that  the 
lymph-capillaries  were  in  open  communication  with  the  tissue  spaces 
it  was  assumed  that  the  absorption  of  lymph  and  its  flow  through 
the  lymph-vessels  was  the  result  of  a  difference  of  pressure  between 
the  lymph  in  the  tissue  spaces  and  the  blood  in  the  innominate  veins. 
But  if  the  lymph-capillaries  are  closed  vessels,  as  recent  investiga- 
tions indicate,  then  additional  factors,  in  explanation  of  lymph  ab- 
sorption,  must  be  sought  for. 

It  is  quite  possible  under  even  normal  conditions  of  pressure  in 
the  tissue  spaces  that  some  of  the  more  diffusible  constituents  of  the 
lymph  should  be  absorbed  by  the  capillary  blood-vessels.  As  to 
whether  the  relatively  feebly  diffusible  colloids  should  be  so  resorbed 
is  as  yet  a  matter  of  investigation. 


ABSORPTION.  223 

ABSORPTION  OF  FOODS. 

The  most  important  of  the  absorbing  surfaces,  especially  in  its 
relation  to  the  absorption  of  new  material,  is  the  mucous  membrane 
of  the  alimentary  canal,  and  more  particularly  that  portion  lining  the 
small  intestine,  provided  as  it  is  with  specialized  absorbing  structures 
— the  villi.  Though  certain  substances  can  be  absorbed  from  the 
mouth,  it  is  not  probable  that  any  food  is  so  absorbed.  From  the 
changes  which  the  food  principles  undergo  in  the  stomach  it  might 
naturally  be  inferred  that  their  absorption  would  promptly  follow. 
Experimental  researches  have  demonstrated,  however,  that  this  takes 
place,  if  at  all,  but  to  a  slight  extent.  If,  however,  solutions  of  inor- 
ganic salts,  sugars,  and  peptones  possessing  a  concentration  of  at 
least  5  per  cent. — a  degree  of  concentration  seldom  realized  under 
normal  conditions — are  introduced  into  the  stomach,  their  absorption 
will  be  effected,  the  rate  of.  absorption  following  in  a  general  way  the 
increase,  within  limits,  in  concentration.  Water  is  practically  not 
absorbed  from  the  stomach.  The  absorption  of  the  products  of 
digestion — i.  <?.,  dextrose,  levulose,  peptones,  soaps,  glycerin,  fatty 
acids,  salts,  along  with  water,  in  which  for  the  most  part  they  are 
held  in  solution — is  therefore  limited  very  largely  to  the  small  intes- 
tine, and  is  accomplished  by  the  villous  processes  projecting  from  the 
surface  of  the  mucous  membrane. 

Structure  of  the  Villi. — The  villi  are  small  filiform  or  conical 
processes,  from  0.5  to  1  mm.  in  length,  and  from  0.2  to  0.5  mm.  in 
breadth,  covering  the  surface  of  the  mucous  membrane  from  the 
pyloric  orifice  to  the  upper  surface  of  the  ileo-cecal  valve.  Each 
villus  consists  of  a  basement  membrane  (see  Fig.  98;  supporting  tall 
columnar  epithelial  cells.  Each  cell  is  composed  of  granular  bio- 
plasm containing  a  distinct  nucleus.  At  its  free  extremity  a  narrow 
border  of  the  cell  presents  a  striated  appearance,  as  if  it  were  com 
posed  of  small  rods  embedded  in  some  cement  substance.  Goblet 
or  mucin-holding  cells  are  also  to  be  found  among  the  columnar 
cells.  The  body  of  the  villus,  that  portion  within  the  basement 
membrane,  consists  of  a  reticulated  connective  tissue  supporting  arte- 
ries, capillaries,  veins,  and  lymphoid  corpuscles.  In  the  center  of  the 
villus  there  is  usually  a  single  though  at  times  a  double  club-shaped 
lymph-capillary,  the  walls  of  which  are  composed  of  epithelioid  cells 
with  sinuous  margins.  This  capillary  probably  begins  by  a  blind  ex- 
tremity and  opens  at  the  base  of  the  villus  into  the  subjacent  lymph- 
vessels.  The  communicating  orifice  is  guarded  by  a  valve.  It  is  also 
surrounded  by  a  layer  of  non-striated  muscle-ribers,  arranged  longitu- 
dinally, derived  from  the  muscularis  mucosae  and  attached  to  the  apex 
of  the  body  of  the  villus. 

The  arteries  which  penetrate  the  villi  are  derived  from  those 
of  the  submucous  coat  of  the  intestine,  which  are  the  ultimate  branches 
of  the  intestinal  artery,  and  serve  the  purpose  of  delivering  nutritive 


224 


TEXT-BOOK  OF  PHYSIOLOGY. 


material  to  the  capillary  plexus  (Fig.  99).  While  passing  through 
the  latter  a  portion  of  the  blood-plasma  transudes  through  the  capil- 
lary walls  into  the  spaces  of  the  reticulated  tissue,  constituting  lymph. 
At  the  same  time  products  of  tissue  metabolism  pass  through  the  capil- 
lary walls  into  the  blood.  The  blood  then  passes  into  the  venules, 
which,  leaving  the  villus  at  its  base,  unite  with  the  veins  of  the  sub- 


Fig.  98. — Longitudinal  Sec- 
tion of  a  Villus  from  In- 
testine of  the  Dog,  Highly 
Magnified,  a.  Columnar  epi- 
thelium containing  goblet-cclls  (b) 
and  migratory  leukocytes  (h).  c. 
Basement  membrane,  d.  Plate- 
like connective-tissue  elements  of 
core,  e,  e.  Blood-vessels.  /.  Ab- 
sorbent radical  or  lacteal.— 
{Pier  sol.) 


Fig.  99. — Section  of  Injected  Small 
Intestine  of  Cat.  a,  b.  Mucosa,  g. 
Villi,  i.  Their  absorbent  vessels,  h.  Sim- 
ple follicles,  c.  Muscularis  mucosae,  j.  Sub- 
mucosa.  g,  e'.  Circular  and  longitudinal 
layers  of  muscle.  /.  Fibrous  coat.  All  the 
dark  lines  represent  blood-vessels  filled  with 
the  injection  mass. — (Piersol.) 


mucous  coat  to  form  the  intestinal  veins.  These  finally  unite  with  the 
gastric  and  splenic  veins  to  form  the  portal  vein,  which  enters  the  liver 
at  the  transverse  fissure  (Fig.  100).  The  excess  of  lymph  within  the 
villus  passes  into  the  club-shaped  lymph-capillary,  to  be  finally  carried 
by  the  lymphatics  of  the  mesentery  into  the  thoracic  duct.  During 
the  intervals  of  digestion  and  in  the  absence  of  food  from  the  intestine 
there  is,  of  course,  no  absorption  of  food  nor  the  removal  from  the  villus 
of  anything  but  the  excess  of  lymph  and  metabolic  products. 

Function  of  the  Villi. — The  villi,  and  especially  the  epithelial 


ABSORPTION. 


225 


cells  covering  them,  are  the  essential  agents  in  the  absorption  of  the 
products  of  digestion.  It  is  by  the  activity  of  these  cells  that  the 
new  materials  are  taken  out  of  the  alimentary  canal  and  transferred 
into  the  lymph-spaces,  in  the  body  of  the  villi,  from  which  they  are 
subsequently  removed  by  the  blood-vessels  and  lymphatics.  As  to 
the  mechanism  by  which  the  epithelial  cells  accomplish  this  result, 
nothing  definite  can  be  asserted. 
Inasmuch  as  the  absorption  of 
food  does  not  take  place  in  ac- 
cordance with  the  laws  of  osmosis 
as  at  present  understood,  it  has 
been  suggested  that  the  cells 
possess  a  "selective  action"  de- 
pendent on  their  organization 
and  living  condition,  an  action 
which  is  to  a  great  extent  con- 
ditioned and  limited  by  the  de- 
gree of  diffusibility  of  the  sub- 
stances to  be  absorbed. 

Absorption  of  Water  and  Inor- 
ganic Salts. — Water  and  inor- 
ganic salts  after  their  absorption 
from  the  intestine  and  transfer- 
ence into  the  lymph-spaces  of 
the  villi  pass  through  the  walls 
of  the  capillary  blood-vessels  and 
are  carried  by  the  way  of  the 
portal  vein  into  the  liver.  Unless 
water  be  present  in  excessive 
amounts,  there  is  no  appreciable 
absorption  of  water  by  the  lym- 
phatics. 

Absorption  of  Sugar. — As  previously  stated,  all  the  carbohydrates, 
with  the  exception  possibly  of  lactose,  are  transformed  by  the. diges- 
tive fluids  into  either  dextrose  or  levulose,  under  which  forms  they 
are  absorbed  by  the  epithelial  cells.  It  is  possible,  however,  that 
soluble  dextrin  may  also  be  absorbed.  Whatever  the  form  under 
which  the  carbohydrates  are  absorbed,  they  never  leave  the  epithe- 
lial cells  except  as  dextrose  and  levulose.  Direct  experimentation 
has  shown  that  the  sugars  are  taken  up  by  the  capillary  blood-vessels 
and  carried  direct  to  the  liver.  Analysis  of  the  blood  of  the  portal 
vein  after  the  ingestion  of  large  quantities  of  sugar  may  reveal  an 
increase  of  0.25  per  cent.;  Avhile  after  the  injection  of  sugar  into  the 
intestine  the  percentage  may  rise  as  high  as  0.4.  As  chemic  analysis 
of  lymph  obtained  from  the  thoracic  duct  shows  no  increase  in  the 
percentage  of  sugar  beyond  that  normally  present  (0.1  per  cent.),  it 
is  assumed  that  sugar  is  not  removed  from  the  villi  by  the  lymphatics. 


Fig.  100. — Diagram  of  the  Portal 
Vein  (pv)  arising  in  the  Alimentary 
Tract  and  Spleen  (s),  and  Carrying  the 
Blood  from  These  Organs  to  the  Liver. 
— (Yeo's   "Text-book   0}  Physiology.'') 


226  TEXT-BOOK  OF  PHYSIOLOGY. 

Absorption  of  Proteins. — Since  most  of  the  proteins  are  transformed 
through  hydration  and  cleavage  by  the  action  of  the  gastric  and  pan- 
creatic enzymes  into  peptones,  there  is  reason  to  believe  that  this 
change  is  necessary  to  their  complete  and  rapid  absorption.  Never- 
theless it  has  been  shown  by  the  results  of  experimentation  that  un- 
changed native  proteins,  such  as  egg-albumin,  and  partially  digested 
proteins,  such  as  acid  and  alkali-albumin,  albumoses,  may  likewise  be 
absorbed  from  the  small  intestine,  though  in  far  less  amounts.  It 
has  also  been  demonstrated  that  native  proteins  can  be  absorbed  from 
the  large  intestine.  Inasmuch  as  chemic  analysis  has  failed  to  detect 
more  than  a  trace  of  either  peptone  or  native  protein  in  the  portal  blood 
or  in  the  lymph  of  the  thoracic  duct,  it  must  be  assumed  that  the 
epithelium  after  absorbing  must  also  synthetize  them  into  some  form 
of  coagulable  protein  (serum-albumin)  which  is  readily  assimilable  by 
the  blood.  That  such  a  reconversion  is  necessary  would  appear  from 
the  fact  that  the  introduction  of  peptones  even  in  small  amounts  into 
the  blood  is  followed  by  their  elimination  unchanged  in  the  urine. 
When  injected  into  the  blood  in  large  amounts,  they  act  as  toxic 
agents,  giving  rise  to  a  fall  of  blood-pressure,  a  diminished  coagula- 
bility of  the  blood,  coma,  and  death. 

After  passing  through  the  epithelium  into  the  spaces  of  the  villi 
they  are  removed  by  the  blood-vessels  and  carried  direct  to  the  liver. 
Even  though  there  is  no  appreciable  increase  in  the  amount  of  pro- 
tein in  the  portal  blood  during  digestion,  there  is  every  reason  to 
think  that  this  is  the  route  by  which  it  reaches  the  general  circulation. 
Ligation  of  the  thoracic  duct  does  not  interfere  with  protein  absorp- 
tion nor  with  the  normal  elimination  of  urea  nor  with  the  weight  of 
the  animal. 

The  foregoing  statements  are  based  on  the  view  that  the  final  stage  in 
the  digestion  of  proteins  is  the  formation  of  peptones.  There  are  reasons 
however  for  believing  that  the  change  is  more  far  reaching  and  com- 
plete, and  that  the  peptones  in  turn  are  disintegrated  and  reduced  to 
still  less  complex  bodies  represented  by  polypeptids,  peptids  and  even 
amino-acids.  The  extent  to  which  this  disintegration  proceeds  will 
doubtless  depend  on  the  quantity  and  variety  of  proteins  consumed. 

If  the  reduction  of  the  protein  molecule  to  this  fragmentary  condi- 
tion is  the  outcome  of  protein  digestion,  as  recent  investigations  in- 
dicate, then  the  problem  of  absorption  is  transferred  to  these  fragment- 
ary bodies  rather  than  to  the  peptone  molecule.  Inasmuch  as  the 
presence  of  the  peptids  and  the  amino-acids  in  the  blood  of  the  portal 
vein  has  not  been  demonstrated  beyond  question,  the  supposition  is  that 
after  their  absorption  by  the  intestinal  epithelium,  they  are  synthesized 
and  a  protein  molecule  constructed,  similar  to  if  not  identical  with,  the 
albumin  of  the  blood-plasma.  This  view  renders  it  much  easier  to 
understand  how  out  of  the  different  proteins,  varying  widely  in  their 
composition,  the  specific  proteins  of  the  blood  are  constructed.  It  is 
only  necessary  to  assume  that  the  epithelial  cell  selects  from  the  variety 


ABSORPTION.  227 

of  fragments  presented  to  it,  those  which  are  necessary  to  the  formation 
of  the  blood  albumin  and  the  blood  globulin,  and  to  synthesize  them. 
The  fate  of  the  unused  fragments  is  various.  Some  are  carried  to  the 
liver  and  transformed  into  urea;  others  are  acted  on  by  intestinal  bac- 
teria, after  which  they  may  be  eliminated  in  the  feces  or  absorbed  and 
carried  to  the  liver  where  they  undergo  other  changes  and  eventually 
appear  in  the  urine. 

Absorption  of  Fat. — As  previously  stated,  there  are  two  views  as 
to  the  changes  which  fats  undergo  during  digestion.  According  as 
the  one  or  the  other  is  accepted  will  depend  the  view  as  to  the  nature 
of  the  absorptive  process.  If  it  be  assumed  that  the  final  stage  in 
the  digestion  of  fat  is  a  purely  physical  one,  the  production  of  an 
emulsion  in  which  the  fats  present  themselves  as  fine  granules,  it  is 
difficult  to  give  any  satisfactory  explanation  of  the  mechanism  by 
which  the  epithelial  cells  take  them  up.  Various  theories  have  been 
advanced  to  explain  the  process,  but  none  are  free  from  serious  ob- 
jections. This  view  of  fat  absorption  has  largely  been  based  on  the 
observation  that  during  digestion  fatty  granules  can  be  seen  in  all 
portions  of  the  cell  apparently  passing  toward  the  interior  of  the 
villus.  If,  on  the  contrary,  it  be  admitted  that  the  final  stage  in  the 
digestion  of  fats  is  the  formation  of  soaps  and  glycerin,  both  of  which 
are  soluble,  their  absorption  can  more  readily  be  accounted  for. 
According  to  this  view,  the  soaps  and  glycerin  are  again  synthesized 
by  a  process  the  reverse  of  that  which  is  produced  by  the  pancreatic 
enzyme,  with  the  appearance  of  minute  granules  of  fat.  That  this 
is  the  more  probable  view  as  to  the  mechanism  of  fat  absorption  is 
evident  from  the  fact  that  when  animals  are  fed  with  alkaline  soaps 
and  glycerin,  or  with  fatty  acids  alone,  globules  of  fat  are  found  in 
the  epithelial  cells  and  in  the  interior  of  the  villus. 

With  the  passage  of  the  fat-granules  into  the  interior  of  the  villus 
they  at  once  enter  the  lymph-radicle  and  become  constituents  of  the 
lymph-stream,  to  which  they  impart  a  white,  milky  appearance.  If 
the  abdomen  of  an  animal  in  full  digestion  be  opened,  the  lymph-ves- 
sels of  the  mesentery  present  themselves  as  distinct  white  threads. 
An  examination  of  the  fluid  they  contain,  known  as  chyle,  shows  the 
presence  of  fat-granules  of  microscopic  size.  With  the  passage  of 
the  chyle  into  the  thoracic  duct  it  also  presents  the  same  milky  ap- 
pearance. For  this  reason  the  lymphatics  of  the  mesentery  were 
erroneously  termed  lacteals.  The  chyle  as  obtained  from  these 
lymph-vessels  possesses  the  same  qualitative  though  not  quantitative 
composition  as  lymph,  the  difference  being  mainly  in  the  large  excess 
of  fat  in  the  former.     Indeed,  chyle  may  be  regarded  as  lymph  plus  fat. 

Routes  for  the  Absorbed  Food. — Physiologic  experiments  have 

demonstrated  that  the  agents  concerned  in  the  removal  of  the  products 

of  digestion  after  their  absorption  from  the  interior  of  the  villus  are: 

1.  The  veins  of  the  gastro-intestinal  tract,  which  converge  to  form  the 

portal  vein. 


228  TEXT-BOOK  OF  PHYSIOLOGY. 

2.  The  lymph- vessels  of  the  small  intestine,  which  converge  to  empty 
into  the  thoracic  duct. 

The  products  of  digestion  find  their  way  into  the  general  circu- 
lation by  these  two  routes,  as  follows:  (See  Fig.  101). 

The  water,  inorganic  salts,  proteins,  and  sugar  after  entering  the 
blood-vessels  of  the  villus  are  carried  by  the  blood  of  the  intestinal 
veins  directly  into  the  liver  by  the  portal  vein;  after  circulating  through 
the  capillaries  of  the  liver  and  being  influenced  by  the  liver  cells, 
they  are  discharged  by  the  hepatic  veins  into  the  inferior  or  ascending 
vena  cava. 

The  fats  after  entering  the  lymph-radicle  of  the  villus  are  carried 
bv  the  lymph-stream  of  the  intestinal  lymph-vessels  and  emptied  into 
the  receptaculum  chyli  from  which  they  ascend  into  the  thoracic  duct, 
and  by  which  they  are  discharged  into  the  blood  at  the  junction  of  the 
left  subclavian  and  internal  jugular  veins. 

Forces  Aiding  the  Movement  of  Lymph  and  Chyle. — The 
force  which  primarily  determines  the  movement  of  the  lymph  has  its 
origin  in  the  beginnings  of  the  lymph-vessels,  the  lymph-spaces, 
and  depends  on  a  difference  in  pressure  here  and  at  the  termination 
of  the  thoracic  duct.  The  rise  of  pressure  in  the  lymph-spaces  is 
due  to  the  continual  production  of  lymph,  either  by  filtration  or  se- 
cretory activity  of  the  capillary  walls.  As  soon  as  the  pressure  rises 
above  that  in  the  thoracic  duct  a  forward  movement  of  lymph  takes 
place.  Other  things  being  equal,  the  rate  of  movement  will  be  pro- 
portional to  the  difference  of  pressure.  The  first  movement  of  the 
chyle,  its  passage  from  the  lymph-capillary  in  the  villus  into  the  sub- 
jacent lymph-vessel,  has  been  attributed  to  a  shortening  of  the  villus 
and  a  compression  of  the  capillary  by  the  contraction  of  the  non- 
striated  muscle-fibers  by  which  it  is  surrounded.  With  the  entrance 
of  the  chyle  into  the  subjacent  lymph-vessel  there  is  a  distention  of  the 
vessel  and  a  rise  in  pressure.  When  the  muscle-fibers  relax,  regurgita- 
tion is  prevented  by  the  closure  of  the  valves  at  the  base  of  the  villus. 
The  elastic  tissue  of  the  lymph-vessel  now  recoils  and  forces  the  chyle 
toward  the  thoracic  duct.  After  the  emptying  of  the  lymph-capillary 
the  conditions  as  far  as  pressure  is  concerned  are  favorable  to  the 
absorption  of  new  material.  The  rhythmic  contractions  of  the  in- 
testinal wall  undoubtedly  aid  in  the  movement  of  lymph  and  chyle. 

It  is  quite  possible  that  the  walls  of  the  general  lymphatic  system 
aid  the  forward  movement  of  lymph  by  more  or  less  rhythmic  con- 
tractions of  their  contained  muscle-fibers. 

Inasmuch  as  the  lymph-vessels  lie  in  situations  in  which  they 
are  subject  to  compression  by  muscles  during  contraction,  it  is  prob- 
able that  the  fluid  in  the  vessels  will  be  forced  onward  toward  the 
thoracic  duct  at  each  compression,  a  backward  movement  being 
prevented  by  the  closure  of  the  valves  which  are  everywhere  present 
in  the  vessels.  Experimental  observations  have  demonstrated  the 
truth  of  this  supposition.     Alternate  contraction  and  relaxation  of 


ABSORPTION. 


229 


Fig.  ioi. — Diagram  Showing  the  Routes  by  which  the  Absorbed  Foods  Reach 
the  Blood  of  the  General  Circulation  (G.  Bachman).  I.  i.,  Loop  of  small  intestine; 
int.,  v.,  intestinal  veins  converging  to  form  in  part,  p.  u.,  the  portal  vein,  which  enters  the 
liver  and  by  repeated  branchings  assists  in  the  formation  of  the  hepatic  capillary  plexus; 
h.  u.  the  hepatic  veins  carrying  blood  from  the  liver  and  discharging  it  into,  inf.  v.  c.  the 
inferior  vena  cava;  int.  I.  v.,  the  intestinal  lymph  vessels  converging  to  discharge  their 
contents,  chyle,  into  rec.  c.  the  receptaculum  chyli,  the  lower  expanded  part  of  the  thoracic 
duct;  th.  d.,  the  thoracic  duct  discharging  lymph  and  chyle  into  the  blood  at  the  juncdon 
of  the  internal  jugular  and  subclavian  veins;  sup.  v.  c,  the  superior  vena  cava. 


230  TEXT-BOOK  OF  PHYSIOLOGY. 

the  muscles  of  the  leg  will,  in  an  animal  at  least,  increase  considerably 
the  flow  as  well  as  the  production  of  lymph  from  the  thoracic  duct. 
Massage  has  a  similar  influence.  The  respiratory  movements  also 
aid  the  flow  of  both  lymph  and  chyle  from  the  thoracic  duct  and 
larger  lymph-vessels  into  the  venous  blood.  During  inspiration  the 
negative  pressure  of  the  thorax  is  increased,  the  increase  being  pro- 
portional to  the  extent  of  the  inspiration.  The  positive  pressure  of 
the  air  within  the  lungs  on  the  thoracic  structures,  venae  cavse,  thoracic 
duct,  etc.,  being  at  the  same  time  diminished,  there  is  an  expansion  of 
and  a  fall  of  pressure  in  the  thoracic  duct  and  vena?  cavas.  As  the  lymph 
in  the  abdominal  portion  of  the  thoracic  duct  is  subjected  to  the  higher 
intra-abdominal  pressure,  its  contents  are  forced  energetically  toward 
the  end  of  the  thoracic  duct.  During  expiration  the  reverse  conditions 
obtain.  As  the  negative  pressure  diminishes  and  the  positive  intra- 
pulmonary  pressure  increases  the  upper  part  of  the  thoracic  duct  is 
compressed  and  the  lymph  is  forced  into  the  subclavian  vein  at  its 
junction  with  the  internal  jugular.  Regurgitation  here  is  prevented 
by  a  closure  of  the  valves. 

DIFFUSION.     OSMOSIS.     FILTRATION. 

As  these  three  factors  are  believed  to  play  an  important  part  in  many 
physiologic  processes,  it  is  essential  to  a  better  understanding  of  these  proc- 
esses, that  certain  elementary  facts  relating  to  these  three  factors  be  known. 

Diffusion. — By  diffusion  is  meant  the  gradual  and  spontaneous  mixture 
of  the  molecules  of  two  or  more  liquids,  or  of  two  or  more  gases,  when  brought 
into  contact  with  each  other,  without  the  application  of  an  external  force. 
The  reason  for  both  processes  lies  in  the  fact  that  the  molecules  of  a  liquid 
and  of  a  gas  are  in  constant  motion,  in  consequence  of  which  a  mutual  inter- 
penetration  of  the  molecules  takes  place,  which  continues  until  a  condition 
of  homogeneity  is  established. 

Again,  when  a  soluble  substance,  inorganic  or  organic,  is  placed  in  water, 
the  molecules  of  the  substance  will  at  once  begin  to  separate  themselves  and 
to  diffuse  throughout  the  water  until  the  solution  becomes  homogeneous, 
and  notwithstanding  the  fact  that  the  dissolved  substance  possesses  weight, 
the  solution  remains  homogeneous.  The  force  of  gravity  is  overcome  by 
the  force  of  diffusion. 

The  velocity  with  which  the  molecules  of  a  substance  will  diffuse  through 
a  solvent  like  water,  varies  considerably.  The  experiments  of  Graham 
show  that  if  the  molecules  of  a  given  weight  of  hydrochloric  acid  diffuse 
completely  in  a  unit  of  time,  the  molecules  of  the  same  weight  of  sodium 
chlorid,  cane-sugar,  albumin  and  caramel,  will  require  for  their  diffusion 
2.33,  7,  48,  and  98  units  of  time  respectively. 

Osmosis. — Osmosis  may  be  defined  as  the  passage  of  the  molecules  of 
water  across  an  intervening  membrane.  If  the  water  on  one  side  of  the 
membrane,  parchment  for  example,  contains  in  solution  substances  such 
inorganic  salts,  their  molecules  will  also  pass  across  the  membrane  though 
the  time  required  for  this  to  take  place  may  be  much  longer  than  in  the  case 
of  the  water  molecules.  The  passage  of  the  dissolved  substance  across 
the  membrane  though  usually  included  under  the  term  osmosis  is  more 
properly  termed  dialysis. 


ABSORPTION.  231 

If  the  two  volumes  of  water  on  opposite  sides  of  the  membrane  are  the 
same  in  amount,  and  if  the  one  volume  contains  a  salt  in  solution,  the  salt 
molecules  will  continue  to  pass  across  the  membrane  until  the  water  on  both 
sides  contains  the  same  number  of  molecules,  or,  in  other  words,  until  it  is 
homogeneous  in  composition.  The  time  required  for  their  passage  being 
longer  than  the  time  required  for  the  passage  of  the  water  molecules,  there 
will  be  (owing  to  factors  which  will  be  explained  later),  a  temporary  increase 
in  the  volume  of  the  water  originally  containing  the  salt,  but  in  time  the  two 
volumes  will  again  become  equal.  Certain  other  substances  which  may  be 
in  solution,  such  as  albumin,  starch,  etc.,  will  not  pass  across  a  membrane, 
because  of  the  large  size  of  their  molecules.  Graham  termed  all  those  sub- 
stances which  by  virtue  of  the  small  size  of  their  molecules  pass  across 
membranes,  crystalloids,  and  all  those  which  by  virtue  of  the  large  size  of 
their  molecules  do  not  pass  across  membranes  or  to  a  very  slight  extent, 
colloids. 

It  was  stated  in  the  foregoing  paragraph  that  if  two  equal  volumes  of 
water  are  separated  by  a  parchment  septum,  one  of  which  contains  in  solu- 
tion an  inorganic  salt,  the  molecules  of  the  salt-free  water  will  osmose  across 
the  septum  into  salt-containing  water,  more  rapidly  than  they  will  in  the 
opposite  direction,  and  as  a  result,  there  will  be  a  temporary  increase  in  the 
volume  of  the  water  containing  the  salt.  If  the  membrane  were  impermeable 
to  the  salt  molecules,  the  difference  in  the  two  volumes  of  the  water  would 
be  far  more  permanent  and  striking.  The  reason  assigned  for  this,  is  that 
the  molecules  of  the  salt  exert  a  pressure  against  the  outer  layer  of  the  water 
molecules  and  these  in  turn  against  the  membrane,  in  consequence  of  which 
there  is  a  more  rapid  osmosis  of  the  water  molecules  towards  the  salt  than  in 
the  reverse  direction.     To  this  pressure  is  applied  the  term — 

Osmotic  Pressure. — Osmotic  pressure  may  be  defined  as  the  pressure 
exerted  by  the  molecules  of  the  substance  in  solution  against  an  enclosing 
wall,  in  consequence  of  which  there  is  an  osmosis  of  the  surrounding  solvent 
towards  it.  The  reason  for  this  pressure  lies  in  the  fact  that,  when  the 
molecules  of  a  substance  are  separated  a  certain  distance,  as  they  are 
when  in  solution,  they  repulse  one  another  as  do  the  molecules  of  a  gas 
and  in  their  flight  strike  against  the  outer  layer  of  the  solvent.  The  pressure 
of  the  molecules  of  a  substance  in  solution  is  therefore  comparable  to  the 
pressure  of  the  molecules  of  a  gas. 

Three  methods  may  be  employed  for  measuring  the  force  of  the  osmotic 
pressure  of  different  substances,  viz.:  1.  Physical.  2.  The  determination  of 
the  freezing  point.     3.  By  calculation. 

1.  Physical  Method. — For  the  purpose  of  measuring  osmotic  pressure 
by  physical  methods,  it  is  customary  to  make  use  of  an  apparatus  similar  to 
that  represented  in  Fig.  102,  which  consists  of  an  earthenware  vessel  (a), 
into  the  upper  open  end  of  which  a  tall  vertical  glass  tube  has  been  hermet- 
ically sealed.  The  pores  of  the  earthenware  vessel  have  been  filled  by  a 
membrane  made  by  precipitating  ferro-cyanid  of  copper  within  them. 
This  membrane  is  freely  permeable  to  water,  but  impermeable  to  certain 
substances  in  solution,  e.  g.,  cane-sugar.  Such  a  membrane,  which  permits 
the  passage  of  the  molecules  of  the  solvent  but  not  the  molecules  of  the  dis- 
solved substance,  is  termed  a  semipermeable  membrane,  and  its  use  is 
absolutely  necessitated  when  it  is  desired  to  obtain  the  actual  pressure 
exerted  by  any  given  substance  in  solution.  An  apparatus  of  this  character 
is  termed  an  osmometer. 


232 


TEXT-BOOK  OF  PHYSIOLOGY. 


When,  therefore,  the  osmometer  containing  a  solution  of  cane-sugar  is 
placed  in  the  vessel  (b)  containing  water,  the  following  phenomena  arise, 
viz.:  an  ascent  of  the  cane-sugar  solution  in  the  vertical  glass  tube,  and  a 
descent  of  the  level  of  the  water  in  the  vessel  b.  These  phenomena  con- 
tinue until  the  level  of  the  fluid  in  the  glass  tube  reaches  a  certain  height 
when  it  becomes  stationary,  and  no  further  effect  takes  place. 

In  explanation  of  the  foregoing  phenomena  it  may  be  said  that  the 
molecules  of  the  sugar  strike  or  press  against  the  outer  layer  of  the  molecules 
of  the  solvent,  which  at  all  points  are  in  contact 
with  the  rigid  walls  of  the  earthenware  vessel,  ex- 
cept at  the  open  extremity  of  the  vertical  glass 
tube.  Inasmuch  as  the  rigid  walls  of  the  osmom- 
eter prevent  any  outward  displacement  of  the 
molecules  of  the  water,  the  force  of  the  impact  of 
the  sugar  molecule  is  directed  against  the  mole- 
cules at  the  extremity  of  the  vertical  tube  which 
are  in  consequence  pressed  or  pushed  upward 
a  certain  distance.  Because  of  the  loss  of  energy 
due  to  the  impact,  the  sugar  molecule  does  not  re- 
bound with  the  same  velocity,  and  hence  time  is 
permitted  for  the  molecules  of  the  water  to  pass 
into  the  sugar  solution,  to  occupy  the  space,  and 
thus  maintain  the  level  of  the  fluid  in  the  vertical 
tube.  (For  the  reason  that  the  osmometer  is  per- 
meable to  water,  the  molecules  will  pass  outward 
as  well  as  inward  though  more  will  pass  in  a  unit 
of  time  in  the  latter,  than  in  the  former  direction, 
until  equilibrium  is  established.)  The  pressure 
of  the  sugar  molecules  continuing,  the  level  of  the 
fluid  in  the  glass  tube  continues  to  rise  and  the 
level  of  the  fluid  in  the  vessel,  b,  continues  to  fall 
until  the  force  of  gravity  prevents  any  further  up- 
ward movement  of  the  molecules  of  sugar  against 
the  outer  film  of  the  molecules  of  the  water.  The 
difference  in  the  level  of  the  two  fluids  expressed 
in  millimeters  of  mercury  is  taken  as  a  measure 
of,  and  equal  to,  the  pressure  of  the  sugar  in  so- 
lution. A  i  per  cent,  solution  of  cane-sugar  at 
a  temperature  of  from  130  C.  to  160  C,  as  de- 
termined by  this  method,  exerts  an  osmotic  pres- 
sure of  about  535  mm.  Hg. ;  a  2  per  cent,  solution 
exerts  an  osmotic  pressure  approximately  twice 
this  amount. 
Experiments  made  with  this  and  similar  osmometers  show — 
That  the  osmotic  pressure  of  any  substance  in  solution  is  proportional 

to  the  concentration,  providing  the  temperature  is  constant. 
That  when  the  concentration  is  constant  the  osmotic  pressure  rises  with, 

and  is  proportional  to,  the  temperature. 
That  when  different  substances  are  present  in  the  same  solvent  the  os- 
motic pressure  is  equal  to  the  sum  of  the  individual  or  partial  pressures. 
That  whatever  the  nature  of  the  substance  in  solution  it  will  exert  the 
same  osmotic  pressure,  providing  always,  the  same  number  of  molecules 


Solution 
Cane  Sugar 


-a 


b- 


Wate, 


Fig.  102. — An  Osmometer. 


ABSORPTION.  233 

are  present;  hence  the  molecular  weights  in  grams  per  liter  of  different 
substances,  exert  the  same  osmotic  pressure  at  the  same  temperature. 
Because  of  the  fact  that  when  certain  substances,  e.  g.,  many  inorganic 
salts,  many  acids  and  bases,  are  dissolved,  some  of  their  molecules  undergo 
ionization,  i.  e.,  separation  into  parts  which  are  charged  with  electricity,  and 
hence  the  two  together,  molecules  and  ions,  exert  a  greater  osmotic  pressure 
than  would  otherwise  be  the  case;  and  because  of  the  further  fact,  that  it  is 
extremely  difficult  to  obtain  absolutely  semipermeable  membranes,  uniform 
results  are  not  obtained  by  the  employment  of  the  three  methods;  therefore, 
the  osmometric  methods  as  well  as  the  calculation  or  arithmetric  method 
have  been  largely  discarded  and  the  method  based  on  the  determination  of 
the  freezing  point  has  been  adopted. 

2.  The  Determination  of  the  Freezing  Point. — Because  of  the  difficulty  in 
obtaining  the  exact  osmotic  pressure  by  means  of  the  osmometer  as  stated 
above,  reliance  is  now  placed  on  the  mathematic  relation  known  to  exist 
between  osmotic  pressure  and  the  freezing  point.  Thus  the  freezing  point 
of  water  holding  any  substance  in  solution  is  lower  than  water  itself  and  is 
indeed  proportional  to  the  number  of  molecules  dissolved.  As  a  standard 
of  comparison  it  is  customary  to  employ  a  gram-molecule  of  a  substance  dis- 
solved in  one  liter  of  water.  (A  gram-molecule  is  the  quantity  of  a  substance 
expressed  in  grams  equal  to  its  molecular  weight.)  The  lowering  of  the 
freezing  point  of  a  gram-molecule  solution  below  that  of  water  is  constant, 
viz.,  1.870  C.  The  osmotic  pressure  therefore  of  such  a  solution,  as  de- 
termined by  calculation  (see  below),  is  equal  to  22.38  atmospheres,  or 
17,008  mm.  of  Hg. 

Therefore  it  is  only  necessary  to  determine  by  means  of  a  differential 
thermometer  the  lowering  of  the  freezing  point  in  degrees  centigrade,  which 
is  usually  expressed  by  the  symbol  A-  Then  the  osmotic  pressure  is  equal 
to  A  divided  by  1.870  C,  and  the  quotient  multiplied  by  22.38  atmospheres, 
or  17,008  mm.  of  Hg.  Thus  if  the  freezing  point  of  any  solution  was  found 
to  be  0.830  C.  lower  than  water,  its  osmotic  pressure  would  be  0.83  -=-  1.87  X 
22.38  atmospheres  or  9.847  atmospheres  =  7,483  mm.  Hg.  If  any  two 
solutions  have  the  same  freezing  point  they  contain  the  same  number  of 
molecules  and  hence  have  the  same  osmotic  pressure.  Blood  plasma  has 
a  freezing  point  of  0.560  C.  Experimentally  it  has  been  determined  that  the 
freezing  point  of  water  is  lowered  to  the  same  level,  when  it  contains  sodium 
chlorid  to  the  extent  of  0.95  per  cent.  Hence  these  two  fluids  have  the  same 
osmotic  pressure  and  are  isotonic;  each  exerts  a  pressure  of  6.696  atmospheres. 

For  this  reason  the  sodium  chlorid  solution  can  be  employed  for  preserv- 
ing, for  a  time  at  least,  the  form  of  blood  corpuscles  or  other  living  mammalian 
cells,  from  which  it  may  be  inferred,  that  the  contents  of  the  cells  have  an 
osmotic  pressure  approximately  equal  to  that  of  the  blood  plasma  or  the 
salt  solution.  If  the  salt  solution  has  a  lower  concentration  and  hence  a 
lower  osmotic  pressure,  water  will  osmose  into  the  corpuscle  and  cause  a 
discharge  of  its  hemoglobin  content.  Such  a  fluid  is  said  to  be  hypoisotonic. 
If  on  the  contrary,  the  salt  solution  has  a  higher  concentration  and  hence  a 
higher  osmotic  pressure,  water  will  osmose  from  the  corpuscle  causing  a 
shrinkage  and  crenation  of  the  corpuscle.  Such  a  fluid  is  said  to  be  hyper- 
isotonic. 

3.  By  Calculation. — The  osmotic  pressure  may  also  be  obtained  by 
calculation  based  on  the  known  pressure  exerted  by  a  gram-molecule  of 
hydrogen — 2  grams — when  compressed  to  a  volume  of  one  liter.     It  is  well 


234  TEXT-BOOK  OF  PHYSIOLOGY. 

known  that  i  gram  of  hydrogen  at  o°  C.  and  at  an  atmospheric  pressure  of  760 
mm.  Hg.  occupies  a  volume  of  11. 19  liters,  and  that  2  grams  under  the  same 
conditions  will  occupy  a  volume  of  22.38  liters,  and  that  when  the  two  grams, 
that  is,  the  gram-molecules  are  compressed  to  a  volume  of  1  liter  the  molecules 
will  exert  a  pressure  equal  to  that  of  22.38  liters  or  22.38  atmospheres  or 
17,008  mm.  of  Hg.  Since  a  gram-molecule  of  any  substance  dissolved  in  1 
liter  of  water  contains  the  same  number  of  molecules  as  a  gram-molecule  of 
hydrogen,  they  have  the  same  osmotic  pressure. 

From  this  it  is  possible  to  calculate  the  osmotic  pressure  of  an  electrolyte, 
if  the  percentage  composition  of  the  substance  in  solution  be  known.  Let 
it  be  supposed,  for  example,  that  it  is  desirable  to  know  the  osmotic  pressure 
of  a  1  per  cent,  solution  of  cane-sugar.  The  procedure  is  as  follows:  A 
gram-molecule  of  cane-sugar  (C12H22Ou)  contains  342  grams;  a  1  per  cent. 
solution  contains  10  grams  to  the  liter;  hence  its  osmotic  pressure  is  10 -f- 
342  X  22.38  atmospheres  or  0.65  atmosphere  which  is  equal  to  494  mm. 
of  Hg. 

Filtration. — Filtration  may  be  defined  as  the  passage  of  water  and  of 
all  substances  dissolved  in  it,  across  a  membrane  as  a  result  of  a  difference 
Df  hydrostatic  pressure  on  the  two  sides.  The  difference  between  the  two 
pressures  constitutes  the  force  of  filtration,  and  hence  the  greater  the  differ- 
ence, the  greater  will  be  the  amount  of  fluid  filtered. 

With  any  given  artificially  prepared  animal  membrane  the  quantities  of 
water  and  crystalloids  in  general  which  pass  across  the  membrane  are  pro- 
portional to  the  filtration  force,  and  hence  the  filtrate  will  have  a  concen- 
tration similar  to,  if  not  identical  with,  that  of  the  original  solution.  The 
passage  of  colloids  in  solution  will  be  proportional  to  the  permeability  of  the 
membrane  and  an  increase  in  the  filtration  force.  The  filtrate,  however, 
will  have  a  lower  degree  of  concentration  than  the  original  solution  for  the 
reason  that  as  the  pressure  rises  the  quantity  of  water  filtered  increases  in  a 
greater  ratio  than  the  quantity  of  colloid  filtered. 

Physiologic  Applications. — In  the  animal  body  the  fluids  are  separated 
by  delicate  membranes  across  which  the  constituents  of  the  fluids,  inorganic 
and  organic,  are  continually  passing.  Thus  prepared  foods  in  the  intestine 
pass  across  the  intestinal  wall  into  blood-  and  lymph-vessels;  the  constituents 
of  the  blood  pass  across  the  wall  of  the  capillary  vessel  into  the  tissue  spaces 
from  which  they  pass  (a)  through  the  walls  of  various  glands  to  take  part 
in  the  formation  of  their  secretion;  (b)  across  the  sarcolemma  into  the  inte- 
rior of  the  muscle  fiber;  (c)  across  the  limiting  surface  of,  and  into  the 
interior  of  all  tissue  cells.  The  waste  products,  the  result  of  tissue  and  food 
metabolism,  pass  from  the  interior  of  cells  across  their  limiting  membranes 
or  surfaces  into  the  tissue  spaces;  thence  across  the  wall  of  the  capillary 
vessel  into  the  blood  and  finally  across  the  wall  of  the  capillary  vessel  and  the 
epithelium  of  the  lung,  the  kidney,  the  liver,  etc.,  to  take  part  in  the  forma- 
tion of  the  excretions.  These  and  other  processes  are  believed  to  be  accom- 
plished by  the  factors,  diffusion,  osmosis  and  filtration. 

The  statements  that  have  been  made  in  foregoing  paragraphs  in  refer- 
ence to  diffusion,  osmosis  and  filtration  have  been  based  on  the  results  of 
experiments  which  have  been  made  with  non-living  membranes,  and  under 
conditions  purely  physical;  and  though  it  is  quite  true  that  in  the  animal 
body  the  fluids  are  separated  by  membranes  more  or  less  permeable  to  all 
their  constituents,  and  that  all  pass  across  these  membranes,  it  is  possible 
that  the  facts  which   have  been  obtained  experimentally  are  not  strictly 


ABSORPTION.  235 

paralleled  in  the  living  body,  and  hence  not  strictly  applicable  to  the  elucida- 
tion of  physiologic  processes.  Nevertheless  there  are  reasons  for  thinking 
that  a  thorough  understanding  of  these  factors  will  eventually  throw  much 
light  on  the  intimate  nature  of  the  process  by  which  organic  as  well  as 
inorganic  substances  in  solution  pass  across  animal  membranes  in  the  living 
condition. 


CHAPTER  XII. 
THE  BLOOD. 

The  blood  is  a  highly  complex  nutritive  fluid,  the  presence  and 
proper  circulation  of  which  in  the  living  organism  are  essential  to  the 
maintenance  and  activity  of  all  physiologic  processes.  The  escape 
of  the  blood  from  the  vessels,  especially  in  the  higher  animals,  is 
.followed  by  a  loss  of  the  physiologic  properties  of  all  the  tissues  within 
a  short  period  of  time.  The  immediate  dependence  of  the  functional 
activities  of  the  tissues  and  organs  on  the  presence  of  the  blood  can  be 
demonstrated  by  the  following  experiment :  If  the  nozzle  of  a  syringe, 
adapted  to  the  size  of  the  animal,  be  introduced  through  the  jugular 
vein  into  the  right  side  of  the  heart  and  the  blood  be  suddenly  with- 
drawn, there  is  an  immediate  cessation  in  the  activity  of  all  the  organs; 
the  return  of  the  blood  to  the  vessels  within  a  limited  period  of  time  is 
promptly  followed  by  a  renewal  of  their  activity. 

Though  contained  within  a  practically  closed  system  of  vessels, 
the  blood  is  brought  into  intimate  relation  with  all  the  tissue  elements 
through  the  intermediation  of  the  capillaries.  As  the  blood  flows 
through  these  delicate  vessels,  portions  of  its  soluble  nutritive  con- 
stituents, including  oxygen,  are  given  up  to  the  tissues,  by  which  they 
are  utilized  for  growth,  repair,  and  functional  activity.  At  the  same 
time  the  tissues  yield  up  to  the  blood  a  series  of  decomposition  prod- 
ucts, resulting  from  their  activity,  which  vary  in  quantity  and  quality 
according  as  the  blood  traverses  the  muscles,  nerves,  glands,  or  other 
tissues. 

The  blood  may  be  regarded,  therefore,  as  a  reservoir  of  nutritive 
materials  prepared  by  the  digestive  apparatus  and  absorbed  from 
the  food  canal;  of  oxygen,  absorbed  from  the  respiratory  surface  of 
the  lungs;  of  decomposition  products,  produced  by  and  absorbed  from 
the  tissues.  Though  the  blood  varies  in  composition  in  different 
parts  of  the  body  in  consequence  of  the  introduction  of  both  nutritive 
material  and  decomposition  products,  it  nevertheless  presents  certain 
average  physical,  morphologic,  and  chemic  properties  which  distin- 
guish it  as  an  individual  tissue. 

Constituents  of  Blood. — A  microscopic  examination  of  the  blood 
as  it  flows  through  the  capillary  vessels  of  the  web  of  the  frog  or  the 
mesentery  of  the  rabbit  shows  that  it  is  not  a  homogeneous  fluid,  but 
that  it  consists  of  two  distinct  portions:  viz.,  (i)  a  clear,  transparent, 
slightly  yellow  fluid,  the  plasma  or  liquor  sanguinis;  (2)  small  par- 
ticles termed  corpuscles  floating  in  it,  of  which  there  are  two  varieties, 
the  red  or  the  erythrocytes  and  the  white  or  the  leukocytes.     By 

236 


THE  BLOOD.  237 

appropriate  methods  it  can  be  shown  that  a  third  corpuscle,  color- 
less in  appearance  and  smaller  in  size  than  the  ordinary  white  corpuscle, 
is  present  in  the  blood-stream  and  known  as  the  blood-plate  or  plaque. 
The  different  constituents  can  be  roughly  separated  by  appropriate 
means  when  the  blood  is  withdrawn  from  the  body.  If  the  blood  of 
the  horse  is  allowed  to  flow  directly  into  a  tall  cylindric  glass  vessel, 
surrounded  by  ice,  it  separates  in  the  course  of  a  few  hours  into  three 
distinct  layers  in  accordance  with  their  specific  gravities.  The  lower 
layer  is  dark  red  and  consists  of  the  red  corpuscles;  the  middle  layer  is 
grayish  in  color  and  consists  of  the  white  corpuscles;  the  upper  layer 
is  clear  and  transparent  and  consists  of  the  plasma.  The  red  corpus- 
cles occupy  almost  one-half,  the  white  one-fortieth,  the  plasma  a  trifle 
more  than  one-half  of  the  height  of  the  entire  blood-column,  which 
indicates  approximately  the  different  volumes  of  each.  The  same  re- 
sult can  be  obtained  with  human  blood  by  the  use  of  the  centrifuge  or 
hematocrit. 

PHYSICAL  PROPERTIES  OF  BLOOD. 

1.  Color. — Within  the  blood-vessels  two  kinds  of  blood  are  dis- 
tinguished— the  arterial,  the  color  of  which  is  a  bright  scarlet,  and 
the  venous,  the  color  of  which  is  a  dark  bluish-red  or  purple.  The 
cause  of  the  color  as  well  as  the  difference  in  color  is  the  presence  in  the 
red  corpuscles  of  a  coloring-matter,  hemoglobin,  in  different  degrees  of 
combination  with  oxygen.  The  intensity  of  the  color  in  either  kind 
of  blood  is  dependent  on  the  thickness  of  the  blood-stream,  for  in  the 
finest  capillaries,  as  seen  under  the  microscope,  there  is  an  almost 
total  absence  of  color.  As  the  arterial  blood  passes  into  and  through 
the  systemic  capillaries,  the  hemoglobin  yields  up  a  portion  of  its 
oxygen  to  the  tissues  and  changes  in  color,  though  the  change  is 
not  appreciable  by  the  eye.  On  passing  into  the  veins,  however, 
the  blood-stream  soon  presents  its  characteristic  dark  bluish  color, 
which  deepens  as  it  approaches  the  lungs.  On  passing  into  and  through 
the  capillary  vessels  of  the  lungs  the  hemoglobin  absorbs  a  new  volume 
of  oxygen,  changes  in  color,  and  on  emerging  from  the  lungs  the  blood 
presents  its  characteristic  scarlet  color. 

2.  Opacity. — Owing  to  the  fact  that  the  corpuscles  have  a  re- 
fracting power  different  from  the  plasma,  the  blood,  even  in  thin 
layers,  is  opaque.  The  repeated  refractions  and  reflections  which 
light  undergoes  in  passing  through  plasma  and  corpuscles  is  attended 
by  such  a  dissipation  that  it  is  impossible  to  see  printed  matter  through 
it.  That  the  opacity  is  due  to  the  shape  of  the  corpuscles  rather  than 
to  their  contained  coloring-matter  is  evident  from  the  fact  that  when 
the  hemoglobin  is  caused  to  separate  from  the  corpuscles  by  the 
addition  of  chemic  reagents,  the  blood,  though  it  deepens  in  color, 
becomes  at  once  transparent. 

3.  Odor. — When   freshly  drawn  the  blood   possesses  a    peculiar 


238  TEXT-BOOK  OF  PHYSIOLOGY. 

characteristic  odor  which  has  been  attributed  to  the  presence  of  a 
volatile  fatty  acid  in  combination  with  an  alkaline  base.  The  in- 
tensity of  the  odor  may  be  increased  by  the  addition  of  concentrated 
sulphuric  acid,  by  means  of  which  the  volatile  acid  is  set  free. 

4.  Specific  Gravity.- — The  specific  gravity  of  blood  lies  within 
the  limits  of  1.05 1  and  1.059,  averaging  in  man  1.056  and  in  woman 
1.053.  Normally,  variations  from  these  values  are  only  temporary  and 
are  connected  with  variations  in  physiologic  processes.  The  specific 
gravity  is  diminished  by  the  ingestion  of  liquids  and  abstinence  from 
solid  food.  It  is  increased  by  abstinence  from  liquids,  by  the  inges- 
tion of  dry  food,  and  by  the  elimination  of  large  quantities  of  water 
by  the  lungs,  skin,  and  kidneys. 

5.  Alkalinity.— The  reaction  of  the  blood  is  alkaline  from  the 
presence  of  the  disodium  phosphate  (Na2HP04)  and  the  sodium  car- 
bonate, Na2C03.  The  alkalinity  can  be  readily  shown  by  allowing 
the  blood  to  remain  for  a  few  seconds  on  slightly  reddened  glazed 
litmus  paper.  On  washing  off  the  blood  a  distinct  blue  color  pre- 
sents itself  against  a  red  or  violet  background.  The  alkalinity  varies 
within  narrow  limits  in  consequence  of  variations  in  physiologic 
processes.  It  is  increased  in  the  early  stages  of  digestion  and  de- 
creased in  the  later  stages.  It  is  decreased  after  muscular  exercise 
in  consequence  of  the  increased  production  and  absorption  of  acids. 
According  to  v.  Jaksch,  the  alkalinity  corresponds  to  from  260  to  300 
milligrams  of  sodium  hydrate,  NaOH,  for  every  100  c.c.  of  blood; 
according  to  Lowy,  from  300  to  325  milligrams.  The  hitherto  un- 
avoidable error  in  these  estimates  is  about  30  milligrams. 

6.  Temperature.— The  temperature  varies  from  36.780  C. 
(98.2 °  F.)  in  the  superior  vena  cava  to  39.7 °  C.  (103.4  °  F.)  in  the 
hepatic  vein,  the  mean  being  about  38 °  C.  (100 °  F.). 

Coagulation  of  the  Blood. — Within  a  few  minutes  after  the 
blood  is  withdrawn  from  the  vessels  of  a  living  animal  it  begins  to 
lose  its  fluidity,  becomes  somewhat  viscid,  and  if  left  undisturbed 
passes  rapidly  into  a  semisolid  or  jelly-like  state.  To  this  change 
in  the  physical  condition  of  the  blood  the  term  coagulation  has  been 
applied.  The  blood,  during  the  progress  of  coagulation,  not  only 
assumes  the  shape  of  the  vessel  in  which  it  is  contained,  but  becomes 
so  firmly  adherent  to  its  walls  that  it  may  be  inverted  without  the 
coagulum  becoming  dislodged.  If  a  portion  of  such  a  jelly-like  mass 
be  examined  microscopically,  it  will  be  found  to  be  penetrated  in  all 
directions  by  a  felt-work  of  extremely  fine  delicate  fibrils,  which, 
having  made  their  appearance  before  the  corpuscles  had  time  to 
settle  to  the  bottom  of  the  fluid,  have  entangled  them  in  the  meshes 
so  that  the  entire  mass  retains  its  characteristic  color.  These  fibrils 
are  collectively  known  as  fibrin  (Fig.  103). 

If  the  coagulated  blood  be  allowed  to  remain  undisturbed,  a 
clear,  transparent,  slightly  yellowish  fluid  makes  its  appearance  on 
the  surface  of  the  mass,  which  as  it  accumulates  forms  a  layer  of 


THE  BLOOD. 


239 


varying  degrees  of  thickness.  Within  a  few  hours  the  blood-mass 
detaches  itself  from  the  sides  of  the  vessel  in  consequence  of  the  re- 
traction of  the  fibrils,  while  at  the  same  time  the  clear  fluid  increases 
in  amount  and  accumulates  along  the  sides  and  bottom  of  the  vessel. 
The  shrinkage  in  the  volume  of  the  red  coagulum  and  the  increase 
of  the  volume  of  the  clear  fluid  which  is  expressed  from  its  meshes 
continue  for  a  period  varying  from  ten  to  fifteen  hours,  according 
to  certain  external  conditions.  The  blood  has  now  become  separated 
into  two  distinct  portions:  viz.,  a  solid  contracted  red  mass,  termed 
clot,  and  a  clear  fluid,  termed  serum.  The  clot  consists  of  the  fibrin 
containing  in  its  meshes  the  red  and  white  corpuscles;  the  serum 
consists  of  all  the  constituents  of  the  plasma  except  the  antecedents 
of  the  fibrin.     The  stages  of  coagulation  are  shown  in  Fig.  103. 


Fig.  103. — Diagram  to  Illustrate  the  Process  of  Coagulation,  i.  Fresh  blood, 
plasma,  and  corpuscles.  2.  Coagulating  blood  (birth  of  fibrin).  3.  Coagulated  blood 
(clot  and  serum).— {Waller.) 


If  the  blood  coagulates  slowly  the  red  corpuscles,  owing  to  their 
greater  specific  gravity,  subside  to  the  bottom  of  the  blood-mass, 
giving  to  it  a  deeper  color;  the  white  corpuscles,  owing  to  their  lesser 
specific  gravity,  remain  near  the  surface  of  the  clot  and  give  to  it  a 
more  or  less  whitish  appearance,  producing  the  so-called  bujjy  coat. 
In  certain  inflammatory  conditions  the  coagulating  power  of  the  blood 
is  much  diminished,  and  the  corpuscles,  having  time  to  subside,  a  well- 
developed  buffy  coat  is  formed  which  at  one  time  had  much  interest 
for  pathologists.  As  the  contraction  of  the  fibrin  takes  place  more 
actively  in  the  center,  there  being  here  less  resistance  than  at  the 
sides  of  the  coagulum,  the  upper  surface  usually  becomes  depressed 
or  cupped. 

Coagulation  of  Plasma. — Clear  plasma  may  be  obtained  by 
means  of  the  centrifuge  from  blood  to  which  sufficient  magnesium 
sulphate  has  been  added  to  prevent  coagulation,  or  from  horse's 
blood  which  has  been  allowed  to  flow  into  a  tall  vessel  surrounded  by 
a  cooling  mixture  so  as  to  prevent  coagulation  and  thus  permit  the 
red  corpuscles  to  subside.  If  such  plasma  be  subjected  to  room-tem- 
perature, it  very  shortly  undergoes  coagulation,  exhibiting  practically 
the  same  phenomena  as  blood  itself.  After  a  variable  length  of  time 
it  also  separates  into  a  soft,  colorless  coagulum  or  clot  consisting  of 
fibrin,  and  a  clear  fluid,  the  serum.  The  presence  of  the  red  cor- 
puscles is  therefore  not  essential  to  the  process  of  coagulation. 


24o  TEXT-BOOK  OF  PHYSIOLOGY. 

Rapidity  of  Coagulation. — The  rapidity  with  which  the  blood 
coagulates  varies  in  different  classes  of  animals  under  the  same  con- 
ditions: e.  g.,  the  blood  of  the  pigeon  coagulates  immediately;  that  of 
the  dog,  in  from  one  to  three  minutes;  that  of  the  horse,  in  from  five 
to  thirteen  minutes;  that  of  man,  in  from  four  to  seven  minutes. 
The  time,  however,  can  be  lengthened  or  shortened  by  either  chang- 
ing the  external  conditions  or  by  altering  temporarily  the  normal 
composition  of  the  blood. 

Coagulation  is  retarded  or  prevented  by  the  following  agents, 
viz.:  (i)  A  low  temperature,  especially  that  of  melting  ice.  (2)  The 
addition  of  magnesium  sulphate  (1  volume  of  a  25  per  cent,  solution 
to  3  volumes  of  blood);  of  sodium  sulphate  (1  volume  of  a  saturated 
solution  to  7  volumes  of  blood).  (3)  The  addition  of  potassium 
oxalate  (1  volume  of  a  1  per  cent,  solution  to  3  volumes  of  blood). 
(4)  The  injection  into  the  blood  of  commercial  peptone.  (5)  The 
mouth  secretion  of  the  leech. 

Coagulation  is  hastened  by  the  following  agents,  viz.:  (1) 
gradually  increasing  temperature  from  38 °  C.  to  500  C.  (2)  The  ad- 
dition of  water  in  not  too  large  amounts.  (3)  The  presence  of  foreign 
bodies.     (4)   Agitation  of  the  blood — e.  g.,  stirring. 

Fibrin  and  Defibrinated  Blood. — If  freshly  drawn  blood  is 
stirred  with  a  bundle  of  twigs  or  glass  rods  for  a  few  minutes,  the 
fibrin  collects  on  them  in  the  form  of  thick  bundles  or  strands;  after 
washing  it  with  water  the  entangled  corpuscles  are  removed,  when 
the  fibrin  assumes  its  natural  white  appearance.  The  strands  can 
be  resolved  into  a  large  number  of  delicate  fibers  which  possess  ex- 
tensibility and  retractibility,  and  are  therefore  elastic.  The  chemic 
features  of  fibrin  have  already  been  considered  (see  page  20).  The 
remaining  fluid,  similar  in  its  physical  appearance  to  the  blood,  is 
termed  defibrinated  blood.  When  such  blood  is  allowed  to  remain 
at  rest  for  a  few  days,  the  remaining  red  corpuscles  gradually  sink 
to  the  bottom  of  the  fluid,  above  which  will  be  found  the  clear  serum. 

COMPOSITION  OF  PLASMA  AND  SERUM. 

Plasma. — The  plasma  obtained  by  any  of  the  methods  previously 
described  is  a  clear,  colorless,  transparent,  slightly  viscid  fluid,  with 
a  specific  gravity  of  1.026  to  1.029.  It  is  composed  largely  of  water 
holding  in  solution  proteids,  sugar,  fatty  matter,  inorganic  salts,  urea, 
cholesterin,  lecithin,  etc.  In  composition  it  is  quite  complex,  con- 
taining as  it  does  not  only  the  nutritive  materials  derived  from  the 
digestion  of  the  food,  but  also  the  substances  resulting  from  the  dis- 
integration of  the  tissues  consequent  on  their  functional  activity. 

Serum. — The  scrum  is  the  clear,  transparent,  slightly  yellow 
fluid  expressed  from  the  coagulated  blood  during  the  contraction  of 
the  fibrin.  It  consists  practically  of  the  ingredients  of  the  plasma, 
with  the  exception  of  those  substances  which  entered  into  the  for- 


THE  BLOOD.  241 

mation  of  fibrin.     The  average  composition  of  plasma  is  shown  in 
the  following  table: 

COMPOSITION  OF  PLASMA. 

Water, ; .  .  90. 00 

f  Serum-albumin, 4.50 

Proteids  <  Paraglobulin, 3.40 

[  Fibrinogen, 0.30 

Fatty  matters, 0.25 

Sugar, o.  10 

Extractives, 0.60 

Inorganic  salts,   0.85 

100.00 

Serum-albumin. — Of  the  proteid  constituents  of  the  blood, 
serum-albumin  is  the  most  abundant,  existing  to  the  extent  of  from 
4  to  5  per  cent.  From  its  similarity  to  egg-albumin  it  is  regarded  as 
holding  an  important  position  as  a  nutritive  agent,  for  it  is  out  of  this 
common  proteid  that  in  all  probability  each  individual  tissue  elabor- 
ates the  special  proteid  characteristic  of  it,  since  during  starvation 
the  albumin  steadily  diminishes  in  amount.  As  it  passes  through 
the  walls  of  the  capillary  vessels  it  is  found  in  the  lymph,  pericardial 
fluid,  and  similar  secretions  in  various  parts  of  the  body,  as  well  as  in 
various  pathologic  transudates.  It  is  also  present  in  serum.  While 
circulating  in  the  lymph-spaces  the  serum-albumin  is  utilized  in 
replacing  the  proteids  which  have  undergone  disintegration  during 
tissue  metabolism.  Its  supply  in  the  blood  is  maintained  by  the 
absorption  of  peptones  which  are  formed  from  the  proteids  of  the  food 
and  which  during  the  time  of  absorption  are  changed  in  some  unknown 
way  into  serum-albumin.  It  is  readily  obtained  from  plasma  or 
serum  by  saturating  either  of  these  fluids  with  magnesium  sulphate, 
when  all  the  proteids  except  serum-albumin  are  precipitated.  After 
their  removal  the  remaining  fluid  is  subjected  to  a  temperature  of 
from  70 °  to  750  C,  when  the  serum-albumin  is  precipitated  in  a 
coagulable  form,  after  which  it  can  be  removed  and  its  chemic  features 
determined. 

Paraglobulin. — -This  proteid,  though  present  in  plasma,  is  best 
obtained  from  serum  when  this  fluid  is  saturated  with  magnesium 
sulphate.  As  the  line  of  saturation  is  approached  the  fluid  becomes 
turbid,  and  after  a  few  minutes  a  fine  white  precipitate  occurs.  It 
can  then  be  collected  on  a  filter,  dried,  and  its  chemic  properties 
determined.  In  its  reactions  it  resembles  the  various  members  of  the 
globulin  class.  The  amount  varies  from  2  to  4  per  cent,  in  the  blood 
of  man.  As  to  the  physiologic  importance  or  antecedents  of  para- 
globulin nothing  is  definitely  known.  Its  constant  presence  in  the 
blood  would  indicate  that  it  plays  an  equally  important,  though  per- 
haps different,  part  with  serum-albumin  in  the  nutrition  of  the  body. 

Fibrinogen. — This  proteid  can  be  obtained  from  plasma,  lymph, 
pericardial,  and  peritoneal  fluids,  as  well  as  from  hydrocele  fluid. 
16 


242  TEXT-BOOK  OF  PHYSIOLOGY. 

It  is,  however,  not  to  be  obtained  from  serum,  as  it  is  removed  from 
the  blood  during  the  formation  of  solid  fibrin.  It  is  normally  present 
in  the  blood  in  very  small  quantity,  amounting  to  not  more  than  2.2 
to  3.3  parts  per  thousand.  Fibrinogen  may  be  obtained  from  plasma 
which  has  been  prevented  from  coagulating,  by  the  addition  of  mag- 
nesium sulphate  in  certain  quantities  or  by  the  addition  of  a  satu- 
rated solution  of  sodium  chlorid.  In  a  few  minutes  a  flaky  precipitate 
occurs.  By  repeated  washing  and  precipitation  with  sodium-chlorid 
solutions  of  varying  strength  the  fibrinogen  may  be  obtained  in  a 
pure  state.  The  history  of  fibrinogen  is  unknown.  Beyond  the  fact 
that  it  contributes  to  the  formation  of  fibrin  there  is  no  positive  knowl- 
edge either  as  to  its  origin,  its  nutritive  value,  or  its  final  disposition 
in  the  blood  under  normal  conditions. 

Fat. — The  plasma  as  well  as  the  serum  contains  a  very  small 
quantity  of  fat  in  the  form  of  microscopic  globules.  Though  the 
percentage  is  normally  not  above  0.25,  yet  just  after  a  meal  rich  in 
fatty  matter  the  amount  may  be  so  great  as  to  give  to  the  blood  a 
milky  or  opalescent  appearance.  Within  a  few  hours,  however, 
this  excess  of  fat  disappears  from  the  blood,  though  its  immediate 
disposition  is  unknown.  Soaps  or  alkaline  salts  of  the  fatty  acids, 
though  formed  during  the  digestion  of  fats,  are  not  present  in  the 
blood.     Lecithin  and  cholesterin  are  present  in  very  small  quantities. 

Sugar. — Sugar  is  present  in  the  blood  in  the  form  of  dextrose, 
and  is  now  regarded  as  a  normal  constituent.  The  amount  is  about 
1  part  per  thousand,  though  it  may  be  present  to  the  extent  of  3  parts 
per  thousand.     Beyond  this,  the  excess  soon  appears  in  the  urine. 

Extractives. — The  blood  contains  a  series  of  nitrogenized  bodies, 
such  as  urea,  uric  acid,  creatin,  creatinin,  xanthin,  etc.,  which  result 
from  the  katabolic  changes  in  nerve-  and  muscle-tissues  as  well  as 
from  subsequent  chemic  combinations  and  decompositions.  Though 
constantly  absorbed  from  the  tissues,  they  seldom  accumulate  beyond 
a  small  amount,  since  they  are  constantly  being  eliminated  from  the 
blood  by  the  various  excretory  organs. 

Inorganic  Salts. — The  inorganic  salts  of  the  plasma  are  chiefly 
sodium  and  potassium  chlorids,  sulphates,  and  phosphates,  together 
with  calcium  and  magnesium  phosphates.  Of  the  salts,  sodium 
chlorid  is  the  most  abundant,  amounting  to  5.5  parts  per  thousand. 
Some  of  the  salts  are  alkaline  and  impart  to  the  blood  its  alkalinity. 
Calcium  phosphate  is  present  in  small  quantity — 2  parts  per  1000. 
This  salt  is  wanting  in  serum  for  the  reason  that  it  became  a  constitu- 
ent of  fibrin  at  the  time  of  coagulation.  In  other  respects  serum 
differs  but  slightly  from  plasma  in  the  proportions  of  its  saline  con- 
stituents. 

HISTOLOGY  OF  THE  RED  CORPUSCLES  OR  ERYTHROCYTES. 

The  histologic  features  of  the  red  corpuscles  are  readily  observed 
in  a  drop  of  freshly  drawn  blood  when  examined  microscopically. 


THE  BLOOD. 


243 


The  field  of  the  microscope  will  be  seen  to  be  crowded  with  red  cor- 
puscles floating  in  a  clear  transparent  fluid — the  plasma.  Here  and 
there  will  also  be  seen  a  white  corpuscle,  round  or  irregular  in  shape, 
and  granular  in  appearance.  Within  a  short  time  a  characteristic 
phenomenon  takes  place:  viz.,  the  arranging  of  the  corpuscles  in  the 
form  of  columns  of  varying  lengths,  resembling  rolls  of  coins.  These 
rolls  interlace  with  each  other 
at  all  angles  and  form  a  net- 
work in  the  meshes  of  which 
lie  individual  red  and  white 
corpuscles.  (See  (Fig.  104.J 
The  cause  of  this  tendency  of 
the  corpuscles  to  adhere  to 
one  another  is  not  definitely 
known.  Since  it  does  not 
take  place  in  circulating  blood, 
and  since  it  is  to  a  great  ex- 
tent prevented  by  defibrinating 
the  blood,  it  has  been  supposed 
to  be  dependent  on  the  forma- 
tion of  some  adhesive  sub- 
stance connected  with  the 
formation  of  fibrin. 

Color. — When  viewed  by 
transmitted  light,  a  single 
corpuscle  is  slightly  yellow  or 

greenish  in  color;  but  wThen  a  number  are  grouped  together,  the  color 
deepens  and  the  corpuscles  appear  red.  In  either  case  the  color  is 
due  to  the  presence  in  the  corpuscle  of  a  specific  coloring-matter, 
hemoglobin. 

Shape. — The   red    corpuscle    is   a   circular,    flattened   disk   with 
rounded   edges.     Each  surface  is  perfectly  smooth  and  presents   a 

shallow  depression  in  its  center,  so 
that  it  is  also  biconcave.  A  longit- 
udinal section  of  a  corpuscle  would 
present,  when  viewed  edgewise,  an 
outline  similar  to  that  of  Fig.  105. 
This  difference  in  the  thickness  of  the 
peripheral  and  central  portions  of  the 
corpuscle  gives  rise  to  differences  in 
optical  appearances  when  examined 
microscopically.  At  a  certain  distance  of  the  object-glass  the 
corpuscle  presents  in  its  peripheral  portion  a  bright  rim,  and  in  its 
central  portion  a  dark  spot.  If  the  objective  be  brought  nearer 
and  the  center  accurately  focused,  the  reverse  appearance  obtains; 
the  central  portion  becomes  bright  and  the  peripheral  portion  dark. 
The    cause   of   this  difference  in  optical  appearance  is  the  unequal 


Fig.  104. — Corpuscles  from  Human 
Subject.  A  few  colorless  corpuscles  axe 
seen  among  the  colored  discs,  many  of 
which  are  arranged  in  rouleaux. — (Funke.) 


Fig.  105. — Ideal  Transverse 
Section  of  a  Human  Red  Corpus- 
cle. (Magnified  5000  times.)  a,  b. 
Diameter,     c,  d.  Thickness. 


244  TEXT-BOOK  OF  PHYSIOLOGY. 

distribution  of  the  transmitted  light  in  consequence  of  the  shape  of  the 
corpuscle. 

Size. — The  diameter  of  a  typical  well-developed  red  corpuscle 
under  normal  conditions  is  0.0075  mm->  its  greatest  thickness  is  0.0019 
mm.  Though  this  may  be  assumed  as  the  average  diameter,  there 
is  a  small  percentage  of  distinctly  smaller  and  a  small  percentage 
of  distinctly  larger  corpuscles.  The  following  table  shows  the  results 
of  measurement  made  by  different  observers: 

Normal  Limits.  Average  Diameter. 

Welcker, diameter  0.0045-0.0095  mm 0.0070  mm. 

Hayem, "  0.0060-0.0088    "    0-0075     " 

Gram, "         0.0067-0.0093    "    0.0078     " 

Melassez, "  0.0076     " 

0.00747 
(32V0  inch) 

Structure. — The  red  corpuscle  of  man  as  well  as  of  all  other  mam- 
mals possesses  neither  a  nucleus  nor  a  limiting  membrane,  but  appears 
to  consist  of  a  homogeneous  substance  more  or  less  semisolid  in  con- 
sistence. Under  the  in- 
fluence of  certain  re- 
agents the  corpuscle 
separates  into  two  dis- 
-y  tinct    portions:     viz.,    a 

colorless       protoplasmic 
stroma  and  a  coloring- 
lb.  matter    which    diffuses 
into     the     surrounding 


I 


5 


r>  ^\\  liquid.     The  presence  of 

the  former  can  be  dem- 
onstrated   by  the  addi- 
-^  tion  of  iodin,  which  im- 


onstrated    by 

'        w  tinn    nf   iodin 


parts  to  it  a  faint  yellow 

c-  d-  color.      The    stroma    is 

Fig.  io6.-The  Shape  of  the  Red  Corpuscle      elastic     and    determines 

in    Different    Mammals.    (Wetdenreich .)    a.    Man.  ,       ,        ,  ,.  , , 

b.  Dog.    c.  Pig.    d.  Rabbit.  not  only  the  shape  of  the 

corpuscle  but  gives  to  it 
the  properties  of  extensibility  and  retractibility. 

The  foregoing  is  the  classic  and  generally  accepted  view  as  to  the 
shape,  size,  and  structure  of  the  red  corpuscle.  Nevertheless  recent  in- 
vestigations render  it  probable  that  the  statements  were  based  on  ob- 
servations of  the  corpuscles  under  artificial  rather  than  natural  condi- 
tions, and  therefore  not  strictly  true.  For  many  years  histologists 
from  time  to  time  have  stated  that  the  red  corpuscle  is  not  circular  and 
biconcave  in  shape,  in  the  circulating  blood,  but  bell  shaped,  similar  to 
that  shown  in  Fig.  106.  It  was  not  until  1902,  after  the  publication  of 
Weidenreich's  investigations  that  this  view  began  to  receive  more  at- 
tention than  bad  hitherto  been  accorded  it.     Weidenreich  preserved 


THE  BLOOD. 


245 


in  a  moist  chamber  a  hanging  drop  of  human  blood,  and  on  examina- 
tion found  that  the  red  corpuscles  were  bell  shaped  though  the  dq3th 
of  the  bell  cavity  varied  considerably.  An  examination  of  the  capil- 
lary circulation  in  the  omentum  of  the 
rabbit  revealed  the  fact  that  the  cor- 
puscles in  their  natural  medium  were 
also  bell  shaped.  The  circular  bicon- 
cave shape  they  ordinarily  present 
when  a  drop  of  blood  is  examined 
microscopically  he  attributes  to  cool- 
ing, evaporation  and  concentration  of 
the  drawn  blood.  Experimentally  it 
was  shown  that  when  blood  was  added 
to  0.6  to  0.65  per  cent,  solution  of 
sodium  chlorid  all  the  corpuscles  were 
bell  shaped;  but  if  the  solution  was 
increased  or  decreased  in  strength, 
this  form  was  at  once  changed. 

The  dimensions  of  the  bell-shaped  cell  according  to  Weidenreich 
are  as  follows: — 


Fig.  107. —  Red  Corpuscles 
Sketched  while  Circulating  in 
the  Vessels  of  the  Omentum  of  a 
Guinea-pig.  (F.  T.  Lewis  in  Stohr's 
Histology.) 


Greatest  diameter 7  microns  0.007  mm. 

Diameter  of  cavity 3         "  0.003 

Height  of  cell 4         "  0.004 

"         "   cavity 2.5      "  0.0025 

Thickness  of  wall  at  apex 2  "  0.002 

"  "     "      "  base 1.5      "  0.0015 


The  foregoing  observations  have  been  confirmed  by  many  subse- 
quent investigators.  Thus  Lewis  states  that  if  a  drop  of  blood  is  placed 
immediately  on  a  warm  slide  and  examined,  the  corpuscles  exhibit 
the  bell  shape,  but  as  the  slide  cools  they  gradually  become  biconcave 
disks  of  the  conventional  form.  He  also  observed  that  the  corpuscles 
in  the  capillary  blood-vessels  of  the  omentum  of  the  guinea-pig  were 
bell  shaped  and  presenting  an  appearance  similar  to  that  shown  in 
Fig.  107.  Radasch  found  on  examination  of  fetal  tissues  such  as  the 
spleen,  kidney,  liver,  placenta,  etc.,  that  the  great  majority  of  the  cor- 
puscles in  all  situations  presented  the  bell  shape  rather  than  the  circu- 
lar biconcave  shape.  This  observer  is  of  the  opinion  that  the  bell 
shape  can  not  be  due  to  the  action  of  the  fixatives  employed  in  the 
preparation  of  the  tissues. 

The  structure  of  the  corpuscle,  according  to  Weidenreich,  differs 
also  from  that  usually  stated.  He  asserts  that  the  corpuscle  is  sur- 
rounded by  a  structureless,  colorless  membrane  enclosing  a  colored  but 
not  nucleated  semi-fluid  mass,  which  consists  chemically  of  protein 
material,  inorganic  salts  and  hemoglobin.  There  is  no  evidence  of 
the  existence  of  a  stroma  in  the  adult  state. 

Number  of  Red  Corpuscles. — In  any  given  specimen  of  blood 


246 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  corpuscles  are  so  numerous  and  the  spaces  between  them  so 
small  that  it  seems  almost  impossible  to  determine  their  number. 
This,  however,  has  been  accomplished  for  a  cubic  millimeter  of 
blood  by  various  observers  employing  different  methods  with  compara- 
tively uniform  results.  The  average  normal  number  of  corpuscles  in 
one  cubic  millimeter  of  blood  is,  for  men,  5,000,000;  and  for  women, 
4,500,000.  This  value,  however,  will  vary  within  slight  limits,  with 
variations  in  the  activity  of  physiologic  processes  and  to  a  large  extent 
at  times  in  pathologic  states  of  the  blood  or  body.  The  number  is 
increased  in  the  cutaneous  veins  by  all  influences  which  cause  a  dim- 
inution in  the  quantity  of  water  in  the  blood — e.  g.,  copious  sweating, 
acute  watery  diarrhea,  fasting,  abstinence  from  liquids;  the  number 
is  diminished  by  influences  which  dilute  the  blood — e.  g.,  the  ingestion 
of  liquids,  the  absorption  of  fluids  from  the  tissue  spaces,  etc.  But 
it  is  well  to  remember  that  these  influences  which  produce  changes  in 
the  number  of  corpuscles  per  cubic  millimeter  do  not  necessarily  pro- 
duce corresponding  changes  in  the  total  number  of  red  corpuscles  in 
the  body.  In  women  lactation,  menstruation,  and  the  act  of  parturi- 
tion diminish  the  number.  High  altitudes  apparently  increase  the 
number  of  corpuscles,  as  shown  by  their  increase  in  the  blood  of  the 
peripheral  vessels.  Whether  this  is  an  indication  that  there  is  a  cor- 
responding increase  of  the  total  number  in  the  general  volume  of  the 
blood  is  uncertain.  The  following  table  will  show  the  increase  in  the 
count  per  cubic  millimeter  at  different  altitudes: 


Place. 

Height  Above  Sea-level. 

Red  Cells. 

Author. 

Christiania 

Gottingen, 

Tubingen, 

Zurich, 

Auerbach, 

Reibaldsgrlin, 

Arosa, 

0  meter 
148  meters 
3i4       " 
414 

425 

700  " 
1800  " 
4392       " 

4,974,000 
5,225,000 
5,322,000 
5,752,000 
5,748,000 
5,900,000 
7,000,000 
8,000,000 

Laache. 

Schaper. 

Reinert. 

Steirlin. 

Koppe. 
it 

Egger. 
Viault. 

(Koppe.) 

The  Cordilleras, 

Moro  cocha. 

This  increase  in  the  number  of  corpuscles  takes  place,  according 
to  Viault's  observations,  within  two  or  three  weeks,  and  is  apparently 
not  connected  with  either  diet  or  mode  of  life,  but  rather  with  dimin- 
ished atmospheric,  if  not  oxygen,  pressure.  On  returning  to  sea- 
level  there  is  a  gradual  reduction,  without  any  apparent  destruction 
of  the  corpuscles,  to  their  normal  number.  The  reason  for  these 
variations  is  not  clear. 

The  method  of  counting  corpuscles  introduced  by  Vierordt  and 
Welcker  has  been  modified  by  different  observers,  and  especially  by 
Thoma.  On  account  of  the  great  number  of  corpuscles  in  1  cubic 
millimeter  of  blood,  it  becomes  necessary  for  purposes  of  enumeration 
that  the  blood  be  diluted  a  definite  number  of  times  and  that  the  di- 


THE  BLOOD. 


247 


luted  mixture  be  placed  in  a  counting  chamber  possessing  a  definite 
capacity.  By  means  of  the  pipette  or  melangeur  of  Potain  and  the 
counting  chamber  of  Thoma  both  these  objects  are  attained. 

The  pipette  consists  of  a  capillary  tube  (Fig.  108)  provided  with  an  en- 
largement containing  a  freely  movable  small  glass  ball,  a.     One  end  of  the 
tube  is  pointed,  while  to  the  other  end  is  attached  a 
rubber  tube  for  the  purpose  of  facilitating  the  introduc-  A 

tion  of  the  blood  and  the  diluting  fluid.  The  capillary 
tube,  which  is  accurately  calibrated,  carries  marks,  %,  1. 
101,  which  signify  that  if  the  tube  be  filled  with  blood 
up  to  the  mark  1  and  the  diluting  fluid  be  sucked  into 
the  tube  up  to  the  mark  101,  the  blood  will  be  diluted 
100  times.  If  the  blood  be  sucked  up  to  the  mark  ^  and 
the  diluting  fluid  to  101,  then  the  blood  will  be  diluted 
200  times.  In  using  the  pipette  the  point  is  introduced 
into  a  drop  of  blood  derived  from  a  small  wound  in  the 
skin  of  the  lobe  of  the  ear  or  finger  and  sucked  into 
the  tube  by  introducing  the  rubber  tube  into  the  mouth. 
The  tube  is  then  quickly  inserted  into  a  solution,  which 
will  preserve  the  shape  and  size  of  the  corpuscles,  such 
as  Gowers's  sodium  sulphate  solution,  sp.  gr.  1.025,  or 
a  3  per  cent,  sodium  chlorid  solution,*  and  the  fluid 
sucked  into  the  tube  up  to  the  mark  101.  On  shaking 
the  pipette  for  a  few  minutes,  the  admixture  will  take 
place,  aided  by  the  movements  of  the  glass  ball. 

Fig.  109  shows  both  a  section  view,  A,  and  a  surface 
view,  C,  of  the  counting  chamber.     This  consists  of  an 
oblong  glass  plate  on  which  are  cemented  two  small 
pieces  of  glass,  one  of  which  has  in  the  center  a  circular 
opening  in  which  is  placed  the  other,  a  circular  disc  or 
stage.      Their  relation  is  such  that  a  narrow  groove  or 
moat  separates  the  one  from  the  other,  the  floor  of  which 
is  formed  by  the  glass  plate.     The  surface  of  the  circular 
stage  is  exactly  0.1  mm.  lower  than  that  of  the  cover- 
glass,  a.     On  the  surface  of  the  glass  stage  a  series  of 
small  squares  is  engraved,  each  one  of  which  has  a  side  length  of  -^  mm. 
and  an  area  of  ±\-$  square  mm.,  B.    To  facilitate  counting,  a  group  of  16 
squares  is  surrounded  by  a  heavy  dark  line.     This  group  is  separated  from 
adjoining  groups,  also  enclosed  by  dark  lines,  by  an  intermediate  light  line, 


Fig.  10S.  —  Me- 
langeur or  P  1- 
p  E  T  T  E .  —  (Landois 
and  Stirling.) 


*  Various  solutions  have  been  devised  for  dilutin 
employed,  e.  g.: 

Hayem's  Fluid: 

Hydrarg.  bichlor., 0.5  gm 

Sodium  sulphate, 5.0    " 

Sodium  chlorid, 2.0    " 

Aqua?  destillat.  , 200.0    " 


blood,  anv  one  of  which  may  be 


Toisson's  Fluid: 

Aqua?  destillat., .  ...  160.00  parts. 

Glycerinar,    30.00      li 

Sodium  sulphate,  .  .  8.00     " 

Sodium  chlorid, ....  1.00     " 

Methyl-violet, 0.025  part.. 


Gowers's  Fluid: 

Sodium  sulphate, gr.  104 

Acid,  acetic, 5j 

Aquae  dest, q.  s.  ad  oiv. 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  serves  as  a  guide  in  passing  from  one  group  to  another.     When  a 
cover-glass  is  accurately  applied  to  the  glass,  b,  each  one  of  the  small  squares 

will  have  a  cubic  capacity  of 
iw  X  o.i,  or  T7TVo  cubic 
millimeter,  and  every  four 
such  squares  will  have  there- 
fore a  capacity  of  1()1()0  cubic 
millimeter. 

Before  placing  the  diluted 
blood  on  the  counting  stage, 
the  fluid  in  the  tube  of  the 
pipette  should  be  blown  out 
and  discarded,  as  it  contains 
no  portion  of  the  blood.  A 
small  drop  is  then  placed  on 
the  glass  stage  and  covered 
with  the  cover-glass.  After 
a  few  minutes  the  corpuscles 
settle  over  the  ruled  spaces 
and  are  ready  for  counting. 
The  number  of  corpuscles  in 
at  least  five  series  of  sixteen 
small  squares  is  then  counted; 
this  number  is  then  multiplied 
by  the  degree  of  dilution  (ioo 
or  200  as  the  case  may  be) 
and  this  by  the  cubic  contents 
of  each  small  square  (4000) ; 
the  product  is  then  divided 
by  the  number  of  squares 
counted  (80  in  the  instance 
given  above):  e.  g.,  five  series 
of  sixteen  small  squares  con- 
tain 500  corpuscles 


a 

h 

mmm 

»"• 

." 

': 

1 0  ° 

Q.« 

'  ° '  0 

1  -  ■ 

, 

■  .'],•", 

-    :   :  s  0 

' 

•»lh"  ■•" 

".    ,%•, 

'- 

o°o° 

v« 

'  f  °° 

B  - 

'.'•  • 

• 

"-: 

\"o 

.-. 

°»  °  »« 

°.      "  ', 

"»"/« 

;*. 

• 

y; 

-,'-", 

°o°" 

.••'.: 

;,  ••. 

;; 

:: 

0° 

°    o° 

0  0° 

0  °°  °  \° 

\  l°°: 

0   V 

0 

%*  ■- 

; 

-:  ~  .v 

_ 

- 

'« 

-  0': 

.' 

' '  '  '! 

.-  ■■  », 

°  I « 

%° 

{ 

°,°° 

'."« 

°  „  ° 

0    °     ° 

'I    ''  o°  • 

\ 

Fig.  109. — Apparatus  of  Thoma  and  Zeiss 
foe  Counting  the  Corpuscles.  A.  In  section. 
C.  Surface  view  without  cover-glass.  B.  Micro- 
scopic appearance  with  the  blood-corpuscles. — 
(Landois  and  Stirling.) 


or, 


■500X200X4000  ,1  . 

g — - —  =  5,000,000  erythrocytes  per  c.mm., 

500X10,000  =  5,000,000,  erythrocytes  per  c.mm. 


The  accuracy  of  the  counting  is  proportional  to  the  number  of  squares 
counted.  If  200  squares  are  counted  with  each  of  two  different  drops,  and 
the  average  taken  the  probable  limit  of  error  will  be  less  than  2  per  cent. 

The  Effects  of  Reagents  on  the  Red  Corpuscles. — Within  the 
blood-vessels  the  physical  conditions  and  chemic  composition  of  the 
plasma  are  such  that  both  the  form  and  the  composition  of  the  cor- 
puscle or  the  relation  of  the  hemoglobin  to  the  stroma,  are  maintained 
in  the  normal  or  physiologic  condition.  The  plasma  is  preservative 
of  jhe  structure  and  function  of  the  corpuscle.  The  reason  assigned 
for  this  is  that  the  osmotic  pressure  of  the  salts  in  the  plasma  and 
of  the  salts  in  the  corpuscle  exactly  balance  one  another  so  that  there 
is  neither  an  absorption  of  water  from,  nor  a  yielding  of  water  to,  the 


THE  BLOOD.  249 

plasma  on  the  part  of  the  corpuscle.     The  plasma  having  an  osmotic 
pressure  equal  to  that  within  the  corpuscle  is  said  to  be  isotonic  with  it. 

When  blood  is  to  be  prepared  for  microscopic  examination  with  a 
view  of  determining  the  histologic  features  of  the  corpuscles  or  for  pur- 
poses of  enumeration,  it  must  be  diluted,  and  unless  special  precau- 
tions are  observed  the  condition  of  equal  osmotic  pressure  will  be  dis- 
turbed by  the  diluting  agent  and  the  corpuscles  will  lose  their  charac- 
teristic form  and  structure  from  either  an  absorption  or  loss  of  water, 
as  the  case  may  be. 

If  distilled  water  is  employed  for  this  purpose,  the  osmotic  pressure 
of  the  plasma  is  of  course  diminished,  and  in  consequence  the  osmotic 
pressure  of  the  constituents  of  the  corpuscles  (particularly  sodium 
chlorid)  causes  an  inflow  of  water.  The  corpuscle  therefore  swells 
and  assumes  a  more  or  less  spheric  form;  the  hemoglobin  is  dissociated 
and  discharged  into  the  surrounding  fluid  throughout  which  it  diffuses. 
Such  an  environment  having  an  osmotic  pressure  less  than  that  of  the 
corpuscle  is  said  to  be  hypotonic,  hypisotonic,  or  hypoisotonic  to  it. 

If  on  the  contrary,  water  containing  inorganic  salts  (particularly 
sodium  chlorid)  in  amounts  which  impart  to  the  plasma  an  osmotic 
pressure  greater  than  that  within  the  corpuscle,  there  will  be  an  outflow 
of  water  from  the  corpuscle,  a  shrinkage  of  the  volume  and  a  crenation 
of  its  surface.  Such  an  environment  having  an  osmotic  pressure 
greater  than  that  of  the  corpuscle  is  said  to  be  hypertonic,  or  hyperisotonic 
to  it.  It  becomes  essential  therefore  in  diluting  the  plasma  with  water, 
that  the  latter  contains  inorganic  salts  in  such  amounts  that  the  result- 
ing mixture  (plasma  and  water)  possesses  an  osmotic  pressure  equal 
to  that  of  the  original  plasma  or  to  that  of  the  corpuscle.  A  diluting 
agent  well  adapted  for  this  purpose  is  the  well-known  Ringer's  mixture. 
Other  solutions  which  preserve  the  form  of  the  corpuscles  during  the 
time  required  for  their  enumeration  are  the  solutions  devised  by  Hayem, 
Toisson  and  Gower  alluded  to  on  a  preceding  page.  Because  of  the 
fact  that  sodium  chlorid  is  the  chief  inorganic  constituent  of  the  plasma 
and  of  the  corpuscle  it  is  common  in  laboratory  work  to  dilute  the 
plasma  of  mammalian  blood  and  of  frog's  blood  with  solutions  of  so- 
dium chlorid  of  0.9  per  cent,  and  0.6  per  cent,  respectively,  which 
though  not  absolutely  are  sufficiently  isotonic  for  the  purpose  desired. 

Many  other  saline  solutions  with  an  osmotic  pressure  greater  or  less 
than  normal  plasma,  dilute  solutions  of  acids  and  alkalies,  bile  salts, 
chloroform,  ether,  ammonium  sulphocyanid,  electricity,  etc.,  also 
destroy  the  physical  and  chemic  integrity  of  the  corpuscle  and  cause 
the  hemoglobin  to  separate  from  the  stroma  and  diffuse  into  the  plasma 
without  itself  undergoing  any  appreciable  change  in  composition. 
With  the  escape  and  diffusion  of  the  hemoglobin  the  blood  becomes  trans- 
parent and  changes  to  a  dark  red  color  to  which  the  term  "lake  color" 
has  been  given.  The  mechanism  by  which  the  hemoglobin  becomes 
dissociated  and  discharged  from  the  corpuscle  by  these  agents  is 
unknown. 


250 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Corpuscles  of  Other  Vertebrated  Animals. — In  all  mam- 
mals, with  the  exception  of  the  camel,  llama,  and  dromedary,  the 
red  corpuscles  present  the  same  shape  and  structure  as  the  corpuscles 
of  man,  and  may  be  described  as  circular,  flattened,  biconcave  disks. 
In  the  animals  excepted  the  corpuscles  are  oval.  The  size,  however, 
varies  in  different  animals  from  0.0092  mm.  (2-7V5  mcn)  m  the-  ele- 
phant to  0.0023  mm.  (i^  inch)  in  the  musk-deer,  while  in  most 
animals  the  average  lies  between  0.0084  mm-  and  0.0050  mm.  Inas- 
much as  the  question  may  arise  as  to  whether  the  corpuscles  of  any 
given  specimen  of  blood  are  those  of  a  human  being  or  of  some  other 
mammal,  a  knowledge  of  the  size  of  the  corpuscles  becomes  a  matter 
of  medicolegal  as  well  as  of  physiologic  interest.  Though  the  dif- 
ferences in  size  are  slight,  yet  it  is  possible  for  skilled  microscopists, 
when  examining  fresh  blood,  to  make  a  diagnosis  between  the  cor- 
puscles of  man  and  those  of  the  domesticated  animals,  with  the  ex- 
ception, perhaps,  of  the  guinea-pig.  The  diagnosis  of  the  corpuscles 
of  dried  blood  which  have  been  altered  by  the  action  of  various  ex- 
ternal agents,  even  though  capable  of  a  certain  degree  of  restoration, 
is  most  difficult,  and  should  not  be  attempted  in  criminal  cases  with- 
out large  experience  in  microscopy,  in  measurements  and  methods 
of  preparation  of  all  kinds  of  blood-corpuscles,  and  a  proper  per- 
ception of  corpuscular  forms  and  sizes.  In  the  following  table  the 
average  results  of  the  measurements  of  the  corpuscles  in  different 
classes  of  animals  are  given  (abstracted  from  Formad's  compilation) : 


Man , 

Guinea  pig. 

Dog 

Rabbit,  .  .  . 

Ox 

Pig 

Horse,  .  .  . 

Cat 

Sheep, 
Goat 


Gulliver. 


Inch.       Mm. 


1.3200 
I-3S38 
1-3532 
1.3607 
1.4267 
1.4230 
1.4600 
1.4404 
1.5300 
1.6366 


0.0079 
0.0071 
0.0071 
0.0070 
0.0060 
0.0060 
0.0057 
0.0058 
0.0048 
0.0040 


WORMLEY. 


Inch.       Mm 


1-3250 
1.3223 
1-3561 
1-3653 
1. 4219 
1.4268 
1.4243 
1-4372 
1. 4912 
1. 6189 


0.0078 
0.0079 
0.0071 
0.0070 
0.0060 
0.0059 
0.0059 
0.0058 
0.0031 
0.0041 


C  Schmidt. 
Mallinin. 


Inch.        Mm 


3300 
3300* 
3636 
3968 
4354 
4098 
4464 
4545 
5649 
6369 


0.0077 
0.0077 
0.0070 
0.0064 
0.0058 
0.0062 
0.0057 
0.0056 
0.0045 
0.0040 


French  Medi- 
colegal Soc. 
Welcker. 


Inch.       Mm 


1-3257 
1-3213! 
1.3485 
1-3653 
1-4545 
1.4098 
1.4545 
1.3922 
1.5076 
1-5525 


0.0078 
0.0079 
0.0073 
0.0069 
0.0056 
0.0062 
0.0056 
0.0065 
0.0059 
0.0046 


FORMAD. 


Inch.       Mm. 


1.3200 
1.3400 
1.3580 
1.3662 
1.4200 
1.4250 
1. 4310 

1.5000 
1. 6100 


0.0079 
0.0075 
0.0071 
0.0069 
0.0060 
0.0060 
0.0059 

0.0051 
0.0042 


In  birds,  reptiles,  and  amphibians  the  corpuscles  are  larger  than 
in  mammals,  are  oval  in  shape,  and  nucleated.  (See  Figs,  no  and 
in.)  As  the  scale  of  animal  life  is  descended  the  corpuscles  increase 
in  size,  until  in  the  proteus  and  amphiuma  the  long  diameter  attains 
an  average  length  of  0.058  mm.  and  0.077  mm-  respectively.  In  fish 
the  corpuscles  are  smaller,  oval,  and  nucleated,  with  the  exception  of 
the  lamprey  eels,  in  which  they  are  circular,  biconcave,  and  nucleated, 
though  the  nucleus  is  generally  concealed  in  the  peripheral  portion  of 
the  corpuscle.  As  in  these  animals,  the  corpuscles  are  almost  twice 
the  size  of  the  human  red  corpuscles,  they  can,  notwithstanding  the 
similarity  of  shape,  be  readily  distinguished  from  them. 

*  Masson.  t  Woodward. 


III. 


THE  BLOOD.  251 

The  Function  of  the  Red  Corpuscles. — The  red  corpuscles,  by 
virtue  of  the  capacity  of  their  contained  hemoglobin  for  oxygen  ab- 
sorption, may  be  regarded  as  carriers  of  oxygen  from  the  lungs  to 
the  tissues,  and  therefore  important  factors  in  the  general  respiratory 
process.  The  size  as  well  as  the  number  of  the  corpuscles  in  different 
classes  of  animals  appears  to  be  directly  related  to  the  activity  of  the 
respiratory  process.  In  those  animals  in  which  the  corpuscles  are 
small  and  numerous  and  the  total  superficial  area  large,  respiration 
is  active,  the  quantity  of  oxygen  absorbed  is 
large,  and  the  energy  evolved  through  oxida- 
tion great.  In  those  animals,  on  the  contrary, 
in  which  the  corpuscles  are  large  and  relatively 
few  in  number,  the  reverse  conditions  obtain. 
This  is  in  accordance  with  the  fact  that  the 
superficial  area  of  any  given  volume  of  sub- 
stance is  increased  in  proportion  to  the  extent 
to  which  it  is  subdivided.  FIG.  no.       Fio. 

The  superficial  area  of  a  single  human  red       Amphibian    Colored 

corpuscle  has  been  estimated  at  0.000128  sq.      Blood-corpuscles.      Fig. 

-rr   xi  -Li!  1  !_•  no,  on  the  flat;   Fig.   in, 

mm.     If  the  number  of  corpuscles  in  1  cubic     on   ecjge  _  (Landots  and 
millimeter  of  blood   averages  5,000,000,  the     Stirling.) 
superficial  area  would  amount  to  640  square 

millimeters;  and  if  the  amount  of  blood  in  the  body  of  a  man  weigh- 
ing 75  kilos  is  taken  as  one- thirteenth  of  this  weight — that  is,  5769 
grams  (5463  c.c.) — the  total  area  of  the  corpuscular  surface  will  amount 
to  3496  square  meters. 

Life-history  of  Red  Corpuscles. — In  the  performance  of  their 
functions  the  red  corpuscles  undergo  more  or  less  disintegration  and 
finally  destruction;  but  as  the  average  number  is  maintained  under 
normal  physiologic  conditions,  there  must  be  a  constant  renewal  of 
corpuscles  from  day  to  day.  The  evidence  of  destruction  of  red 
corpuscles  is  furnished  by  the  presence  in  the  blood,  in  various  situ- 
ations of  the  body,  of  a  pigment  containing  iron  and  the  presence  of 
pigments  in  the  bile  and  urine,  all  of  which  are  believed  to  be  deriv- 
atives of  effete  hemoglobin.  The  blood-pigment  (hematin),  which 
contains  the  iron  of  the  hemoglobin,  is  found  in  the  capillaries  of 
the  liver,  in  the  cells  of  the  splenic  pulp,  and  in  the  marrow  of  the 
bones.  Whether  the  presence  of  the  pigment  in  these  organs  is  a 
proof  that  the  corpuscles  are  destroyed  here,  or  whether  they  are  to 
be  regarded  merely  as  agents  concerned  in  the  further  reduction 
and  elimination  of  the  hematin,  is  uncertain.  The  genetic  rela- 
tionship between  bile-pigment  and  hemoglobin  is  shown  by  the  fact 
that  any  artificial  destruction  of  hemoglobin  or  its  injection  into  the 
blood  is  attended  by  an  increase  in  the  quantity  of  bile-pigment  elim- 
inated. It  appears  also  from  chemic  considerations  that  the  hemo- 
globin will  undergo  cleavage  into  a  globulin  body  and  hematin,  which 
by  the  loss  of  its  iron  is  readily  converted  into  the  bile-pigment,  bili- 


252  TEXT-BOOK  OF  PHYSIOLOGY. 

rubin.  The  amount  of  this  latter  pigment  may  therefore  be  taken  as 
an  index  of  the  extent  of  corpuscular  destruction. 

This  gradual  decay  of  corpuscles  as  well  as  the  losses  occasioned 
by  hemorrhages  necessitate  a  continuous  formation  of  new  corpuscles, 
so  that  the  normal  number  may  be  maintained.  The  rapidity  with 
which  corpuscles  may  be  renewed,  in  the  woman  at  least,  is  shown 
by  a  computation  of  Mr.  Charles  L.  Mix.  A  woman  loses  during  a 
menstrual  period  150  c.c.  of  blood.  At  the  end  of  twenty-eight  or 
thirty  days  this  volume  is  restored,  so  that  in  one  day  5  c.c,  or  5000 
c.mm.,  of  blood  must  be  formed,  or  208  c.mm.  per  hour  and  3^  c.mm. 
per  minute.  That  is,  during  a  certain  number  of  years  15,750,000 
corpuscles  must  be  formed  every  minute,  and  this  independent  of  the 
daily  loss  due  to  functional  activity. 

At  the  present  time  there  is  a  general  agreement  among  histolo- 
gists  that  in  adult  life  the  red  corpuscles  are  derived  from  embryonic 
forms,  the  so-called  erythroblasts,  which  are  found  chiefly  in  the 
red  marrow  of  the  long  bones.*  In  this  situation  both  arterial  and 
venous  capillaries  are  relatively  large  and  the  blood  is  separated  from 
the  surrounding  marrow  by  extremely  thin  walls.  In  the  passages  of 
this  capillary  network  the  erythroblasts  make  their  appearance  most 
probably  by  a  transformation  of  pre-existing  marrow  cells.  At  first 
they  are  large,  homogeneous,  colorless,  perhaps  slightly  tinged  with 
hemoglobin  and  distinctly  nucleated.  They  increase  in  number  by 
karyokinesis  and  at  the  same  time  increase  in  their  hemoglobin  con- 
tent. The  nucleus  is  finally  extruded,  earrying  with  it  a  portion  of 
the  perinuclear  cytoplasm,  after  which  the  remainder  of  the  cor- 
puscle assumes  the  shape  and  size  of  the  adult  corpuscle  and  is  carried 
out  into  the  general  circulation.  After  severe  hemorrhage  the  forma- 
tive processes  in  the  marrow  may  become  so  active  that  erythroblasts 
make  their  appearance  in  the  blood-stream  before  the  extrusion  of  the 
nucleus  has  taken  place. 

CHEMIC  COMPOSITION  OF  RED  CORPUSCLES. 

Hemoglobin. — The  red  corpuscle  consists  of  a  stroma  and  a 
coloring-matter,  hemoglobin.  The  amount  of  hemoglobin  in  the 
normal  corpuscle  is  about  30  per  cent,  of  the  total  weight,  the  re- 
maining 70  per  cent,  consisting  of  68  per  cent,  of  water  and  2  per 
cent,  of  solid  matter,  e.  g.,  lecithin,  cholesterin  and  inorganic  salts. 
When  the  corpuscle  is  freed  from  water  the  hemoglobin  constitutes 
about  94  per  cent,  of  the  total  solids,  32  per  cent.  In  the  normal 
condition  of  the  corpuscle  the  hemoglobin  is  in  an  amorphous  con- 
dition and  is  combined  in  some  unknown  way  with  the  stroma. 

If  blood  Which  has  been  rendered  laky,  by  water  or  any  other 

*  For  an  admirable  resume  of  the  various  views  regarding  the  origin  and  formation 
of  red  corpuscles  see  the  paper  of  Mr.  Charles  L.  Mix,  Boston  Med.  and  Surg.  Journal, 
1892,  Nos.  n  and  12;  also  paper  by  Prof.  W.  H.  Howell,  Journal  of  Morphology,  vol. 
iv,  1802. 


THE  BLOOD. 


253 


of  the  known  agencies,  be  allowed  so  slowly  evaporate,  the  dissolved 
hemoglobin  undergoes  crystallization.  The  rapidity  with  which  the 
crystals  form  varies  in  the  blood  of  different  animals  under  similar  con- 
ditions. According  to  the  ease  with  which  crystallization  takes  place, 
Preyer  has  classified  various  animals 
as  follows:  (i)  Very  difficult — calf, 
pigeon,  pig,  frog;  (2)  difficult — man, 
monkey,  rabbit,  sheep;  (3)  easy — 
cat,  dog,  mouse,  horse;  (4)  very  easy 
— guinea-pig,  rat. 

The  hemoglobin  crystals  vary  in 
shape  according  to  the  blood  from 
which  they  are  obtained  (Fig.  112). 
Those  obtained  from  the  guinea-pig 
are  tetrahedral;  those  from  man 
and  most  mammals  are  prismatic 
rhombs;  those  from  the  squirrel  are 
in  the  form  of  hexagonal  plates. 
Notwithstanding  these  slight  differ- 
ences, all  forms  belong  to  the  same 
crystal  system,  with  the  exception 
of  those  from  the  squirrel. 

A  simple  but  very  effective 
method  of  obtaining  blood-crystals 
suggested  by  Reichert  is  to  lake 
denbrinated  blood,  especially  that 
of  the  dog,  rat,  guinea-pig,  and 
horse,  with  acetic  or  ethylic  ether 
and  then  add  a  solution,  1  to  5  per 
cent.,  of  ammonium  oxalate.  A 
drop  of  this  mixture  placed  under 
the  microscope  will  show  crystal 
formation  in  a  very  few  minutes. 

Chemic  Composition  of  Hemoglobin. — By  appropriate  methods 
hemoglobin  can  be  obtained  in  a  practically  pure  form,  and  when 
subjected  to  a  temperature  of  100 °  C.  its  water  of  crystallization  is 
driven  off,  after  which  it  can  be  analyzed.  In  the  subjoined  table 
the  results  of  several  analyses  are  given  for  100  parts  of  hemoglobin. 


Fig.  112.  —  Crystallized  Hemo- 
globin, a,  b.  Crystals  from  venous 
blood  of  man.  c.  From  blood  of  cat. 
d.  Of  guinea-pig.  e.  Of  marmot.  /. 
Of  squirrel. — (Gaulier). 


C, 
O, 
H, 
N, 
S,. 
Fe, 


Dog. 

Horse. 

Dog. 

Guinea-pig. 

53-91 

22.62 

6.62 

15.98 

0.54 

°-33 
Jaquet. 

5"5 

23-43 
6.76 

17.94 
o-39 

.  °-33 

Zinoffsky. 

53-85 
21.84 

7-32 

16.17 

°-39 
0.43 

54-12 

20.6S 

7-36 
16.78 

0.58 
0.48 

Hoppe 

Seyler. 

254  TEXT-BOOK  OF  PHYSIOLOGY. 

The  elementary  composition  of  hemoglobin  is  thus  seen  to  vary 
slightly  in  different  animals,  suggesting  that  there  may  be  different  kinds 
of  hemoglobin.  The  rational  molecular  formula  is  not  known.  On 
the  assumption  that  each  molecule  contains  one  atom  of  iron,  Preyer 
suggested  the  following  empirical  formula:  C6ooH96oNIS40179S3Fe, 
with  a  molecular  weight  of  13,332;  Jaquet  has  suggested  a  different 
formula:  viz.,  C7s8HI203NI9502l8S3Fe,with  a  molecular  weight  of  16.669. 
It  is  very  evident  from  this  that  the  molecule  is  of  enormous  size  and 
exceedingly  complex. 

Quantity  of  Hemoglobin.— The  quantity  of  hemoglobin  in  blood 
as  determined  by  chemic,  chromometric,  and  spectro-photometric 
methods  amounts  to  about  14  per  cent,  in  man  and  13  per  cent,  in 
woman.  Of  the  chemic  methods,  that  based  on  the  amount  of  iron 
is  the  most  familiar.  Chemic  analysis  has  shown  that  hemoglobin 
contains  0.43  per  cent,  and  blood  0.056  per  cent,  of  iron;  with  these 
two  factors  the  quantity  of  hemoglobin  can  be  determind  by  thee 
following  formula:  x=iooox°2°56  =  13.33  per  cent.  The  total  quantity 
of  hemoglobin  in  the  blood,  assuming  the  latter  to  be  about  5769 
grams  (one-thirteenth  of  the  body- weight,  75  kilos)  will  therefore 
amount  to  769  grams;  e.g.,  x  =  5769**3"33  =  769.  The  total  amount 
of  iron  in  the  blood  is  obtained  by  the  following  formula:  viz., 
^=^7^=3.23  grams. 

Under  normal  physiologic  conditions  the  percentage  of  hemo- 
globin undergoes  but  slight  variation.  In  pathologic  states  there  is 
frequently  a  great  diminution  in  the  amount,  especially  in  chlorosis, 
splenic  leukemia,  and  pernicious  anemia,  diseases  in  which  it  dimin- 
ishes to  2 \  per  cent,  in  many  instances.  For  the  determination  of 
these  variations  in  the  hemoglobin  for  clinical  purposes  two  chromo- 
metric methods  are  at  present  largely  employed,  that  of  Gowers  and 
v.  Fleischl.  All  chromometric  methods  are  based  on  the  principle 
that  if  two  equally  thick  and  equally  well-illuminated  solutions  pre- 
sent the  same  intensity  of  color,  their  richness  in  coloring-matter  is 
the  same.  There  are  two  methods  by  which  this  can  be  done:  (1)  By 
diluting  the  blood  to  be  examined  with  water  until  the  shade  of  color 
corresponds  to  that  of  a  solution  of  hemoglobin  of  known  strength 
(Gowers).  (2)  Diluting  a  given  quantity  of  blood  with  a  given 
quantity  of  water  and  then  finding  an  identical  color  which  repre- 
sents a  previously  determined  quantity  of  hemoglobin  (v.  Fleischl). 

Gowers'  hemoglobinometer  consists  (Fig.  113)  of  two  glass  tubes 
of  exactly  the  same  size.  One,  D,  contains  glycerin  jelly  colored 
with  picro-carmine  the  shade  of  which  corresponds  to  that  of  normal 
blood  diluted  100  times,  20  c.mm.  in  2000  c.mm.  of  water  repre- 
senting a  1  per  cent,  solution.  The  other  tube,  C,  is  ascendingly 
graduated  with  120  divisions,  each  one  of  which  corresponds  to  20 
c.mm.  With  a  graduated  pipette  B  20  cubic  millimeters  of  blood  are 
accurately  measured  and  blown  into  the  bottom  of  the  tube  C,  in 


THE  BLOOD. 


255 


which  a  few  drops  of  distilled  water  have  been  placed  so  as  to  prevent 
coagulation.  Water  is  then  added  drop  by  drop  until  the  color  of 
the  diluted  blood  is  exactly  that  of  the  standard.  The  division  of 
the  scale  reached  by  the  dilution  will  represent  the  relative  percentage 
of  hemoglobin.  If  this  tint  is  not  obtained  until  the  dilution  reaches 
100  divisions,  the  quantity  of  hemoglobin  is  normal.  If  more  water 
must  be  added,  it  is  in  excess;  if  less,  it  is  diminished.  If,  for  example, 
the  20  cubic  millimeters  of  blood  from  an  anemic  patient  gave  the 
standard  tint  at  60  divisions,  the  blood  contained  but  60  per  cent, 
of  the  normal  amount  of  hemoglobin. 


Fig.  113. — Gowers'  Hemoglobinometer.  A.  Pipette  bottle  for  distilled  water. 
B.  Capillary  pipette.  C.  Graduated  tube.  D.  Tube  with  standard  dilution.  F. 
Lancet  for  pricking  the  finger. — [Landois  and  Stirling.) 


Von  Fleischl's  hemometer  consists  of  a  metallic  cell  divided  into 
two  compartments,  a  and  a',  by  a  vertical  partition  (Fig.  114).  In 
the  former  a  definite  quantity  of  blood  is  placed  and  diluted  with  a 
known  quantity  of  water.  Beneath  the  compartment  a'  is  placed 
a  glass  wedge  stained  with  the  golden  purple  of  Cassius  or  simi- 
lar pigment,  the  color  of  which  passes  from  a  deep  red  at  one  end  to 
clear  glass  at  the  other  (Fig.  115).  To  the  side  of  this  wedge  is  placed 
a  scale  ranging  from  o  to  120.  By  means  of  the  screw,  R  T,  the  glass 
wedge  is  moved  until  the  color  of  the  glass  and  diluted  blood  is  identical. 
The  illumination  of  the  blood  and  glass  wedge  is  accompanied  by  lamp- 
light reflected  from  the  white  reflecting  surface  beneath.  The  depth 
of  color  of  the  glass  opposite  100  on  the  scale  corresponds  to  that  of 
normal  blood.  If,  therefore,  the  colors  are  identical  at  75  divisions, 
the  blood  contains  but  75  per  cent,  of  hemoglobin. 

Very  frequently  the  diminution  of  corpuscles  and  hemoglobin 
proceeds  along  parallel  lines,  in  which  case  the  amount  of  hemoglobin 


256 


TEXT-BOOK  OF  PHYSIOLOGY. 


per  corpuscle  is  supposed  to  be  normal  and  the  color-index  =  1. 
If  the  hemoglobin  diminution  is  greater  than  the  corpuscles,  as  is 
the  case  in  many  pathologic  conditions,  the  color-index  is  less  than 
unity.     If  the  percentage  of  corpuscles  is  determined  by  the  method 


Fig.  114. — Von  Fleischl's  Hemometee.  K.  Red  colored  wedge  of  glass  moved 
by  R.  G.  Mixing  vessel  with  two  compartments,  a  and  a'.  M.  Table  with  hole  to  read 
off  the  percentage  of  hemoglobin  on  the  scale  P.  T.  To  move  K.  S.  Mirror  of  plaster- 
of-Paris. 


of  counting  to  be  80  per  cent.  (4,000,000  per  cubic  millimeter)  and  the 
percentage  of  hemoglobin  60,  the  color-index  is  obtained  by  dividing 
the  latter  by  the  former;  e.  g.,  -§-$■  =  0.75.  In  other  words,  each  cor- 
puscle has  but  0.75  per  cent,  of  the  normal  amount  of  hemoglobin. 
Absorption  Spectra. — Both  oxyhemoglobin  and  reduced  hemo- 
globin, like  other  soluble  pig- 
ments, have  an  absorbing  influ- 
ence on  certain  waves  of  light, 
and  hence  give  rise  to  absorption 
bands  which  can  be  studied  with 
the  spectroscope,  and  which  are 
so  characteristic  as  to  serve  for 
their  identification. 

In  principle  a  spectroscope 
consists  of  a  prism  which  decom- 
poses the  light  from  a  narrow  slit  into  a  band  of  all  the  spectral  colors. 
A  form  of  spectroscope  in  common  use  is  that  shown  in  Fig.  116. 
It  consists  of  a  tube,  B,  which  has  at  one  end  a  slit  that  can  be  nar- 
rowed or  widened  by  means  of  a  screw.  The  light,  having  passed 
through  it,  falls  on  an  achromatic  convex  lens  (called  the  collimator) 


Fig.  115.  —  Tinted  Glass  Wedge  of 
the  von  Fleischl  Hemometer. — (Da 
Costa's  Hematology.) 


{Triacid  Stain.) 

i,   2,  3,  4.   Small  Lymphocytes. 

Contrast  the  faintly  colored  protoplasm  of  these  cells  in  the  triple  stained  specimen, 
with  their  intensely  basic  protoplasm  in  the  film  stained  with  eosin  and  methylene- 
blue,  17  and  18.  The  cell  body  of  1  is  invisible.  Note  the  kidney -shaped  nucleus 
in  4. 

5,  6.  Large  Lymphocytes. 

With  this  stain  the  nucleus  reacts  more  strongly  than  the  protoplasm;  with  eosin 
and  methvlene-blue  (19,  20),  on  the  contrary,  the  protoplasm  is  so  deeply  stained 
that  the  nucleus  appears  pale  by  contrast.  This  peculiarity  is  also  observed  in 
the  smaller  forms  of  lymphocytes. 

7,  8.  Transitional  Forms. 

Note  the  moderately  basic  and  indented  nucleus,  and  the  almost  hyaline  non-granular 
protoplasm.  Compare  8  with  the  myelocyte,  7,  Plate  I,  these  cells  differing  chiefly 
in  that  the  myelocyte  contains  neutrophile  granules. 

9,  10,  11.  Polynuclear  Neutrophils. 

These  cells  are  characterized  by  a  polymorphous  or  polynuclear  nucleus,  surrounded 
bv  a  cell-body  filled  with  fine  neutrophile  granules.  In  11  the  nuclear  structure  is 
obviously  separated  into  four  parts;  in  9  it  is  moderately,  and  in  10  markedly,  poly- 
morphous. 

12,  13.  Eosinophiles. 

The  nuclei  are  not  unlike  those  of  the  polynuclear  neutrophile,  except  that  they  are 
somewhat  less  convoluted,  and  poorer  in  chromatin,  staining  less  intensely.  The 
protoplasm  is  filled  with  coarse  eosinophile  granules,  the  characteristics  of  which 
are  clearly  illustrated  by  13,  a  "fractured"  eosinophile. 

14.  Eosinophilic  Myelocyte. 
Compare  with  15. 

15,  16.   Myelocytes.      {Neutrophilic.) 

These  cells  are  morphologically  similar  to  14,  except  that  they  contain  neutrophile 
instead  of  eosinophile  granules.  Note  that  the  granules  of  the  myelocyte  are  identical 
with  those  of  the  polynuclear  neutrophile.  A  dwarf  form  of  myelocyte  is  repre- 
sented by  16. 

(Eosin  and  Methylene -blue.) 

17,  18.  Small  Lymphocytes. 

Note  the  narrow  rim  of  pseudo-granular  basic  protoplasm  surrounding  the  nucleus, 
and  the  pale  appearance  of  the  latter. 

19,  20.  Large  Lymphocytes. 

Budding  of  the  basic  zone  of  protoplasm  is  represented  by  20.  Both  of  these  cells 
belong  to  the  same  type  as  3  and  6. 

21,  22.  Large  Mononuclear  Leukocytes. 

Compared  with  19  and  20,  these  cells  have  a  decidedly  less  basic  protoplasm,  but 
a  somewhat  more  basic  nucleus.  In  the  triple  stained  film  these  differences  can- 
not be  detected,  so  that  they  must  be  classed  as  large  lymphocytes. 

23.  Transitional  Form. 

The  distinction  between  this  cell  and  24  is  not  marked;  the  nucleus  of  the  latter 
simply  being  somewhat  more  basic  and  convoluted. 

24,  25,  26,  27.  Polynuclear  Neutrophiles. 

With  this  stain  these  cells  show  a  feebly  acid  protoplasm,  and  lack  granules.     Note 
that  the  more  twisted  the  nucleus  the  deeper  it  is  stained.     Compare  with  9,    10, 
and  1  r. 
28,  29.  Eosinophiles. 

Compare  with   12  and  13. 

30.  Eosinophilic  Myelocyte. 
Compare  with  14 

31.  Basophile.      (Finely  granular.) 

This  (ell  is  characterized  by  the  presence  of  exceedingly  fine  5-granuIes,  staining 
the-  pure  color  of  the  basic  dye.  The  nucleus  is  markedly  convoluted  and  deficient 
in  chromatin.     The  cell  here  shown  was  found  in  normal  blood. 

32.  t,t,,  34,  35,  36.  Mast  Cells. 

The  granules  take  a  modified  basic  color,  as  shown  by  their  royal-purple  tint  in  this 
illustration.  Note  their  unusually  large  size  and  ovoid  shape  in  t,^,  their  peculiar 
distribution  in  35  and  36,  and  their  irregularity  in  size  in  32  and  36.  With  the  triacid 
mixture  these  granules,  as  well  as  those  of  the  finely  granular  basophile,  31,  remain 
unstained,  showing  as  dull-white  stippled  areas  in  the  cell-body.  The  nuclear  chro- 
matin of  the  mast  cell  is  so  delicate  and  so  feebly  stained  that  it  is  barely  visible. 
These  cells  were  found  in  the  blood  of  a  case  of  splcnomcdullary  leukemia. 


PLATE  I. 


1  Wk 


.»>       0|e°o         i^ 

■fog  j?it'->>sP 


f,4*&% 


"*>%%*? 


%p 


•     •si'V-''!-". 


^SSsjs*  \hJ? 


The  Leukocytes. 

(2-16,   Triacid  Stain;  17-36,  Eosin  and  Methylene-blue.) 

(E.  F.  Faber,  /ec.) 

(From  DaCosla's  "Clinical  Hematology.") 


THE  BLOOD. 


257 


at  the  opposite  end  of  the  tube  which  renders  the  divergent  rays  of 
light  parallel.  These  parallel  rays  subsequently  fall  on  the  prism,  by 
which  they  are  dispersed  and  directed  into  the  tube,  A,  which  is 
nothing  more  than  a  small  telescope.  On  looking  into  it  at  the  ocular 
end  the  spectral  colors  are  seen.  If  the  light  has  been  derived  from 
the  sun,  the  spectrum  will  present  vertical  dark  lines,  the  so-called 
Fraunhofer's  lines.  They  are  given  from  A  to  F  in  Fig.  117.  If  a 
colored  medium  be  held  in  front  of  the  slit  so  that  the  light  has  to  pass 


Fig.  116.  —  The  Spectroscope.  A.  Telescope.  B.  Tube  for  the  admission  of 
light  and  carrying  the  collimator.  C.  Tube  containing  a  scale,  the  image  of  which  when 
illuminated  is  reflected  above  the  spectrum.  D.  The  fluid  examined. — (Landois  and 
Stirling.) 


through  it  first,  certain  dark  bands  will  appear  in  the  spectrum,  owing 
to  the  absorption  of  certain  rays. 

Dilute  solutions  of  arterial  blood  show  two  absorption  bands 
between  the  Fraunhofer  lines,  D  and  E,  in  the  green  and  yellow 
portion  of  the  spectrum.  (See  Fig.  117.)  The  band  nearest  D 
frequently  designated  as  alpha  is  dark  in  the  center  and  sharply 
defined.  The  band  which  lies  toward  E  is  broader  and  less  sharply 
defined. 

As  the  amount  of  light  absorbed  varies  with  the  concentration  of 
the  solution  as  well  as  its  thickness,  and  gives  rise  to  absorption  bands 
of  different  widths  and  intensities,  it  becomes  necessary,  in  order  to 
obtain  the  characteristic  bands,  to  employ  only  dilute  solutions. 

The  absorption  spectra,  as  seen  with  different  strengths  of  solution 
one  centimeter  thick,  are  shown  graphically  in  Fig.  118.  It  will  be 
observed  that  solutions  varying  in  strength  from  0.1  per  cent,  to  0.6 
17 


258 


TEXT-BOOK  OF  PHYSIOLOGY. 


per  cent,  give  rise  to  the  two  characteristic  bands,  but  with  gradually 
increasing  breadths.  With  a  percentage  greater  than  0.65  per  cent, 
the  light  between  D  and  E,  the  yellow-green,  becomes  extinguished 
and  the  two  bands  fuse  together,  forming  a  single  band  overlapping 
slightlv  the  lines  D  and  E.  At  the  same  time  there  is  a  progressive 
darkening  of  the  violet  end  of  the  spectrum.  At  0.85  per  cent.,  all 
the  light  is  absorbed  with  the  exception  of  a  small  amount  of  the  red. 


Red.     Orange.  Yellow. 


Green. 


Cyan  Blue. 


f 

\    a 
,,1,1, 

B 

50 
.Y..I....I-. 

c 

fao 
nil..'. 

1 

70 
1 1 n  1 1 11 

E   1 

80 

linil 

) 

Mill 

00 

III    MM 

100                 1 

in  il  1 1 1  1 1  in  1 

D 

L 

II 

HIM 

2 

II 

1     1 

| 

• 

1 

Hi    J 

I 

4. 

J 

1 

1 

Oxy 

hemoglobin 

0.8%. 


Oxy- 

hemoglocin 
0.28%. 


Carbon  * 
Monoxid 
Hemoglobin 


Reduced 
Hemoglobin 


FiGi  117. — Spectra  of  Hemoglobin  and  Some  of  its  Compounds. 
— (Landois  and  Stirling.) 


Solutions  less  than  0.01  per  cent,  to  0.003  Per  cent-  show  but  a  single 
absorption  band — that  nearest  D. 

A  solution  of  venous  blood  or  of  reduced  hemoglobin  shows  but  a 
single  absorption  band  (see  Fig.  117),  frequently  designated  as  gamma, 
broader  and  less  marked  between  the  lines  D  and  E,  but  extending 
slightly  beyond  D.  Fig.  117  shows  in  the  same  graphic  manner  the 
increasing  breadth  of  the  absorption  band  with  increasing  strengths 
of  solution,  as  well  as  the  simultaneous  absorption  of  light  at  both 
the  red  and  violet  ends  of  the  spectrum. 

Compounds  of  Hemoglobin.— The  coloring-matter  of  the 
blood  is  characterized  by  the  property  of  combining  with  and  of  again 
yielding  up  oxygen.  The  union  is  a  chemic  one,  taking  place  under 
certain  pressure  conditions.  It  therefore  may  exist  in  two  states  of 
oxidation,  distinguished  by  a  difference  in  color  and  their  absorption 
spectra.  If  hemoglobin  either  in  blood  or  in  solution  be  shaken  with 
air,  it  at  once  combines  with  oxygen  and  is  converted  into  oxyhemo- 
globin, which  imparts  to  the  blood  or  solution  a  bright  red  or  scarlet 
color.     If  the  blood  or  solution  be  now  deprived  of  oxygen,  the  oxy- 


THE  BLOOD. 


259 


hemoglobin  is  converted  into  reduced  hemoglobin,  which  imparts  to 
the  blood  or  solution  a  dark  bluish  or  purple  color. 

The  quantity  of  oxygen  absorbed  by  1  gram  of  hemoglobin  is 
estimated  at  1.56  c.c.  measured  at  o°  C.  and  760  mm.  of  mercury. 
The  compound  formed  by  the  union  of  oxygen  and  hemoglobin  is  a 
very  feeble  one;  for  when  the  pressure  is  lowered  the  union  becomes 
less  stable,  and  as  the  zero  point  is  approached,  as  in  the  Torricellian 
vacuum,  a  rapid  dissociation  of  the  oxygen  takes  place.  This,  how- 
ever, is  not  due  entirely  to  a  fall  of  pressure  but  partly  to  the  dis- 
sociating force  of  heat,  which  increases  in  power  as  the  pressure  falls. 


Fig.  118. — Graphic  Representa- 
tion of  the  Absorption  of  Light  in 
a  Spectrum  by  Solutions  of  Oxy- 
hemoglobin of  Different  Strengths. 
The  shading  indicates  the  amount  of 
absorption  of  the  spectrum,  and  the 
numbers  at  the  side  the  strength  of  the 
solution. 


Fig.  119 — Graphic  Representation 
of  the  Absorption  of  Light  in  a 
Spectrum  by  Solutions  of  Hemoglobin 
of  Different  Strengths.  The  shad- 
ing indicates  the  amount  of  absorption 
of  the  spectrum,  and  the  numbers  at 
the  side  the  strength  of  the  solution. 


The  same  dissociation  of  oxygen  can  be  brought  about  by  passing 
through  blood  indifferent  gases,  such  as  hydrogen,  nitrogen,  carbon 
dioxid,  which  lower  oxygen  pressure,  or  by  the  addition  of  reducing 
agents,  such  as  ammonium  sulphid  or  Stokes'  fluid. 

These  experimental  determinations  of  the  relation  of  oxygen  to 
hemoglobin  partly  explain  the  oxidation  and  deoxidation  of  the 
hemoglobin  in  the  lungs  and  tissues.  As  the  blood  passes  through 
the  lungs  and  is  subjected  to  the  oxygen  pressure  there,  the  hemoglo- 
bin combines  with  a  definite  quantity  of  oxygen,  and  on  emerging 
from  the  lungs  exhibits  a  bright  red  or  scarlet  color;  as  the  blood 
passes  through  the  systemic  capillaries  where  the  oxygen  pressure 
in  the  surrounding  tissues  is  low,  the  oxyhemoglobin  yields  up  a  por- 
tion of  its  oxygen,  becoming  deoxidized  or  reduced,  and  the  blood 
on  emerging  from  the  tissues  exhibits  a  dark  bluish  color.  The  portion 
of  oxygen  given  up  to  the  tissues  is  termed  respiratory  oxygen.  In 
100  parts  of  arterial  blood  the  coloring-matter  presents  itself  almost 
exclusively  in  the  form  of  oxyhemoglobin.     In  passing  through  the 


26o  TEXT-BOOK  OF  PHYSIOLOGY. 

capillaries  about  5  per  cent,  only  gives  up  its  oxygen  and  becomes 
reduced,  so  that  both  kinds  are  present  in  venous  blood.  In  asphyx- 
iated blood  only  reduced  hemoglobin  is  present.  It  is  this  capa- 
bility of  combining  with  and  of  again  yielding  up  oxygen,  that  enables 
hemoglobin  to  become  the  carrier  of  oxygen  from  the  lungs  to  the 
tissues. 

Carbon  Monoxid  Hemoglobin. — Carbon  monoxid  is  a  con- 
stituent of  both  coal-gas  and  water-gas.  From  either  source  it  is 
likely  to  accumulate  in  the  air,  and  if  inspired  for  any  length  of  time 
produces  a  series  of  effects  which  may  eventuate  in  death.  If  blood 
be  brought  into  contact  with  this  gas,  it  assumes  a  bright  cherry-red 
color,  which  is  quite  persistent  and  due  to  the  displacement  of  the 
loosely  combined  oxygen  and  the  union  of  the  carbon  monoxid  with 
the  hemoglobin.  The  compound  thus  formed  is  very  stable  and  resists 
the  action  of  various  reducing  agents.  The  passage  of  air  or  of  some 
neutral  gas  through  the  blood  for  a  long  period  of  time  will  gradually 
displace  the  carbon  monoxid  and  enable  the  hemoglobin  to  again 
absorb  oxygen.  It  is  for  this  reason  that  partial  poisoning  with  the 
gas  is  not  fatal.  It  is  to  its  power  of  displacing  oxygen  and  form- 
ing a  stable  compound  with  hemoglobin  and  thus  interfering  with 
its  respiratory  function  that  carbon  monoxid  owes  its  poisonous 
properties.  Examined  spectroscopically,  solutions  of  carbon  monoxid 
hemoglobin  exhibit  two  absorption  bands  closely  resembling  in  position 
and  extent  those  of  oxyhemoglobin;  but  careful  examination  shows 
that  they  are  slightly  nearer  the  violet  end  of  the  spectrum  and  closer 
together.  (See  Fig.  117.)  A  useful  test  for  CO  blood  is  the  addition 
of  caustic  soda,  which  produces  a  cinnabar  red  precipitate. 

Methemoglobin. — This  is  a  pigment,  closely  related  to  oxy- 
hemoglobin, found  in  the  blood  after  the  administration  of  various 
drugs,  in  cysts  and  in  the  urine  in  hematuria  and  hemoglobinuria. 
It  is  also  produced  when  a  solution  of  hemoglobin  is  exposed  to  the 
air  and  becomes  acid  in  reaction  and  brown  in  color.  The  spectrum 
shows  two  absorption  bands  similar  to  oxyhemoglobin,  but  in  addition 
a  new  and  quite  distinct  band  near  the  line  C,  in  the  red.  If  the 
acid  solution  be  rendered  alkaline  by  the  addition  of  ammonia,  this 
band  disappears  and  another  makes  its  appearance  near  the  line  D. 
The  addition  of  ammonium  sulphid  develops  reduced  hemoglobin, 
which,  on  the  absorption  of  oxygen,  produces  again  oxyhemoglobin, 
as  shown  by  the  spectroscope. 

Hematin. — Boiling  hemoglobin  or  adding  to  it  acids  or  alka- 
lies decomposes  it  and  develops  one  or  more  proteid  bodies  to  which 
the  general  term  globulin  has  been  given,  and  an  iron-holding  pig- 
ment termed  hematin.  This  is  regarded  as  an  oxidation  product  of 
hemoglobin  and  constitutes  about  4  per  cent,  of  its  composition. 
When  obtained  in  a  pure  state,  it  is  a  non-crystallizable  blue-black 
powder  with  a  metallic  luster.  According  as  it  is  treated  with  acidskor 
alkalies,  two  forms  of  hematin  can  be  obtained  (acid  and  alkaline), 


THE  BLOOD.  261 

each  of  which  has  special  properties,  giving  rise  to  different  absorp- 
tion bands. 

Hemochromogen. — This  pigment  is  derived  from  hemoglobin, 
of  which  it  constitutes  about  96  per  cent.,  during  decomposition  in 
the  absence  of  oxygen.  In  solution  it  produces  a  purple  color,  but 
soon  absorbs  oxygen  and  is  converted  into  hematin. 

Hemin. — This  pigment  is  a  derivative  of  hematin,  presenting 
itself  in  the  form  of  microscopic  rhombic  plates  or  rods  (Teichmann's 
crystals),  which  are  so  characteristic  as  to  serve  as  tests  for  blood- 
stains in  medicolegal  inquiries.  These  crystals  are  readily  obtained 
by  adding  to  a  small  quantity  of  dried  blood  on  a  glass  slide  a  few 
drops  of  glacial  acetic  acid  and  a  crystal  of  sodium  chlorid;  after 
heating  gently  for  a  few  minutes  over  a  spirit  lamp  and  then  allowing 
the  mixture  to  cool,  crystallization  of  the  hemin  soon  takes  place. 

Hematoidin. — This  term  has  been  applied  to  a  pigment  which 
occurs  in  the  form  of  yellow  crystals  in  old  blood-clots  or  in  blood 
which  has  been  extravasated  into  the  tissues.  In  its  chemic  com- 
position and  in  its  reactions  it  closely  resembles  bilirubin,  the  pigment 
of  the  bile,  exhibiting  the  same  characteristic  play  of  colors  on  the 
addition  of  nitric  acid. 

The  Stroma. — The  stroma  of  the  red  corpuscles  obtained  by  the 
methods  which  dissolve  out  the  hemoglobin  has  been  shown  by  analysis 
to  consist  of  from  60  to  70  per  cent,  of  water  and  40  to  30  per  cent, 
of  solid  material,  containing  a  proteid  resembling  cell-globulin,  lecithin, 
cholesterin,  and  inorganic  salts,  among  which  potassium  phosphate 
is  especially  abundant. 

HISTOLOGY  OF  THE  WHITE  CORPUSCLES  OR  LEUKOCYTES. 

The  presence  of  white  corpuscles  in  the  blood  can  be  readily 
observed  under  the  same  conditions  as  the  red  corpuscles  are  observed. 
Thus  when  the  mesentery  of  the  frog  or  the  guinea-pig  is  examined 
with  the  microscope  the  white  corpuscles  are  seen  adhering  to  the  walls 
of  the  blood-vessels;  in  a  drop  of  freshly  drawn  blood  they  are  found 
in  the  spaces  between  red  corpuscles  (Fig.  104). 

Shape  and  Size. — In  the  resting  condition,  whether  seen  in  the 
vessel  or  on  the  stage  of  the  microscope,  the  white  corpuscle,  as  its 
name  implies,  is  grayish  in  color,  round  or  globular  in  form,  though 
often  presenting  a  more  or  less  irregular  surface.  Its  diameter  varies 
from  0.0004  to  0.0013  rnm.,  though  the  average  is  about  0.0011  mm. 
or  about  -5-5V0  inch- 
Structure. — A  typical  white  corpuscle  consists  of  a  ground- 
substance  uniformly  transparent  and  apparently  homogeneous,  in 
which  are  embedded  a  number  of  granules  of  varying  size,  some  of 
which  are  very  fine,  while  others  are  larger.  By  various  reagents  it 
has  been  demonstrated  that  the  granules  are  fatty,  proteid,  and 
carbohydrate  (glycogen)  in  character.     In  the  fresh  cells  the  existence 


262  TEXT-BOOK  OF  PHYSIOLOGY. 

of  a  nucleus  is  difficult  of  detection,  though  its  presence  can  be  demon- 
strated by  the  addition  of  acetic  acid,  which  renders  the  perinuclear 
cytoplasm  more  transparent  and  makes  the  nucleus  conspicuous  and 
sharply  defined.  From  its  structure  it  is  apparent  that  the  white 
corpuscle  belongs  to  the  group  of  undifferentiated  tissues  and  re- 
sembles the  cells  of  the  embryo  in  its  earliest  stages  as  well  as  the 
unicellular  organism,  the  amoeba. 

Chemic  Composition. — The  chemic  composition  of  the  white 
corpuscles  has  been  inferred  from  an  analysis  of  pus-corpuscles, 
with  which  they  are  practically  identical,  and  of  lymph-corpuscles 
from  the  lymph-glands.  Of  the  corpuscle  about  go  per  cent,  is 
water  and  the  remainder  solid  matter  consisting  mainly  of  proteids, 
of  which  nuclein,  nucleo-albumin,  and  cell  globulin  are  the  most 
abundant.  The  two  former  are  characterized  by  the  presence  of  a 
considerable  quantity  of  phosphorus,  amounting  to  as  much  as  10 
per  cent.  Lecithin,  fat,  glycogen,  and  earthy  and  alkaline  phosphates 
are  also  present. 

Number  of  White  Corpuscles. — The  number  of  white  cor- 
puscles per  cubic  millimeter  of  blood  is  much  less  than  the  number 
of  red  corpuscles,  the  ratio  being  in  the  neighborhood  of  i  white  to 
700  red.  This  ratio,  however,  varies  within  wide  limits  in  different 
portions  of  the  body  and  under  normal  variations  in  physiologic 
conditions.  In  the  blood  of  the  splenic  artery  there  is  but  1  white 
to  2260  red,  while  in  the  splenic  vein  there  is  1  white  to  every  60  red; 
or  about  thirty-eight  times  as  many  as  in  the  artery.  In  the  portal 
vein  there  is  1  white  to  740  red,  while  in  the  hepatic  vein  there  is  1 
white  to  170  red. 

The  total  number  of  white  corpuscles  per  cubic  millimeter  has 
been  estimated  at  from  5000  to  10,000,  though  the  average  is  about 
7500.  The  number,  however,  is  influenced  by  a  variety  of  physio- 
logic conditions.  The  ingestion  of  food  rich  in  proteid  material 
raises  the  count  from  30  to  40  per  cent.,  as  compared  with  the  count 
before  the  meal.  Fasting  for  a  few  days  always  lowers  the  count, 
and  in  a  case  of  total  abstinence  of  food  for  a  week,  reported  by  Luciani, 
the  count  fell  to  861  per  cubic  millimeter,  after  which  it  rose  to  1530, 
where  it  practically  remained  for  the  succeeding  three  weeks  of  the 
fasting  period.  In  the  new-born  the  number  is  greater  than  in  adults 
—17,000  to  20,000  per  cubic  millimeter.  Cabot  states  that  30,000 
is  never  a  high  count  after  a  meal  in  infants  under  two  years.  In  the 
later  months  of  pregnancy,  especially  in  primiparas,  the  number  in- 
creases to  16,000  to  18,000.  Many  pathologic  conditions  of  the  body 
also  influence  the  count  very  considerably. 

The  method  for  counting  the  white  corpuscles  is  similar  to  that 
used  in  counting  the  red.  The  given  volume  of  blood  should,  however, 
be  diluted  with  10  or  20  volumes  of  a  one  per  cent,  solution  of  acetic 
acid,  which  disintegrates  the  red  corpuscles  and  thus  facilitates  the 
counting  of  the  white.     The  pipette  should  have  a  larger  bore  than 


THE  BLOOD. 


263 


that  used  for  the  red,  and  a  much  greater  number  of  squares  in  the 
counting  chamber  should  be  counted,  so  as  to  diminish  the  percent- 
age of  error. 

Physiologic  Properties. — The  white  corpuscles  possess  the 
characteristic  property  of  exhibiting  movements  similar  to  those 
observed  in  the  amoeba,  and  are  therefore  termed  amoeboid.  These 
movements  consist  in  alternate  protrusions  and  retractions  of  portions 
of  the  cell  body,  as  a  result,  of  which  they  exhibit  a  great  variety  of 
forms.  (See  Fig.  120.)  The  protruded  process  can  also  attach  itself 
to  some  point  of  the  surface  on  which  it  rests,  and  then  draw  the  body 
of  the  corpuscle  after  it. 
By  a  repetition  of  this  .....        ) 

process  the  corpuscle  can 
slowly  creep  about  and 
change  its  position  in 
space.  In  virtue  of  these 
ameboid  movements  the 
corpuscle  can  appropri- 
ate small  particles  of  pig- 
ment, such  as  indigo  or 
carmine,  and  after  a 
short  time  eliminate 
them  from  various  parts 
of  the  surface.  It  is  also 
capable  of  thrusting  a 
process  into  and  through 
the  wall  of  the  capillary 
vessel,  after  which  the 
remainder  of  the  corpuscle  follows  (Fig.  121).  This  continues  until 
the  corpuscle  is  outside  the  vessel  and  in  the  lymph-space,  where 
it  resumes  its  original  shape  and  movement.  This  process  is  best 
observed  in  inflammatory  conditions,  when  the  blood  has  come  to 
rest  and  the  vessels  are  occluded  with  both  red  and  white  corpuscles. 
To  this  passage  of  the  white  blood-corpuscles  through  the  capillary 
wall  the  term  diapedesis  is  given.  The  movements  of  the  white 
corpuscles  are  increased  by  a  rise  in  temperature  up  to  40 °  C,  beyond 
which  they  cease,  owing  to  the  coagulation  of  the  cell-substance.  A 
low  temperature  also  arrests  the  movements.  Induced  electric  cur- 
rents also  cause  contraction  and  death  of  the  cell.  Moisture  and 
oxygen  are  necessary  to  their  activity.  From  their  similarity  to  lower 
organisms  the  white  corpuscles  may  be  regarded  as  independent 
organisms  living  in  the  animal  fluids,  just  as  the  amoeba  lives  in  its 
natural  liquid  medium. 

Varieties  of  Leukocytes. — A  detailed  study  of  the  blood  with  the 
aid  of  the  triacid  staining  fluid  of  Ehrlich  or  any  of  the  various  eosin 
and  methylene-blue  stains,  reveals  the  presence  of  five  distinct  varieties 
of  leukocvtes  and  transitional  forms  which  may  be  classified  as  follows: 


Fig.  120- 
of  Shape 
Bachman.) 


-Human  Leukocytes  Showing  Changes 
Due    to    Amceboid    Movements.     (G. 


264 


TEXT-BOOK  OF  PHYSIOLOGY. 


1.  Small  lymphocytes,  so  called  from    their  resemblance  to  the  cor- 

puscles of  the  lymph-glands,  consisting  of  a  deeply  staining  and 
relatively  large  round  nucleus,  encircled  by  a  narrow  rim  of 
cytoplasm.     Found  in  from  20  to  25  per  cent,  of  all  leukocytes. 

2.  Large  lymphocytes  or  hyaline  cells,  which  are  believed  by  some  to 

represent  the  preceding  type  at  a  later  stage  of  development,  by 
others  to  have  an  independent  origin,  are  distinguished  by  a 
round  or  ovoid  nucleus  staining  faintly  n 
and  surrounded  by  a  relatively  larger 
layer  of  cytoplasm  than  is  seen  in  the 
small  lymphocyte.  The  large  lymphocyte 
is  present  to  the  extent  of  from  4  to  8  per 
cent.  The  transitional  forms  are  very 
much  like  the  large  lymphocyte  in  appear- 
ance and  size,  with  the  exception,  how- 
ever, that  they  possess  a  crescentic  or  in- 
dented nucleus  having  a  somewhat  greater 
affinity  for  basic  dyes.  They  are  usually 
counted  with  the  large  lymphocytes. 

Both  varieties  of  lymphocytes  are  char- 
acterized by  a  cytoplasm  which  is  devoid 
of  granules.  Rarely,  basophilic  granules 
may  be  present. 

3.  Polymorphonuclear    leukocytes    or    neutro- 

philes.  The  nucleus  of  this  cell  is  irreg- 
ular and  assumes  a  great  variety  of  shapes 
in  different  cells,  a  feature  which  has 
suggested  the  name  given  to  the  cell. 
The  perinuclear  cytoplasm  contains  a 
large  number  of  fine  granules  which  are 
neutrophilic  or  faintly  acidophilic  in  their 
staining  reaction.  They  make  up  about 
60  to  70  per  cent,  of  the  whole  number 
of  the  white  blood-cells. 

4.  Eosinophile  cells.     The  nucleus  resembles 

in  many  respects  that  of  the  preceding 
variety;  it  is,  however,  less  apt  to  stain 
so  deeply.  It  is  also  very  irregular  in 
possess  several  apparently  distinct  nuclei. 

defined  but  its  presence  is  easily  revealed  through  the  large,  in- 
tensely acidophilic  granules  which  it  possesses. 
It  is  present  to  the  extent  of  0.5  to  2  per  cent. 

5.  Basophile  cells,  the  nucleus  of  which  is  round  or  slightly  irregular. 

The  granules  which  may  be  large  or  small  are  basophilic  and 
stain  more  deeply  than  the  nucleus,  though  they  have  the  same 
color.  It  is  rare  for  this  cell  to  be  present  above  0.5  per  cent, 
of  all  leukocytes. 


Fig.  121. — Small  Ves- 
sel showing  Various 
Stages  in  the  Diapedesis 
of  Leukocytes.  {G.Bach- 
man.) 

shape  and  many  cells 
The  cytoplasm  is  ill- 


THE  BLOOD.  265 

Origin  of  the  White  Corpuscles. — Of  the  various  theories  ad- 
vanced to  explain  the  origin  of  leukocytes,  that  formulated  by  Ehrlich 
has  found  the  most  credence.  According  to  this  theory  the  leukocytes 
may  genetically  be  classed  into  two  groups.  In  the  first  group  are 
the  large  and  small  lymphocytes  which  take  their  origin  entirely  from 
the  lymph-adenoid  tissues  of  the  body,  e.g.,  the  lymph-glands,  solitary 
and  agminated  follicles  of  the  intestines,  etc.  As  the  lymph  flows 
through  these  structures  the  lymph-corpuscles,  as  the  future  lymph- 
ocytes of  the  blood  are  called  in  these  situations,  are  washed  out  and 
carried  by  way  of  the  lymph-stream  into  the  general  circulation. 

In  the  second  group  are  the  transitional  forms,  the  polymorpho- 
nuclear, eosinophile  and  basophile  leukocytes  which  originate  from 
the  bone-marrow  only.  The  immediate  ancestors  of  these  cells  are 
known  as  myelocytes  and  are  normally  found  in  the  red  bone-marrow. 
These  cells,  through  transitional  stages,  assume  the  characteristics 
of  the  leukocytes  just  mentioned  and  pass  directly  into  the  capillaries 
of  the  marrow  whence  they  are  distributed  throughout  the  body. 

Several  attempts  have  been  made  by  different  investigators  to  trace 
all  varieties  of  leukocytes  to  a  common  mother  cell.  While  this  is 
believed  to  take  place  during  embryonal  life,  the  proofs  of  such  an 
origin  of  leukocytes  in  the  normal  adult  are  insufficient  and  uncon- 
vincing. 

After  an  unknown  period  of  life  the  leukocytes  undergo  dissolution 
and  disappear. 

Functions. — The  functions  of  the  white  corpuscles  are  but  im- 
perfectly known,  and  at  present  no  positive  statements  can  be  made. 
It  has  been  suggested  that  wherever  found  in  the  body,  whether  in 
blood  or  tissues,  they  are  engaged  in  the  removal  of  more  or  less  in- 
soluble particles  of  disintegrated  tissues,  in  attacking  and  destroying 
more  or  less  effectively  various  forms  of  invading  bacteria  and  thus 
protecting  the  body  against  their  deleterious  activity.  This  they  do 
by  surrounding,  enveloping,  and  incorporating  either  the  tissue  par- 
ticle or  bacterium  and  digesting  it.  On  account  of  this  swallowing 
action  these  cells  were  termed  by  Metchnikoff  phagocytes  and  the 
process  phagocytosis.  The  cells  engaged  in  this  process  are  the 
polymorphonuclear  leukocytes  and  the  large  and  the  small  lymphocytes. 
He  regards  them  as  the  general  scavengers  of  the  body.  It  has  been 
suggested  that  they  are  also  engaged  in  the  absorption  of  fat  from 
the  lymphoid  tissue  of  the  intestine.  In  their  dissolution  they  con- 
tribute to  the  blood-plasma  certain  proteid  materials  which  assist 
under  favorable  circumstances  in  the  coagulation  of  the  blood. 

HISTOLOGY  OF  THE  BLOOD-PLATES. 

The  blood-plates  or  plaques  are  small  histologic  elements  circu- 
lating in  the  blood-plasma.  They  were  discovered  by  Hayem,  who 
applied  to  them  the  term  hematoblasts,  on  the  supposition  that  they 


266  TEXT-BOOK  OF  PHYSIOLOGY. 

were  the  early  stages  in  the  development  of  the  red  corpuscles.  This 
is  now  known  to  be  erroneous.  On  account  of  their  specific,  distinct 
characters,  and  their  constant  presence  in  the  blood  of  living  animals 
(guinea-pig  and  bat),  they  are  now  regarded  as  normal  constituents 
of  the  blood  and  designated  as  the  third  corpuscle.  When  blood  is 
freshly  drawn  from  the  body,  the  plaques  rapidly  undergo  disintegra- 
tion and  disappear;  but  by  treating  the  blood  with  osmic  acid,  the 
form  and  structure  of  the  plaque  may  be  retained. 

The  blood-plaque  may  be  defined  as  a  colorless,  grayish-white, 
homogeneous  or  finely  granular  protoplasmic  disk,  varying  in  diam- 
eter from  1.5  to  3.5  micro-millimeters.  The  edges  are  rounded  and 
well  defined,  but  it  is  not  certain  whether  they  are  only  flattened  or 
are  slightly  biconcave.  There  is,  however,  no  nucleus.  The  ratio  of 
the  plaques  to  the  red  corpuscles  is  1  to  18  or  20,  and  the  total  number 
per  cubic  millimeter  has  been  estimated  to  be  250,000  to  300,000. 

When  blood  is  shed  they  tend  to  adhere  to  each  other  and  form 
irregular  masses  known  as  Schultze's  granular  masses.  If  threads 
are  suspended  in  blood,  the  plaques  accumulate  in  enormous  numbers 
upon  them  and  appear  to  form  a  center  from  which  fibrin  filaments 
radiate  as  coagulation  proceeds.  The  white  thrombi  which  form  in 
blood-vessels  in  consequence  of  diseased  states — e.  g.,  endocarditis, 
atheromatous  ulceration,  etc. — are  composed  very  largely  of  blood- 
plaques  and  fibrin  threads.  The  function  of  the  blood-plaques  is 
unknown,  but  it  has  been  surmised  that  in  some  way  they  are,  like 
the  leukocytes,  concerned  in  the  coagulation  of  the  blood.  When- 
ever they  are  diminished  in  number,  as  in  purpura  and  hemophilia, 
coagulation  takes  place  very  slowly. 

The  blood-plaques  can  be  seen  with  high  powers  of  the  micro- 
scope in  the  blood-vessels  of  the  omentum  of  the  guinea-pig  and  rat, 
especially  when  the  blood-stream  begins  to  slow.  They  are  also 
readily  seen  in  the  blood-vessels  of  subcutaneous  connective  tissue 
of  various  animals,  and  especially  in  that  of  the  new-born  rat.  A 
small  quantity  of  this  tissue  moistened  with  normal  saline  and  exam- 
ined microscopically  with  suitable  powers  will  show  large  numbers 
of  plaques  within  the  blood-vessels. 

THE  TOTAL  QUANTITY  OF  THE  BLOOD;  ITS  GENERAL 
COMPOSITION. 

The  determination  of  the  total  quantity  of  the  blood  in  an  animal 
is  best  made  by  the  chromometric  method,  somewhat  modified  at 
present,  of  Welcker.  This  consists,  first,  in  bleeding  an  animal, 
collecting  all  the  blood  it  yields,  and  weighing  it;  second,  in  washing 
out  the  vessels  with  a  normal  saline  solution  until  the  fluid  comes 
from  the  veins  clear  and  free  from  blood;  third,  in  mincing  the  tissues 
of  the  body,  after  removal  of  the  contents  of  the  alimentary  canal, 
soaking  them  in  water  for  twenty-four  hours,  and  then  expressing 
them.     All  the  washings  are  collected  and  weighed.     A  given  volume 


THE  BLOOD.  267 

of  the  normal  defibrinated  blood,  treated  with  carbon  monoxid  so  as 
to  give  it  uniform  color,  is  then  diluted  with  water  until  its  tint  is 
identical  with  that  of  the  washings  similarly  treated  with  carbon 
monoxid.  From  the  quantity  of  water  necessary  to  dilute  the  blood 
the  quantity  of  blood  in  the  washings  is  readily  determined.  The 
animal  having  been  previously  weighed  and  the  weight  of  the  contents 
of  the  alimentary  canal  deducted,  the  ratio  of  the  total  weight  of  the 
blood  to  the  weight  of  the  body  at  once  becomes  apparent.  By  this 
method  it  has  been  shown  that  the  ratio  of  blood  to  body-weight  in  a 
human  adult  is  1:13;  in  an  infant,  1:19;  in  a  dog,  1:13;  in  a  cat, 
1:21.  Thus  an  adult  man  of  75  kilos  weight  would  have  5769  grams 
of  blood. 

The  amount  of  blood  in  the  different  organs  has  been  determined 
by  ligating  the  blood-vessels  in  the  living  animal,  removing  the  organ, 
and  after  allowing  the  blood  to  escape  subjecting  the  tissues  to  the 
chromometric  methods  described  above.  According  to  Ranke,  the 
volume  of  the  blood  is  distributed  as  follows:  Heart,  lungs,  arteries, 
and  veins,  \;  liver,  \\  muscles,  \;  other  organs,  -J. 

General  Composition. — The  results  of  the  analyses  of  the  blood 
will  vary  with  the  animal  and  the  methods  employed.  The  following 
table,  taken  from  Gad,  shows  the  average  composition,  expressed  in 
whole  numbers,  of  horse's  blood.  In  essential  respects  the  ratio  of 
the  constituents  in  human  blood  would  not  be  materially  different. 

One  thousand  parts  of  blood  contain: 

f  Water, 200 200 

Cells, 328  \                                        f  Hemoglobin,   116 

[  Solids, 128  <  Other  organic  matter, 10 

[  Salts, 2 

f  Water, 604 604 

plasma> 6?2  f  Sbumin;.' :::::.::::::::::::::::::  $1 

*-  Solids, 68  •{  .~ .,' 

]  Other  organic  matter, 3 

]  Potassium  and  sodium  salts, 4 

[  Calcium  and  magnesium  salts. 1 

CHEMISTRY  OF  COAGULATION. 

The  changes  which  eventuate  in  the  formation  of  fibrin,  and 
hence  all  the  subsequent  phenomena  of  coagulation,  are  chemic  in 
character;  but  as  these  changes  take  place  in  organic  compounds  the 
composition  of  which  is  but  imperfectly  known,  the  intimate  nature 
of  the  process  is  quite  obscure:  All  the  theories  which  have  been 
advanced  in  explanation,  though  approximating  the  truth,  are  more 
or  less  incomplete  and  in  some  respects  contradictory.  Since  the 
coagulation  is  coincident  with  the  appearance  of  the  fibrin,  the  ante- 
cedents of  this  substance,  the  physical  and  chemic  conditions  which 
condition  its  development,  and  the  succession  of  chemic  changes  in- 
volved must  be  determined,  before  any  consistent  theory  can  be 
established. 


268  TEXT-BOOK  OF  PHYSIOLOGY. 

Extra-vascular  Coagulation. — At  present  it  is  generally  be- 
lieved that  the  immediate  factors  concerned  in  extra-vascular  coagu- 
lation are  fibrinogen,  a  calcium  salt,  and  a  ferment-body.  As  to  the 
manner  in  which  these  three  bodies  react  one  with  another  there  is  a 
diversity  of  opinion.  At  least  five  different  theories  are  current  at 
the  present  time,  all  of  which  have  some  features  in  common,  though 
presenting  points  of  difference. 

Alexander  Schmidt  long  contended  that  fibrin  was  the  result  of 
a  union  of  fibrinogen  and  paraglobulin;  that  the  union  was  brought 
about  by  a  ferment-body;  that  the  presence  of  the  neutral  salts  of  the 
plasma  was  necessary  to  the  activity  of  the  ferment.  Previous  to  his 
death  in  1893  Schmidt  modified  his  view  as  follows:  The  insoluble 
fibrin  is  developed  out  of  a  soluble  fibrin  derived  from  paraglobulin, 
which  in  turn  is  a  product  of  general  cell  disintegration;  the  conver- 
sion of  the  fibrinogen  into  fibrin  is  due  to  the  activity  of  a  ferment, 
thrombin,  a  derivative  of  pro-thrombin,  a  product  of  the  disintegra- 
tion of  leukocytes,  lymph-cells,  etc.;  that  the  production  of  thrombin 
is  conditioned  by  the  presence  of  the  neutral  salts  of  the  plasma  in 
normal  percentages;  that  no  one  of  these  salts,  calcium  included,  acts 
in  a  specific  manner;  finally,  that  fibrin  is  not  a  compound  of  a  pro- 
teid  and  calcium. 

Hammersten,  as  a  result  of  many  years  of  investigation,  believes 
that  paraglobulin  is  not  necessary  to  the  process,  fibrinogen  alone 
being  transformed  into  fibrin  under  the  influence  of  the  ferment,  in  the 
presence  of  a  neutral  salt,  especially- calcium,  which  acts  specifically 
in  a  manner  different  from  the  sodium  salts.  Inasmuch  as  the  quan- 
tity of  fibrin  produced  is  always  less  than  the  quantity  of  fibrinogen 
previously  present,  Hammersten  concludes  that  the  latter  substance, 
under  the  influence  of  the  ferment,  undergoes  a  cleavage  into  two 
unequal  portions,  one  of  which  remains  in  solution,  the  other  solidify- 
ing as  fibrin.  While  admitting  that  the  calcium  salts  act  specifically, 
he  believes  that  they  are  concerned  rather  with  the  production  of  the 
ferment  than  the  fibrin,  for  if  the  ferment  is  present  in  sufficient 
quantity  coagulation  takes  place  in  a  typical  manner  even  in  the  total 
absence  of  calcium. 

Arthus  and  Pages  conclude  that  for  the  transformation  of  fibrin- 
ogen into  fibrin  the  calcium  salts  are  absolutely  essential  and  act  in  a 
specific  manner;  that  the  ferment  causes  a  cleavage  of  fibrinogen  into 
two  substances,  one  of  which  remains  in  solution,  the  other  com- 
bines with  calcium  to  form  fibrin.  They  offer  in  support  of  this 
view  the  fact  that  if  a  1  per  cent,  solution  of  potassium  oxalate  be 
added  to  blood  in  quantity  sufficient  to  precipitate  the  calcium,  coagu- 
lation will  not  take  place;  but  if  calcium  is  restored  coagulation  pro- 
ceeds in  the  usual  manner.  They  transfer  the  sphere  of  influence  of 
calcium  to  the  formation  of  the  fibrin  rather  than  to  the  formation 
of  the  ferment. 

Pekelharing's   researches   led   him   to   the   conclusion   that   there 


THE  BLOOD.  269 

arises  from  the  disintegration  of  the  leukocytes  a  nucleo-proteid, 
pro-thrombin,  which  combining  with  the  calcium  salt  forms  the 
ferment  thrombin.  This  compound  then  transfers  the  calcium  to 
the  fibrinogen,  which  in  turn  becomes  fibrin;  the  latter  is  therefore 
a  proteid-calcium  compound. 

Lilienfeld  asserts  that  fibrin  formation  is  a  cleavage  process  by 
which  fibrinogen  is  separated  into  two  bodies,  one  an  albumose  which 
remains  in  solution,  the  other  a  proteid  to  which  he  has  given  the 
name  thrombosin.  This  cleavage  is  attributed  to  the  action  of  the 
usual  ferment,  a  product  of  the  disintegration  of  leukocytes.  Throm- 
bosin combines,  according  to  Lilienfeld,  with  calcium  to  form  fibrin. 

In  a  critical  examination  of  these  different  theories  Hammersten 
denies  that  fibrin  is  a  compound  of  a  proteid  and  calcium;  for  chemic 
analysis  of  both  fibrinogen  and  fibrin  shows  that  the  former  contains 
as  much  calcium  as  the  latter,  and  that  therefore  the  view  of  coagu- 
lation according  to  which  fibrinogen  unites  with  calcium  to  form 
fibrin  is  without  foundation.  On  the  contrary,  he  maintains  that  the 
specific  influence  of  the  calcium  is  directed  toward  the  production  of 
the  ferment,  for  if  this  be  present  in  sufficient  quantity  coagulation 
takes  place  in  a  typical  manner,  no  matter  whether  the  blood  has 
been  decalcified  by  potassium  oxalate  or  not. 

Intra-vascular  Coagulation. — So  long  as  the  relations  of  the 
blood  and  the  vascular  system  remain  physiologic  no  coagulation 
occurs  in  the  vessels.  The  reason  assigned  for  this  is  that  the  fer- 
ment, though  continually  being  produced,  is  as  rapidly  being  de- 
stroyed, and  hence  never  accumulates  in  amount  sufficient  to  develop 
fibrin.  This  view  is  supported  by  the  fact  that  if  a  solution  of  cell- 
protoplasm,  leukocytes,  lymph-corpuscles,  etc.,  presumably  contain- 
ing a  large  amount  of  the  ferment,  be  injected  into  the  blood-vessels, 
extensive  intra-vascular  coagulation  promptly  follows.  It  is  also 
believed  that  the  lining  of  the  blood-vessel  in  some  unknown  way 
restrains  the  coagulation  process  even  though  the  circulation  has 
come  to  rest. 

Under  pathologic  conditions  of  the  circulatory  apparatus,  espe- 
cially of  the  internal  lining,  intra-vascular  coagulation  frequently 
arises,  though  the  process  can  not  be  considered  as  identical  with 
extra-vascular  coagulation.  Many  pathologists  assert  that  in  its 
origin,  mode  of  formation,  and  structure  the  intra-vascular  coagulum 
or  thrombus  is  not  a  true  coagulum  as  ordinarily  understood,  but 
rather  a  conglutination  of  blood-plaques  and  leukocytes.  Whenever 
the  integrity  of  the  internal  wall  of  the  vessel  is  impaired  by  disease 
or  by  the  introduction  of  foreign  bodies,  there  is  primarily  a  deposition 
and  accumulation  of  blood-plaques  at  the  injured  area  or  on  the 
foreign  body  which  constitutes  to  a  large  extent  the  mass  of  the  throm- 
bus which  at  once  forms.  The  thrombi  which  form  on  the  surface 
of  atheromatous  ulcers,  on  the  valves  of  the  heart,  and  in  the  veins 
in  consequence  of  diseased  states,  on  threads  or  needles  passed  through 


27o  TEXT-BOOK  OF  PHYSIOLOGY. 

the  vessels,  at  the  orifices  of  torn  blood-vessels,  consist  largely  of 
blood-plaques.  A  thrombus  so  formed  may  contain  a  number  of 
delicate  fibrin  threads,  which,  however,  present  a  different  appearance 
from  the  fibrin  of  the  extra-vascular  clot.  In  the  thrombi  which 
form  around  foreign  bodies  there  is  always  a  larger  quantity  of  fibrin 
than  in  those  originating  from  causes  wholly  within  the  vessel. 


CHAPTER   XIII. 
THE  CIRCULATION  OF  THE  BLOOD. 

Each  organ  and  tissue  of  the  body  is  the  seat  of  a  more  or  less 
active  metabolism,  the  maintenance  of  which  is  essential  to  its  physio- 
logic activity.  This  metabolism  is  characterized  by  the  assimilation 
of  food  materials  and  the  production  of  waste  products;  that  it  may 
be  maintained  it  is  imperative  that  there  shall  be  a  continuous  supply 
of  the  former  and  a  continuous  removal  of  the  latter.  Both  condi- 
tions are  subserved  by  the  blood.  In  order,  however,  that  this  fluid 
may  fulfil  these  functions  it  must  be  kept  in  continuous  movement, 
must  flow  into  and  out  of  the  tissues  in  volumes  varying  with  their 
activity,  under  a  given  pressure  and  with  a  certain  velocity. 

The  apparatus  by  which  these  results  are  attained  is  termed 
the  circulatory  apparatus.  This  consists  of  a  central  organ,  the 
heart;  a  series  of  branching  diverging  tubes,  the  arteries;  a  network 
of  minute  passageways  with  extremely  delicate  walls,  the  capillaries; 
a  series  of  converging  tubes,  the  veins.  These  structures  are  so 
arranged  as  to  form  a  closed  system  of  vessels  within  which  the  blood 
is  kept  in  continuous  movement  mainly  by  the  pressure  produced  by 
the. pumping  action  of  the  heart,  though  aided  by  other  forces.  (See 
Fig.  122.) 

In  this  system  a  particle  of  blood  which  passes  any  given  point 
will  eventually  return  to  the  same  point,  no  matter  how  intricate  or 
tortuous  the  route  may  be  through  which  it  in  the  meanwhile  travels; 
for  this  reason  the  blood  is  said  to  move  in  a  circle,  and  the  movement 
itself  is  termed  the  circulation. 

In  order  to  understand  the  reasons  for  the  movement  of  the  blood 
in  one  direction  only,  as  well  as  for  many  other  phenomena  connected 
with  the  circulation,  a  knowledge  of  the  structure  of  the  heart  and 
its  internal  mechanism  is  of  primary  importance. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  HEART. 

The  heart  is  a  cone  or  pyramid-shaped  hollow  muscular  organ 
situated  in  the  thorax  just  behind  the  sternum.  The  base  is  directed 
upward  and  to  the  right  side;  the  apex  downward  and  to  the  left  side, 
extending  as  far  as  the  space  between  the  cartilages  of  the  fifth  and 
sixth  ribs.  In  this  situation  the  heart  is  enclosed  and  suspended 
in  a  fibroserous  sac,  the  pericardium,  attached  to  the  great  vessels  at 
its  base. 

The  heart  is  a  hollow,  double  organ,  consisting  of  a  right  and  a 


272 


TEXT-BOOK  OF  PHYSIOLOGY. 


left  half,  separated  by  a  musculo-membranous  septum.     The  general 
cavity  of  each  side  is  subdivided  by  an  incomplete  transverse  fibrous 

septum  into  two  smaller  cavities,  an  upper 
and  a  lower,  known  respectively  as  the 
auricle  and  the  ventricle.  The  heart 
may  therefore  be  said  to  consist  of  four 
cavities,  the  walls  of  which  are  composed 
of  muscle-tissue.  Of  these  four  cavities, 
the  right  auricle  and  the  right  ventricle 
constitute  the  venous  heart;  the  left 
auricle  and  the  left  ventricle,  the  arterial 
heart. 

The  right  auricle  is  quadrangular  in 
shape  and  presents  on  its  posterior  aspect 
two  large  openings,  the  terminations  of 
the  two  final  trunks  of  the  venous  system, 
the  superior  and  inferior  vena  cava  (Fig. 
123).  Below,  the  auricle  communicates 
with  the  ventricle  by  a  large  opening 
which,  from  its  position,  is  termed  the 
auriculo-ventricular  opening.  The  walls 
of  the  auricle  are  extremely  thin,  not 
measuring  more  than  two  millimeters  in 
thickness. 

The  right  ventricle,  as  shown  on 
cross-section,  is  crescentic  in  shape  owing 
to  the  projection  of  the  ventricular  sep- 
tum. It  presents  at  its  upper  left  angle 
a  cone-shaped  prolongation,  the  conus 
arteriosus.  From  this  prolongation,  and 
continuous  with  it,  arises  the  pulmonary 
artery.  The  wall  of  the  ventricle  meas- 
ures in  the  middle  about  four  millimeters 
in  thickness.  The  inner  surfaces  of  the 
ventricle  show:  (1)  a  complicated  system 
of  muscle  ridges  and  bands,  the  columnce 
carnea  (fleshy  columns),  and  (2)  a  set  of 
muscle  projections,  the  musculi  papillares 
(papillary  muscles),  which  arise  by  a 
broad  base  from  the  walls  of  the  ventricle 
and  project  upward  toward  the  auriculo- 
ventricular  opening.  From  the  apex  of 
each  papillary  muscle  there  are  given  off 
fine  tendinous  cords,  the  chorda  tendinece, 
which  .become  attached  above  to  the 
under  surface  of  the  auriculo-ventricular  valve. 

The  left  auricle,  similar  in  general  shape  to  the  right,  presents 


Fig.   122. — Diagram  of  Cir- 
culation.    1.  Heart.     2.  Lungs. 

3.  Head    and    upper    extremities. 

4.  Spleen.  5.  Intestine.  6.  Kid- 
ney. 7.  Lower  extremities.  8.  Liver. 
— (Dalton.) 


THE  CIRCULATION  OF  THE  BLOOD. 


273 


posteriorly  four  openings,  the  terminations  of  the  four  final  trunks 
of  the  venous  system  of  the  lungs,  the  pulmonary  veins.  Below  is 
found  the  corresponding  auriculo-ventricular  opening.  The  wall 
of  the  auricle  measures  about  3  mm.  in  thickness.  The  left  ventricle 
(Fig.  124)  is  conic  in  shape  from  above  downward  and  oval  or  cir- 


Fig.  123. — The  Right  Auricle  and  Ventricle  Opened,  and  a  Part  of  Their 
Right  and  Anterior  Walls  Removed,  so  as  to  show  Their  Interior,  h — 1.  Supe- 
rior vena  cava.  2.  Inferior  vena  cava.  2'.  Hepatic  veins  cut  short.  3.  Right  auricle. 
3'.  Placed  in  the  fossa  ovalis,  below  which  is  the  Eustachian  valve.  3".  Is  placed  close 
to  the  aperture  of  the  coronary  vein.  +  +.  Placed  in  the  auriculo-ventricular  groove, 
where  a  narrow  portion  of  the  adjacent  walls  of  the  auricle  and  ventricle  has  been  preserved. 
4,  4.  Cavity  of  the  right  ventricle;  the  upper  figure  is  immediately  below  the  semilunar 
valves.  4'.  Large  columna  carnea  or  musculus  papillaris.  5,  5',  5".  Tricuspid  valve. 
6.  Placed  in  the  interior  of  the  pulmonary  artery,  a  part  of  the  anterior  wall  of  that  vessel 
having  been  removed,  and  a  narrow  portion  of  it  preserved  at  its  commencement,  where 
the  semilunar  valves  are  attached.  7.  Concavity  of  the  aortic  arch  close  to  the  cord  of 
the  ductus  arteriosus.  8.  Ascending  part  or  sinus  of  the  arch  covered  at  its  commence- 
ment by  the  auricular  appendix  and  pulmonary  artery.  9.  Placed  between  the  innom- 
inate and  left  carotid  arteries.  10.  Appendix  of  the  left  auricle.  11,  n.  The  outside  of 
the  left  ventricle,  the  lower  figure  near  the  apex. — (Alien  Thomson.) 

cular  in  shape  on  cross-section.  At  its  upper  right  angle  it  presents 
a  circular  orifice,  the  margins  of  which  give  attachment  to  the  walls 
of  the  aorta,  the  main  arterial  trunk  of  the  systemic  circulation.  The 
inner  surfaces  of  the  ventricle  show  a  similar  though  better  devel- 
oped system  of  columnae  carnea?,  musculi  papillares,  chorda?  tendinea?, 
18 


274 


TEXT-BOOK  OF  PHYSIOLOGY. 


etc.     The  wall  of  the  left  ventricle  measures  about  11.5  mm.  in  thick- 
ness in  the  middle. 

The  Endocardium. — The  cavities  of  both  the  right  and  left  sides 
of  the  heart  are  lined  by  a  thin  firm  connective-tissue  membrane, 


\  1  \  wn"1""""    wmmm^. 


■  ■■■■■  T  Jnw    *# 

l  ,tJ,*inP      :/ 


Fig.  124. — The  Left  Auricle  and  Ventricle  Opened  and  a  Part  of  Their 
Anterior  and  Left  Walls  Removed.  \. — The  pulmonary  artery  has  been  divided 
at  its  commencement;  the  opening  into  the  left  ventricle  is  carried  a  short  distance  into  the 
aorta  between  two  of  the  segments  of  the  semilunar  valves;  and  the  left  part  of  the  auricle 
with  its  appendix  has  been  removed.  The  right  auricle  is  out  of  view.  i.  The  two 
right  pulmonary  veins  cut  short;  their  openings  are  seen  within  the  auricle,  i'.  Placed 
within  the  cavity  of  the  auricle  on  the  left  side  of  the  septum  and  on  the  part  which  forms 
the  remains  of  the  valve  of  the  foramen  ovale,  of  which  the  crescentic  fold  is  seen  toward 
the  left  hand  of  i'.  2.  A  narrow  portion  of  the  wall  of  the  auricle  and  ventricle  preserved 
round  the  auriculo-ventricular  orifice.  3,  3'.  The  cut  surface  of  the  walls  of  the  ventricle, 
seen  to  become  very  much  thinner  toward  3",  at  the  apex.  4.  A  small  part  of  the  anterior 
wall  of  the  left  ventricle  which  has  been  preserved  with  the  principal  anterior  columna 
carnea  or  musculus  papillaris  attached  to  it.  5,  5.  Musculi  papillares.  5'.  The  left  side 
of  the  septum,  between  the  two  ventricles,  within  the  cavity  of  the  left  ventricle.  6,  6'. 
The  mitral  valve.  7.  Placed  in  the  interior  of  the  aorta,  near  its  commencement  and  above 
the  three  segments  of  its  semilunar  valve  which  are  hanging  loosely  together.  7'.  The 
exterior  of  the  great  aortic  sinus.  8.  The  root  of  the  pulmonary  artery  and  its  semilunar 
valves.  8'.  The  separated  portion  of  the  pulmonary  artery  remaining  attached  to  the 
aorta  by  9,  the  cord  of  the  ductus  arteriosus.  10.  The  arteries  rising  from  the  summit 
of  the  aortic  arch. — {Allen  Thomson?) 


THE  CIRCULATION  OF  THE  BLOOD. 


275 


closely  adherent  to  the  muscle-tissue,  termed  the  endocardium.  It 
also  contains  elastic  fibers  and  smooth  muscle-fibers.  Its  entire  surface 
is  covered  with  a  layer  of  polygonal  endothelial  cells.  This  membrane 
partially  serves  to  resist  undue  distention  of  the  heart  during  contrac- 
tion and  to  prevent  separation  of  the  muscle-fibers.  The  endocar- 
dium is  continuous  with  the  lining  membrane  of  the  blood-vessels. 

The  inter-auricular  septum  is  quite  thin  and  composed  of  the  two 
layers  of  the  endocardium  between  which  is  a  layer  of  muscle-fibers. 
It' presents  at  its  lower  portion  an  oval  depression,  the  fossa  ovalis. 

The  inter-ventricular  septum 
is  quite  thick  and  well  developed, 
and  composed  of  the  two  layers 
of  the  endocardium  enclosing  the 
muscle-fibers.  In  the  upper  and 
central  portion  of  the  septum, 
there  is,  however,  a  small  region 
which  is  thin  owing  to  the  ab- 
sence of  muscle-tissue  and  com- 
posed of  endocardium  only. 
This  region  is  known  as  the  pars 
membranacea  septi. 

The  Cardio- pulmonary 
Vessels. — Though  the  two  sides 
of  the  heart  are  separated  from 
each  other  by  the  auriculo-ven- 
tricular  septum,  they  are  anatom- 
ically and  physiologically  con- 
nected by  the  intermediation  of 
the  pulmonary  system  of  vessels: 
viz.,  the  pulmonary  artery,  capil- 
laries, and  veins  (Fig.  125). 

The  pulmonary  artery  arises 
from  the  conus  arteriosus  of  the 
right  ventricle.  After  a  short  upward  course  it  divides  into  a  right  and 
a  left  branch,  which  enter  corresponding  lungs.  The  vessel  at  once 
divides  and  subdivides  into  a  number  of  branches,  which,  after  fol- 
lowing the  bronchial  tubes  to  their  termination,  give  origin  to  capil- 
laries that  surround  the  air-cells  of  the  pulmonary  lobules. 

The  capillaries  in  this  situation  are  extremely  abundant  and  well 
developed.  They  lie  close  to  the  inner  surfaces  of  the  air-cells.  The 
blood  is  thus  brought  into  intimate  relationship  with  the  intra-pul- 
monary  air,  and  the  exchange  of  gases — the  excretion  of  carbon  dioxid 
and  the  absorption  of  oxygen — for  which  the  cardio-pulmonary  vessels 
exist,  is  readily  accomplished. 

The  pulmonary  veins  which  return  the  blood  to  the  heart  are 
formed  by  the  convergence  and  union  of  the  small  veins  which  emerge 
from  the  capillary  system.     The  final  trunks  thus  formed,  the  four 


Fig.  125. — Diagram  of  the  Cardio- 
pulmonary Vessels,  a.  Right  auricle,  b. 
Right  ventricle,  c.  Pulmonary  artery,  d. 
Lungs,  e.  Pulmonary  vein.  /.  Left  auricle. 
g.  Left  ventricle,  h.  Aorta,  i.  Vena  cava. 
—(Datton.) 


276 


TEXT-BOOK  OF  PHYSIOLOGY. 


pulmonary  veins- — two  from  each  lung — enter  the  posterior  wall  of 
the  left  auricle. 

The  Course  of  the  Blood  through  the  Heart.— There  is  thus 
established  a  pathway  between  the  venae  cavae  on  the  right  side  and  the 
aorta  on  the  left  side,  by  way  of  the  right  side  of  the  heart,  the  cardio- 
pulmonary vessels,  and  the  left  side  of  the  heart. 

The  venous  blood  flowing  toward  the  heart  is  emptied  by  the  supe- 
rior and  inferior  venae  cavae 
into  the  right  auricle,  from 
which  it  passes  through  the 
auriculo-ventricular  open- 
ing into  the  right  ventricle 
(Fig.  126);  thence  into  and 
through  the  pulmonary 
artery  and  its  branches  to 
the  pulmonary  capillaries, 
where  it  is  arterialized  by 
the  exchange  of  gases — the 
giving  up  of  a  portion  of 
carbon  dioxid  to  the  lungs 
and  the  absorption  of  oxy- 
gen— and  changed  in  color 
from  bluish-red  to  scarlet- 
red .  The  arterialized 
blood,  flowing  toward  the 
heart,  is  emptied  by  the 
pulmonary  veins  into  the 
left  auricle,  from  which  it 
passes  through  the  auriculo- 
ventricular  opening  into  the 
left  ventricle;  thence  into 
the  aorta  and  its  branches 
to  the  systemic  capillaries, 
where  it  is  de-arterialized 
by  a  second  but  opposite 
exchange  of  gases — the  giv- 
ing up  of  a  portion  of  its 
oxygen  to  the  tissues  and 
the  absorption  of  carbon  dioxid  from  the  tissues — and  changed  in  color 
from  scarlet  to  bluish-red.  The  venous  blood  is  again  returned  by 
the  systemic  veins  to  the  venae  cavae.  Though  the  blood  is  thus 
described  as  flowing  first  through  the  right  side  and  then  through  the 
left  side,  it  must  be  kept  in  mind  that  the  two  sides  fill  synchronously; 
that  while  the  blood  is  flowing  into  the  right  side  from  the  venae  cavae, 
it  is  also  flowing  from  the  pulmonary  veins  into  the  left  side  in  equal 
quantities  and  velocities. 

Though  there  is  but  one  set  of  capillaries,  as  a  rule,  between 


Fig.  126. — Diagram  of  Course  of  Blood 
through  the  Heart,  i,  2.  Superior  and  inferior 
venae  cavae,  3.  Right  auricle.  4.  Right  ventricle. 
5,  5,  5.  Pulmonary  artery  and  branches.  6,  6. 
Pulmonary  veins.  7.  Left  auricle.  8.  Left  ven- 
tricle. 9.  Aorta.  10.  Innominate  artery-  11. 
Left  carotid  artery.  12.  Left  subclavian  artery. — 
(After  Moral  and  Doyon.) 


THE  CIRCULATION  OF  THE  BLOOD. 


277 


arteries  and  veins,  there  is  an  exception  in  the  case  of  the  arteries  and 
veins  of  some  of  the  abdominal 
viscera.  Thus  the  veins  emerg- 
ing from  the  capillaries  of  the 
stomach,  intestines,  pancreas, 
and  spleen,  instead  of  passing 
directly  to  the  inferior  vena  cava, 
unite  to  form  a  large  vein — the 
portal  vein — which  enters  the 
liver.  In  this  organ  the  portal 
vein  divides  to  form  a  second 
capillary  system  which  is  in  close 
relation  to  the  liver  cells  and 
from  which  arise  the  veins  which 
unite  to  form  the  hepatic  veins. 
These  latter  vessels  empty  and 
discharge  the  blood  into  the  in- 
ferior vena  cava  just  below  the 
diaphragm. 

From     the     fore^oin^     facts  FlG'    127'~ RlGHT    Cavities    of    the 

rrom      me      I0reb0ino      IdClS      Heart      Auriculo-ventricular    valves    open. 

physiologists     frequently     divide       arterial  valves  closed.— (Dalton.) 

the  general  circulation  into: 

1.  The  pulmonary  circulation,  which  includes  the  course  of  the  blood 

from  the  right  side  of  the 
heart  through  the  lungs, 
to  the  left  side  of  the 
heart. 

2.  The  systemic  circulation, 
which  includes  the  course 
of  the  blood  from  the  left 
side  of  the  heart  through 
the  aorta  and  its  branches, 
through  the  capillaries 
and  veins  to  the  right  side 
of  the  heart. 

3.  The  portal  circulation, 
which  includes  the  course 
of  the  blood  from  the 
capillaries  of  the  stomach, 
intestines,  pancreas,  and 
spleen  through  the  portal 
vein  to  the  liver. 

Orifices  and  Valves. — 
The  movement  of  the  blood 
along  the  path  of  the  circle 
above 'outlined  is  accomplished  by  the  alternate  contraction  and  relax- 
ation of  the  muscle  walls  of  the  heart.     That  the  movement  mav  be  a 


Fig.  12S. — Right  Cavities  of  the  Heart. 
Auriculo-ventricular  valves  closed,  semilunar 
valves  open. — {Dalton.) 


!78 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  129. — Valves  of  the  Heart,  i. 
Right  auriculo-ventricular  orifice,  closed  by  the 
tricuspid  valve.  2.  Fibrinous  ring.  3.  Left 
auriculo-ventricular  orifice,  closed  by  the  mitral 
valve.  4.  Fibrinous  ring.  5.  Aortic  orifice  and 
valves.  6.  Pulmonic  orifice  and  valves.  7,  8, 
9.  Muscular  fibers. — (Bonamy  and  Beau.) 


progressive  one,  that  there  shall  be  no  regurgitation  during  either  the 
contraction  or  the  relaxation,  it  is  essential  that  some  of  the  orifices 
of  the  heart  be  closed  during  each  of  these  periods.  This  is  accom- 
plished by  the  heart  valves. 

The  right  auriculo-ven- 
tricular opening  is  surrounded 
and  strengthened  by  a  ring  of 
fibrous  tissue  to  which  is  at- 
tached a  membrane  partially 
subdivided  into  three  portions 
or  cusps,  which  during  the 
period  of  relaxation  are  di- 
rected into  the  ventricle  (Fig. 
127);  during  the  period  of 
contraction  they  are  raised 
and  placed  in  complete  appo- 
sition, when  they  act  as  a 
valve  preventing  a  backward 
flow  into  the  auricle  (Fig. 
128).  In  the  former  position 
the  valve  is  open;  in  the  latter, 
shut.  For  these  reasons  this 
structure  is  known  as  the  tri- 
cuspid valve.  This  valve  is  formed  of  fibrous  tissue  derived  from  the 
fibrous  ring,  some  muscle-fibers,  covered  over  by  a  reduplication  of 
the  endocardium.  To  the  under  surface  and  to  the  edges  of  this  valve 
the  tendinous  cords  of  the  papillary  muscles  are  firmly  and  intricately 
attached.  These  cords  are  just  sufficiently  long 
to  permit  closure  of  the  valve  and  to  prevent 
their  being  floated  into  the  auricle. 

The  orifice  of  the  pulmonary  artery  is  also 
surrounded  by  a  ring  of  fibrous  tissue  to  which 
are  attached  three  semilunar  or  pocket-shaped 
membranes,  the  semilunar  valves.  Each  valve 
is  formed  by  a  reduplication  of  the  endocardium 
strengthened  by  fibrous  tissue.  In  the  center  of 
the  free  edge  of  the  valve  there  is  a  small  nodule  of 
fibro-cartilage  (the  corpus  Aurantius) .  The  outer 
edge  of  the  valve  is  strengthened  by  a  delicate 
fibrous  band.  A  similar  band  strengthens  the 
convex  attached  portion  of  the  valve  just  where 
it  is  joined  to  the  fibrous  ring.     A  third  set  of 

fibers  pass  toward  the  nodule,  interlacing  in  all  directions.  Two 
narrow  crescentic-shaped  areas  (the  lunulae)  near  the  free  edge  are 
devoid  of  these  fibers.  During  the  period  of  relaxation  of  the  heart 
the  edges  of  the  valves  are  in  close  apposition  and  prevent  a  return 
of  the  blood  into  the  ventricle  (Fig.  127);  during  the  contraction  they 


Fig.  130.  —  Muscle- 
fibers  from  the 
Heart  of  a  Mammal. — 
(Landois  and  Stirling.) 


THE  CIRCULATION  OF  THE  BLOOD. 


79 


are  directed  into  the  artery  (Fig.  128).  In  the  former  position  they 
are  shut;  in  the  latter,  they  are  open. 

The  left  auriculo-ventricular  opening  is  provided  .with  a  similar 
though  better  developed  fibrous  ring  and  membranous  valve.  It  is, 
however,  subdivided  into  but  two  portions  or  cusps,  and  is  therefore 
termed  the  bicuspid  valve,  or,  from  its  fancied  resemblance  to  a  bishop's 
mitre,  the  mitral  valve.  The  general  arrangement,  connections, 
and  mode  of  action  of  this 
valve  are  similar  in  all  re- 
spects to  those  of  the  tri- 
cuspid valve.  The  orifice 
of  the  aorta  is  also  sur- 
rounded by  a  ring  of  fibrous 
tissue  to  which  are  attached 
three  semilunar  or  pocket- 
shaped  valves  (Fig.  124), 
which  in  their  arrangement, 
connections,  and  mode  of 
action  are  similar  in  all  re- 
spects to  those  at  the  orifice 
of  the  pulmonary  artery. 
The  anatomic  relations  of 
the  cardiac  orifices  one  to 
the  other  and  the  appear- 
ance presented  by  the  valves 
when  closed  are  represented 
in  Fig.  129. 

The  Heart  Muscle- 
fibers  and  Their  Arrange- 
ment.— The  muscle-fibers 
of  the  heart,  though  trans- 
versely striated  and  nu- 
cleated, differ  in  shape  and 
arrangement  from  those 
found  in  any  other  situa- 
tion. The  individual  fiber 
is    short    and    broad     and 

usually  divided  at  one  or  both  ends.  By  this  means  the  fibers  are 
united  not  only  longitudinally,  but  laterally.  (See  Fig.  130.)  The 
fibers  are  devoid  of  a  sarcolemma  and  united  one  to  the  other  by  a 
cement  material.  The  entire  musculature  is  permeated  and  supported 
by  connective  tissue  which  is  so  arranged  as  to  group  the  fibers  in 
bundles  or  fasciculi  of  varying  size. 

The  arrangement  of  the  muscle-bundles  is  quite  complicated  and 
in  accordance  with  the  functions  of  the  individual  portions  of  the 
heart.  In  the  auricles  the  bundles  are  arranged  in  two  sets:  an  outer 
transverse  set,  which  pass  from  auricle  to  auricle,  and  an  inner  longit- 


Fig.  131. — Muscle-fibers  of  the  Ventricles. 

1.  Superficial   fibers,    common   to   both   ventricles. 

2.  Fibers  of  the  left  ventricle.  3.  Deep  fibers, 
passing  upward  toward  the  base  of  the  heart.  4. 
Fibers  penetrating  the  left  ventricle. — {Sappcy,  after 
Bonamy  and  Beau.) 


28o  TEXT-BOOK  OF  PHYSIOLOGY. 

udinal  set,  which  pass  over  the  auricles  and  are  attached  anteriorly 
and  posteriorly  to  the  connective  tissue  of  the  transverse  auriculo- 
ventricular  septum.  The  longitudinal  fibers  of  each  auricle  are 
practically  independent  of  each  other.  Circularly  arranged  fibers  are 
present  near  the  terminations  of  the  venae  cavae  and  pulmonary  veins. 
In  the  ventricles  the  muscle-bundles  are  also  arranged  in  two 
sets,  a  superficial  longitudinal  and  a  deep  transverse,  though  their 
arrangement  is  somewhat  more  complicated  than  that  observed  in 
the  auricles.  In  a  general  way  it  may  be  said  that  the  superficial 
longitudinal  fibers  on  both  the  anterior  and  posterior  surfaces  take 
their  origin  in  the  connective  tissue  of  the  auriculo-ventricular  septum. 
The  superficial  fibers  on  the  anterior  surface  of  the  heart  pass  obliquely 
downward  and  forward  from  right  to  left  toward  the  apex,  where  they 
turn  backward  and  inward  in  a  vortex  manner  after  which  they  ascend 
to  terminate  in  the  wall  of  the  septum,  the  columnse  carneae  and  musculi 
papillares.  The  superficial  fibers  of  the  posterior  surface  of  the  heart 
pass  obliquely  downward  from  left  to  right,  wind  around  the  apex, 
turn  upward  and  end  in  the  same  structures  as  do  the  fibers  from 
the  anterior  surface.  The  fibers  from  the  base  of  the  right  ventricle 
terminate  in  the  structures  of  the  left  ventricle  while  those  from  the 
left  ventricle  terminate  in  the  structures  of  the  right  ventricle.  Longit- 
udinal fibers  are  also  found  on  the  inner  surface.  The  transverse 
fibers  are  very  abundant  and  surround  each  ventricle  separately  though 
they  are  continuous  with  each  other  across  the  septum.  Between  the 
superficial  longitudinal  and  deep  transverse  fibers  there  are  several 
layers  of  fibers  which  possess  varying  degrees  of  obliquity.  The 
general  arrangement  of  the  fibers  is  such  as  to  ensure  a  complete  and 
simultaneous  discharge  of  blood  from  both  auricles  and  ventricles 

(Fig-  131)- 

THE  MUSCLE  CONNECTION  BETWEEN  THE  AURICLES 
AND  VENTRICLES. 

The  Muscle  Band  of  His,  or  the  Auriculo-ventricular  Bundle. 

— In  the  mammalian  heart  there  is  no  continuity  of  the  muscle-fibers 
across  the  auriculo-ventricular  groove,  uniting  auricles  and  ventricles 
such  as  exists  in  the  frog  or  turtle  heart.  The  muscle-fibers  of  the 
auricles  and  ventricles  are  completely  separated  from  each  other  by 
the  transverse  fibrous  septum  to  which  they  are  attached.  This  fact 
has  for  a  long  time  made  it  difficult  to  understand  how  the  contraction 
process  which  begins  in  the  auricles  (to  which  there  will  be  occasion 
to  refer  in  subsequent  paragraphs)  is  conducted  to  the  ventricles. 
The  physiologic  necessity  for  the  existence  of  a  muscle  connection 
between  the  auricles  and  ventricles  led  to  a  series  of  investigations 
which  have  resulted  in  the  discovery  of  an  elaborate  system  of  muscle- 
fibers  by  which  they  are  united  both  anatomicly  and  physiologicly. 
In  1893  Wilhelm  His,  Jr.,  discovered  the  existence  of  a  band  or  bundle 


THE  CIRCULATION  OF  THE  BLOOD.  281 

of  muscle-fibers  which  apparently  took  its  origin  from  the  posterior 
part  of  the  right  side  of  the  auricular  septum,  and  from  which  point  it 
passed  forward  just  above  the  auriculo-ventricular  septum  to  a  point 
near  the  aortic  opening,  where  it  divided  into  two  portions,  a  right  and 
a  left,  of  which  the  latter  apparently  ended  in  the  basis  of  the  aortic 
leaflet  of  the  mitral  valve.  This  bundle  has  been  termed  "the 
muscle  band  of  His."  In  1904  Retzer  and  Braunig,  working  inde- 
pendently, corroborated  the  existence  of  this  bundle  and  described  its 
anatomic  course  more  completely.  The  investigations  of  Braunig 
led  to  the  conclusion  that  this  bundle  of  muscle-fibers  which  was 
constantly  present  in  all  animals  examined,  including  man,  began 
on  the  right  side  of  the  auricular  wall  below  the  fossa  ovalis  from 
which  point  it  passed  forward,  and  anteriorly  penetrated  the  auriculo- 
ventricular  septum  to  become  connected  with  the  musculature  of  the 
ventricular  septum  just  below  the  pars  membranacea  septi.  Though 
both  these  observers  state  that  the  bundle  divides  into  a  right  and  left 
limb  as  it  enters  the  ventricular  septum,  the  ultimate  distribution  and 
termination  of  these  limbs  was  not  clearly  determined.  Retzer 
estimated  that  this  bundle  was  18  mm.  long,  2.5  mm.  broad,  and  1.5 
mm.  thick.  By  these  investigators  this  bundle  was  termed  the 
"  auriculo-ventricular  bundle." 

In  1906  Tawara  published  the  results  of  an  extended  series  of 
investigations  made  on  the  embryonic  and  adult  hearts  of  many 
mammals  including  man,  which  resulted  in  a  further  increase  of 
knowledge  concerning  the  development,  anatomic  course  and  histo- 
logic features  of  this  bundle,  and  established  beyond  doubt  that  it  is 
the  pathway  along  which  the  contraction  process  is  conducted  from 
the  auricles  to  the  ventricles. 

A  brief  summary  of  Tawara's  account  of  this  bundle  is  as  follows: 
It  arises  near  the  opening  of  the  coronary  sinus  where  it  is  connected 
with  the  true  auricular  fibers.  It  then  passes  forward  on  the  right 
side  of  the  auricular  septum  between  the  lower  edge  of  the  fossa  ovalis 
and  the  auriculo-ventricular  septum;  just  above  the  insertion  of  the 
median  cusp  of  the  tricuspid  valve  the  bundle  presents  a  very  com- 
plicated network  of  muscle-fibers  which  has  been  designated  as  a  knot; 
from  the  anterior  portion  of  the  knot,  a  bundle  of  fibers  turns  down- 
ward and  penetrates  the  auriculo-ventricular  septum,  beyond  which 
it  passes  through  the  pars  membranacea  septi  to  the  upper  limit  of  the 
muscle  portion  of  the  ventricular  septum.  It  then  divides  into  two 
limbs  or  branches  which  descend  on  either  side  of  the  septum  under 
the  endocardium,  the  right  limb  lying  somewhat  deeper  than  the  left. 
Each  of  these  limbs  is  enclosed  by  a  layer  of  connective  tissue  which 
isolates  it  from  the  musculature  of  the  ventricular  septum  as  far 
as  the  lower  third  of  the  ventricular  cavities.  In  this  region  they 
divide  into  a  number  of  bundles  some  of  which  enter  the  papillary 
muscles,  while  others  forming  tendon-like  strands  branch  freely 
beneath  the  endocardium  and  spread  in  all  directions  over  the  entire 


282  TEXT-BOOK  OF  PHYSIOLOGY. 

inner  surface  of  the  ventricle  and  enter  into  histologic  connection  with 
the  true  cardiac  muscle-fibers. 

The  fibers  composing  this  system,  and  termed  by  Tawara  from  its 
supposed  function  the  "conduction  system"  are  histologicly  different 
from  the  cardiac  fibers,  insofar  as  they  are  poorer  in  sarcoplasm  and 
similar  in  their  appearance  to  embryonic  muscle-fibers.  In  the  auric- 
ular portion  of  the  bundle  the  fibers  exhibit  a  more  or  less  reticular 
arrangement;  in  the  ventricular  portion,  the  fibers  are  more  regularly 
arranged,  are  richer  in  sarcoplasm  and  present  a  number  of  fibrilla 
near  their  periphery. 

The  ultimate  termination  of  the  system,  beneath  the  endocardium, 
constitutes  the  so-called  Purkinje  fiber  layer.  In  the  sheep,  calf  and 
in  other  animals  these  fibers  are  abundant  and  readily  recognized; 
though  they  are  not  so  well  developed,  they  are  nevertheless  present 
and  extensively  distributed  in  the  human  heart. 

More  recently  the  connection,  in  the  human  heart,  between  the 
distal  extremities  of  the  two  limbs  of  the  muscle  bundle  and  the  sub- 
endocardial layer  of  Purkinje  fibers  has  been  questioned  by  Fahr,  who 
has  been  unable  to  verify  Tawara' s  statements  in  this  respect.  His 
findings  led  him  to  the  opinion  that  the  terminations  of  the  limbs 
merge  directly  without  any  division  into  the  musculature  of  the  ven- 
tricle. 

THE  MECHANICS  OF  THE  HEART. 

The  immediate  cause  of  the  movement  of  the  blood  through  the 
vessels  is  the  contraction  and  relaxation  of  the  muscle-walls  of  the 
heart,  and  more  particularly  of  the  walls  of  the  ventricles,  each  of 
which  plays  alternately  the  part  of  a  force-pump,  and  to  a  slight 
extent  of  a  suction-pump.  The  motive  power  is  furnished  by  the 
heart  itself,  by '  the  transformation  of  potential  energy,  stored  up 
during  the  period  of  rest,  into  kinetic  energy — i.  e.,  heat  and  mechanic 
motion. 

The  contraction  of  any  part  of  the  heart  is  termed  the  systole; 
the  relaxation,  the  diastole.  As  each  side  of  the  heart  has  two  cavities 
the  walls  of  which  contract  and  relax  in  succession,  it  is  customary  to 
speak  of  an  auricular  systole  and  diastole,  and  a  ventricular  systole 
and  diastole.  As  the  two  sides  of  the  heart  are  in  the  same  anatomic 
relation  to  each  other,  they  contract  and  relax  in  the  same  periods 
of  time. 

The  movements  of  the  heart,  as  well  as  many  phenomena  con- 
nected with  the  flow  of  blood  through  its  cavities,  have  been  deter- 
mined by  observation  of,  and  experiment  on,  the  exposed  heart  of  a 
mammal — e.  g.,  dog,  cat,  rabbit — supplemented  and  corrected  by 
experiments  on  the  heart  in  its  normal  relations.  Valuable  informa- 
tion as  to  the  heart-beat  and  the  influences  which  modify  it  has  been 
obtained  from  experiments  made  on  the  isolated  heart  of  the  turtle, 
frog,  and  allied  animals. 


THE  CIRCULATION  OF  THE  BLOOD.  283 

If  the  thorax  of  a  dog,  completely  anesthetized,  is  opened  and 
artificial  respiration  established,  the  heart  will  be  observed  in  active 
movement  inside  the  pericardium.  If  this  sac  is  divided  and  turned 
aside,  the  heart  will  be  fully  exposed  to  view.  At  the  normal  rate 
of  movement  characteristic  of  the  dog  it  will  be  almost  impossible 
to  determine  either  the  succession  of  events  or  their  duration.  But 
by  observing  the  heart  under  different  conditions  at  different  rates  of 
movement  and  with  instrumental  aids  physiologists  have  succeeded 
not  only  in  analyzing  the  movements,  but  in  describing  their  sequence 
and  in  estimating  their  time  duration. 

Thus  it  has  been  determined  that  the  heart  presents  two  distinct 
movements  which  alternate  with  each  other  in  quick  succession.  One 
is  the  movement  of  contraction,  or  the  systole,  by  which  the  blood 
contained  within  its -cavities  is  ejected  into  the  arteries — pulmonary 
artery  and  aorta;  the  other  is  the  movement  of  relaxation,  or  the  dias- 
tole, followed  by  a  pause  during  which  the  cavities  again  fill  up  with 
the  blood  from  the  venae  cava?  and  pulmonary  veins. 

Sequence  of  Events. — It  has  been  ascertained  that  the  contrac- 
tion of  the  auricles  and  ventricles  as  well  as  their  subsequent  relaxa- 
tions, though  occurring  with  extreme  rapidity,  do  not  take  place 
simultaneously  but  successively;  that  the  contraction  process  passes 
over  the  heart  in  the  form  of  a  wave;  that  it  begins,  indeed,  at 
the  terminations  of  the  great  veins,  then  passes  to  and  over  the 
auricles,  thence  to  and  over  the  ventricles  from  base  to  apex  with  great 
rapidity,  but  occupying  in  these  different  regions  unequal  periods  of 
time;  that  the  relaxation  immediately  succeeds  the  contraction,  in 
the  same  order,  and  that  at  the  close  of  the  ventricular  relaxation 
there  is  a  period  during  which  the  whole  heart  is  in  repose,  passively 
filling  with  blood. 

Changes  in  Position  and  Form. — In  passing  from  the  diastolic 
to  the  completed  systolic  condition  the  exposed  heart  undergoes  changes 
both  of  position  and  form  as  the  contraction  rises  to  its  maximum. 
This  having  been  attained,  the  heart  undergoes  reverse  changes  until 
the  original  diastolic  condition  is  regained.  Thus  at  the  time  of  the 
venticular  systole  the  apex  is  tilted  upward,  the  entire  heart  is  twisted 
on  its  axis  from  left  to  right  and  forced  downward  by  the  expansion 
and  elongation  of  the  pulmonary  artery  and  aorta.  At  the  time  of  the 
diastole,  the  reverse  movements  take  place. 

It  is  probable,  however,  that  these  movements  are  not  permitted  to 
the  same  extent  in  the  unopened  chest,  for  the  following  reasons: 
the  heart  is  enclosed  in  the  pericardium,  is  supported  posteriorly  by 
the  expanded  lungs,  and  both  posteriorly  and  inferiorly  by  the  dia- 
phragm, all  of  which  cooperate  in  keeping  the  heart,  and  more  particu- 
larly the  right  ventricle,  in  close  contact  with  the  chest-wall  and  limit- 
ing its  movements.  By  means  of  needles  inserted  into  the  apex  of  the 
heart,  through  the  chest-walls,  it  has  been  shown  by  their  slight  move- 
ment that  the  apex  is  practically  a  fixed  point. 


284  TEXT-BOOK  OF  PHYSIOLOGY. 

In  the  diastolic  condition  the  shape  of  the  heart  near  the  base  is 
elliptic  on  cross-section,  the  long  diameter  extending  from  side  to  side. 
In  the  completed  systolic  condition  the  shape  of  the  same  cross-section 
is  that  of  a  circle.  In  passing  from  the  diastolic  to  the  systolic  con- 
dition the  transverse  diameter  diminishes  while  the  antero-posterior 
diameter  increases,  while  the  whole  heart  becomes  somewhat  more 
conic  in  shape.  It  is  questionable  if  the  vertical  diameter  percept- 
ibly shortens.  During  the  systole  the  heart  hardens,  increases  in 
convexity,  and  is  more  forcibly  pressed  against  the  chest-wall.  As 
this  takes  place  suddenly,  it  gives  rise  to  a  marked  vibration  of  the  chest- 
wall,  known  as  the  cardiac  impulse.  This  is  principally  observed  in 
the  space  between  the  fourth  and  fifth  ribs,  between  the  left  edge  of 
the  sternum  and  a  line  drawn  vertically  through  the  nipple.  The  car- 
diac impulse  is  synchronous  with  the  cardiac  systole. 

The  cardiac  impulse  may  be  recorded  with  an  appropriate  appa- 
ratus known  as  a  cardiograph;  the  record  obtained  with  it  is  known 

as  a  cardiogram.  A  cardiograph  con- 
sists of  a  tambour  covered  with  a  thin 
rubber  membrane  provided  with  a 
button.  The  tambour  is  supported 
by  a  metallic  frame  which  permits  of 
an  easy  and  accurate  adjustment  of 
the  button  over  the  seat  of  the  cardiac 
impulse.  A  rubber  tube  connects  the 
Fig.  132.— a  Cardiogram.  cardiography  tambour  with  a  second 

tambour  provided  with  a  recording 
lever  and  thus  transmits  all  variations  in  the  pressure  of  the  air  in  the 
former  to  the  latter. 

When  all  adjustments  are  carefully  made  a  tracing  similar  to  that 
shown  in  Fig.  132  will  be  obtained  in  which  the  slight  elevation  a 
represents  the  contraction  of  the  auricles  which,  completing  the  filling 
of  the  ventricles,  causes  the  apex  of  the  heart  to  press  more  vigorously 
against  the  chest  wall;  b-c  represents  the  contraction  of  the  ventricles 
at  which  moment  the  apex  is  suddenly  and  forcibly  driven  against  the 
chest  wall;  c-d  represents  the  systolic  plateau,  the  time  during  which 
the  ventricle  is  discharging  blood  into  the  aorta;  d-e  represents  the 
relaxation  of  the  ventricle  while  e-j  represents  the  time  of  the  diastole 
during  which  the  heart  cavities  are  enlarging  with  the  incoming  of  a 
new  volume  of  blood  and  in  consequence  of  which  the  heart  is  press- 
ing against  the  chest  walls.  The  systolic  plateau  is  characterized 
by  one  or  more  elevations  and  depressions,  the  true  cause  of  which  is 
unknown. 

If  the  heart  sounds  are  listened  to  at  the  same  time  the  record 
is  being  taken  it  becomes  possible  to  indicate  by  means  of  an  electro- 
magnet signal  the  time  during  the  beat  that  they  occur.  While  it  is 
generally  admitted  that  the  first  sound  is  heard  at  b,  there  is  some 
difference  of  opinion  as  to  the  time  the  second  sound  is  heard.     Thus 


THE  CIRCULATION  OF  THE  BLOOD. 


285 


while  Landois  places  it  as  occurring  at  d,  Edgren  places  it  at  e,  while 
Marey  places  it  at  a  point  midway  between  d  and  e. 

The  Cardiac  Cycle. — The  entire  period  of  the  heart's  pulsation 
may  be  divided  into  three  phases,  viz. : 

1.  The  auricular  contraction. 

2.  The  ventricular  contraction. 

3.  The  pause  or  period  of  repose,  during  which  both  auricles    and 

ventricles  are  at  rest. 
These  three  phases  collectively  constitute  a  cardiac  cycle  or  a 
cardiac  revolution.  The  duration  of  a  cycle,  as  well  as  the  duration 
of  each  of  its  three  phases,  varies  in  different  animals  in  accordance 
with  the  number  of  cycles  which  recur  in  a  unit  of  time.  In  human 
beings  in  adult  life  there  are  about  72  cycles  to  the  minute;  the  average 
duration  therefore  is  0.83  second. 
From  this  it  follows  that  the  time 
occupied  by  any  one  of  the  three 
phases  must  be  extremely  short  and 
difficult  of  determination.  From 
observations  made  on  human  beings 
and  from  experiments  on  animals 
the  following  estimates  have  been 
made  and  accepted  as  approxi- 
mately correct: 

1.  The     auricular     systole,     0.16       soun 

second. 

2.  The    ventricular    systole,    0.32 

second. 

3 .  The  period  of  rest  for  both  auric- 

les and  ventricles,  0.32  second. 
The  relations  of  these  three  phases  to  one  another  may  be  illus- 
trated by  the  accompanying  diagram  (Fig.  133),  in  which  the  space  1-2  is 
the  duration  of  a  cardiac  cycle  divided  into  eight  equal  spaces,  each 
of  which  represents  one-tenth  of  a  second.  The  line  A  represents  the 
auricular,  the  line  V  the  ventricular  phase.  The  rise  in  the  line  A 
represents  the  contraction;  the  fall  and  subsequent  continuation,  the 
relaxation  and  pause.  The  rise  in  the  line  V  and  its  continuation 
represent  the  contraction;  the  fall  and  subsequent  continuation,  the 
relaxation  and  the  pause.  From  this  it  is  apparent  that  the  auric- 
ular contraction  or  systole  has  a  brief  duration,  0.16  second,  while 
the  relaxation  or  diastole  has  a  long  duration,  0.64  second;  that  the 
ventricular  contraction  immediately  following  the  auricular  has  a 
duration  of  0.32  second,  while  the  relaxation  and  diastole  have  a 
duration  of  0.48  second;  that  the  pause  of  the  entire  heart,  that  is,  the 
period  between  the  termination  of  the  ventricular  systole  and  the  be- 
ginning of  the  next  auricular  systole,  is  only  0.32  second. 

The  frequency  of  the  heart-beat  varies  with  a  variety  of  con 
ditions:  e.  g.,  age,  sex,  posture,  exercise,  etc. 


Fig.  133. — The  Phases  of  the  Heart's 
Pulsation. 


286  TEXT-BOOK  OF  PHYSIOLOGY. 

Age. — The  most  important  normal  condition  which  modifies  the 
activity  of  the  heart  is  age.     Thus: 

Before  birth,  the  number  of  beats  a  minute  averages 140 

During  the  first  year  it  diminishes  to 128 

During  the  third  year  it  diminishes  to 95 

From  the  eighth  to  the  fourteenth  year  it  averages 84 

In  adult  life  it  averages 72 

Sex. — The  heart-beat  is  more  rapid  in  females  than  in  males. 
Thus  while  the  average  beat  in  males  is  72,  in  females  it  is  usually 
8  or  10  beats  more. 

Posture. — Independent  of  muscle  efforts  the  rate  of  the  beat  is 
influenced  by  posture.  It  has  been  found  that  when  the  body  is 
changed  from  the  lying  to  the  sitting  and  to. the  standing  position, 
the  heart  will  vary  as  follows — from  66  to  71  to  81  on  the  average. 

Exercise  and  digestion  also  temporarily  increase  the  number  of  beats. 

A  rise  in  blood-pressure  from  any  cause  whatever  is  usually  attended 
by  a  decrease,  while  a  jail  in  blood-pressure  is  attended  by  an  increase 
in  the  rate. 

The  Action  of  the  Valves. — As  previously  stated,  the  forward 
movement  of  the  blood  is  permitted  and  regurgitation  prevented  by 
the  alternate  action  of  the  auriculo-ventricular  and  the  semilunar 
valves.  As  a  point  of  departure  for  a  consideration  of  the  action  of 
the  valves  and  their  relation  to  the  systole  and  diastole  of  the  heart, 
the  close  of  the  ventricular  systole  may  be  conveniently  selected. 

At  this  moment,  if  the  blood  is  not  to  be  returned  to  the  ventricles, 
the  semilunar  valves  must  be  instantly  and  completely  closed.  This 
is  accomplished  in  the  following  manner:  During  the  outflow  of  blood 
from  the  ventricles  the  valves  are  pushed  outward  toward  the  walls 
of  the  vessels,  though  not  coming  into  contact  with  them,  for  behind 
them  are  the  pouches  of  Valsalva,  containing  blood,  continuous  with 
and  under  the  same  pressure  as  that  in  the  vessels  themselves.  With 
the  cessation  of  the  outflow  and  the  beginning  of  the  relaxation  the 
pressure  of  the  blood  behind  the  valves  suddenly  forces  them  inward 
until  their  free  edges,  including  the  lunulas,  come  into  complete  appo- 
sition. By  this  means  the  orifices  of  the  pulmonary  artery  and  aorta 
are  securely  closed  and  a  return  flow  prevented.  Reversal  of  the 
valves  is  prevented  by  their  mode  of  attachment  to  the  fibrous  rings 
of  the  orifices. 

During  the  ventricular  systole  the  relaxed  auricles  have  been 
filling  with  blood.  With  the  ventricular  relaxation  this  volume,  or 
its  equivalent,  flows  readily  into  the  empty  and  easily  distensible 
ventricles,  its  place  being  taken  by  an  additional  volume  of  blood 
flowing  from  the  venae  cavae  and  pulmonary  veins.  Whether  the 
ventricles  exert  a  suction  power  at  the  moment  of  their  relaxation  is 
an  undecided  question.  A  steady  stream  of  blood  into  the  auricles 
and  ventricles  continues  throughout  the  entire  period  of  rest  until 
both   cavities  are  filled.     The  tricuspid  and  bicuspid  valves  which 


THE  CIRCULATION  OF  THE  BLOOD. 


287 


hang  down  into  the  ventricular  cavities  are  now  floated  up  by  cur- 
rents of  blood  welling  up  behind  them  until  they  are  nearly  closed. 
The  auricles  now  contract,  forcing  their  contained  volumes,  or  at 
least  the  larger  portions  of  them,  into  the  ventricles,  which  become 
fully  distended. 

With  the  cessation  of  the  auricular  systole  the  ventricular  systole 
begins.  If  the  blood  is  not  to  be  returned  to  the  auricles  at  this 
moment,  the  tricuspid  and  mitral  valves  must  be  suddenly  and  com- 
pletely closed.     This  is  readily  accomplished  by  reason  of  the  position 


S.a.-D.v 


D.a.-S.v 


Fig.  134.  —  Diagrammatic  Representation  of  the  Auricular  Systole,  S.a., 
with  the  Ventricular  Diastole,  D.v.,  and  of  the  Auricular  Diastole,  D.a.,  with 
the  Ventricular  Systole,  S.v.  C.s.  andC.i.  Superior  and  inferior  cavae;  A.d.  (atrium  dex- 
trum)  right  auricle;  A.s.  (atrium  sinistrum)  left  auricle;  V.d.  (ventriculus  dexter)  right  ven- 
tricle; Y.s.  (ventriculus  sinister)  left  ventricle;  P.  pulmonary  artery;  A.  aorta;  P.P.  papillary 
muscles. — (Landois.) 


of  the  valves,  which  have  been  floated  up  and  placed  almost  in  apposi- 
tion by  the  blood  itself.  With  the  beginning  of  the  ventricular  pressure 
the  blood  is  forced  upward  against  the  valves  until  their  free  edges 
are  brought  together  and  the  orifices  closed.  Reversal  of  these 
valves  is  prevented  by  the  contraction  and  shortening  of  the  papil- 
lary muscles,  which  in  consequence  exert  a  traction  on  their  under 
surfaces.  The  blood  now  confined  in  the  ventricle  between  the 
closed  auriculo-ventricular  and  semilunar  valves  is  subjected  to 
pressure  from  all  sides.  As  the  pressure  rises  proportionately  to  the 
vigor  of  the  contraction,  there  comes  a  moment  when  the  intra-vcn- 
tricular  pressure  exceeds  that   in  the  aorta  and  pulmonary  artery. 


288  TEXT-BOOK  OF  PHYSIOLOGY. 

Immediately  the  semilunar  valves  of  both  vessels  are  thrown  open  and 
the  blood  discharged.  Both  contraction  and  outflow  continue  until 
the  ventricles  are  practically  empty,  after  which  ventricular  relaxa- 
tion sets  in,  attended  by  a  rapid  fall  of  pressure.  Under  the  influence 
of  the  positive  pressure  of  the  blood  in  the  sinuses  of  Valsalva  the 
semilunar  valves  are  again  closed,  the  column  of  blood  supported, 
and  regurgitation  is  prevented.  With  the  accumulation  of  blood  in 
the  auricles  the  cardiac  cycle  is  completed. 

The  changes  in  the  shape  of  the  heart,  the  variations  in  the 
size  of  its  cavities  and  in  that  of  the  blood-vessels  arising  from  them, 
the  relative  position  of  the  valves  during  systole  and  diastole  are  shown 
in  Fig.  134. 

Relative  Functions  of  Auricles  and  Ventricles. — Though 
both  auricles  and  ventricles  are  essential  to  the  continuous  movement 
of  blood,  they  possess  unequal  values  in  this  respect.  The  passage 
of  the  blood  through  the  pulmonary  and  systemic  vessels  is  accom- 
plished by  the  driving  power  of  the  right  and  left  ventricles  respec- 
tively, aided,  however,  by  minor  extra-cardiac  forces.  They  may 
be  regarded  therefore  as  force-pumps. 

If  the  heart  consisted  of  ventricles  only,  the  flow  of  blood  from 
the  vense  cavse  and  pulmonary  veins  would  be  temporarily  arrested 
during  their  systole  and  their  subsequent  refilling  delayed.  This  is 
obviated,  however,  by  the  addition  of  the  auricles;  for  during  the 
ventricular  systole  the  blood  continues  to  flow  into  the  auricles,  in 
which  it  is  temporarily  stored  until  the  ventricular  relaxation  sets  in. 
With  this  event  the  accumulated  blood  passes  into  the  ventricles,  which 
are  thus  practically  filled  before  the  auricular  systole  occurs  by  which 
the  filling  is  completed.  By  this  means  there  is  no  delay  in  the  filling 
of  the  ventricles,  and  hence  their  effective  working  as  force-pumps 
is  more  readily  secured.  The  auricles  may  therefore  be  regarded 
as  feed-pumps.  For  this  reason  it  is  probable,  notwithstanding  the 
contraction  of  the  circular  muscle-fibers  at  the  terminations  of  the 
venous  system,  the  flow  of  blood  into  the  auricles  is  never  entirely 
arrested.  Regurgitation  in  these  vessels  does  not  occur  for  the  reason 
that  the  pressure  in  the  auricles  is  not  higher  than,  if  as  high  as,  in 
the  great  veins. 

Synchronism  of  the  Two  Sides  of  the  Heart. — If  the  balance 
of  the  circulation  is  to  be  maintained,  the  two  sides  of  the  heart  must 
act  synchronously.  That  they  do  so  can  be  shown  by  attaching 
levers  to  their  walls,  and  thus  recording  their  activities.  The  syn- 
chronism is  so  perfect  that  until  recently  it  was  generally  believed  to 
be  dependent  on  nerve  connections;  but  Porter  has  shown  that  if  the 
ventricles  are  cut  away  from  the  auricles,  in  which  the  nerve  mechan- 
ism seemed  to  lie,  the  synchronism  of  the  former  is  not  interfered 
with;  that  the  apical  halves  of  the  ventricles  will  beat  synchronously 
if  perfused  with  blood  through  an  artery;  that  a  very  small  bridge  of 
muscle-tissue  will  carry  the  wave  of  excitation  from  one  part  to  neigh- 


THE  CIRCULATION  OF  THE  BLOOD. 


to  manometer 


mm  valve 


boring  parts  of  the  ventricle.  It  is  therefore  probable  that  the  syn- 
chronism is  accomplished  through  muscle  connections  only.  The 
left  ventricle,  in  keeping  with  the  greater  work  it  has  to  do,  has  a 
greater  development  than  the  right,  and  therefore  contracts  more 
energetically.  The  ratio  between  the  energy  of  the  left  and  right 
sides  is  approximately  3  to  1. 

Intra-cardiac  Pressure. — It  has  been  stated  that  during  the 
pause  of  the  heart  when  its  cavities  are  rilling  with  blood  the  semilunar 
valves  are  kept  closed  by  the  pressure  of  the  blood  in  the  pulmonary 
artery  and  aorta,  a  pressure  clue  to  the  resistance,  as  will  be  explained 
later,  offered  to  the  flow  of  the  blood  mainly  by  the  smaller  arteries 
and  capillaries;  that  they  are  opened  only  when  the  pressure  of  the 
blood  within  the  ventricle  exceeds  that 
in  the  arteries.  It  becomes,  therefore, 
a  matter  of  importance  to  determine 
the  extent  of  this  pressure  as  well  as 
its  variations  during  the  course  of  a 
cardiac  cycle.  This  can  be  done  by 
inserting  a  long  catheter  into  either  the 
right  or  left  ventricle,  through  the 
jugular  vein  or  the  carotid  artery  re- 
spectively, and  connecting  its  free  ex- 
tremity with  a  mercurial  manometer. 
By  the  interposition  of  a  double  valve 
such  as  represented  in  Fig.  135,  it 
becomes  possible,  according  to  the 
direction  the  blood  is  permitted  to 
flow,  to  obtain  either  the  maximal  or 
the  minimal  pressure  that  occurs  in 
the  heart  during  a  series  of  cycles. 
Thus  Goltz  found  in  the  left  ventricle 
of  the  dog  a  maximal  pressure  of  114 
to   135    mm.;   in    the    right    ventricle, 

Minimal  pressures  of  — 2$  to  — 52  mm.  for  the  left  ventricle  have  also 
been  obtained. 

The  maximal  pressure  in  the  ventricles  during  the  systole,  though 
always  higher  than  that  in  the  arteries,  is  not  a  fixed  or  an  invariable 
pressure,  as  it  rises  and  falls  with  the  latter  from  moment  to  moment. 
Within  limits  the  cardiac  power,  and  therefore  the  intra-cardiac 
pressure,  is  capable  of  considerable  increase.  The  function  of  the 
heart  is  to  drive  the  blood  through  the  vessels  with  a  given  velocity. 
This  is  only  possible  by  first  overcoming  the  resistance  to  the  flow 
offered  by  the  vessels,  as  indicated  by  the  arterial  pressure.  As  this 
is  a  variable  factor,  rising  and  falling  very  considerably  at  times,  the 
heart  must  meet  and  exceed  each  rise,  if  the  circulation  is  to  be  main- 
tained. The  power  put  forth  by  the  heart  is  proportional  to  the 
work  it  has  to  perform.     If  the  arterial  pressure  continues  higher 

TQ 


to  heart 

Fig.  135. — v.  Frank's  Valve. 
This  is  placed  in  the  course  of  the 
tube  between  heart  and  manometer, 
so  that  the  latter  may  be  used  as  a 
maximum,  minimum,  or  ordinary 
manometer  according  to  the  tap  which 
is  left  open. — (Starling.) 

a   pressure   of    35  to  62  mm. 


290 


TEXT-BOOK  OF  PHYSIOLOGY. 


than  the  average  for  any  length  of  time,  the  heart  meets  the  condition 
by  an  hypertrophy  of  its  walls. 

The  Intra-ventricular  Pressure  Curve. — An  accurate  interpre- 
tation of  the  play  of  the  heart  mechanism  necessitates  the  obtaining  of 
a  graphic  record  of  the  course  of  the  intra-ventricular  pressure,  its  varia- 
tions and  time  relations.  With  such  a  record  may  be  compared  the 
records  of  the  pressures  in  the  venae  cava?,  on  the  one  hand,  and  in  the 
aorta,  on  the  other  hand,  and  their  relations  one  to  another  accurately 
defined. 

The  intra-ventricular  pressure  has  been  obtained  by  specially  de- 
vised manometers  or  tonometers  or  tonography,  as  they  are  variously 
termed,  the  construction  of  which  is  such  as  to  enable  them  to  respond 
instantly  to  the  very  rapid  variations  of  the  pressure  which  occur 
during  the  brief  cardiac  cycle.     One  of  the  best  is  that  of  Hiirthle. 

This  consists  of  a  small  metallic  tam- 
bour 5  or  6  millimeters  in  diameter, 
covered  by  a  thin  rubber  membrane. 
A  small  button  resting  on  the  mem- 
brane plays  against  an  elastic  steel 
spring,  by  the  tension  of  which  the 
pressure  of  the  blood  is  counterbal- 
anced. The  movements  of  the  mem- 
brane are  taken  up,  magnified,  and 
recorded  by  a  suitable  lever.  A  long 
cannula  is  inserted  into  the  right 
ventricle  through  the  jugular  vein  or 
into  the  left  ventricle  through  the 
carotid  artery.  Both  cannula  and 
tambour  are  filled  with  an  alkaline 
solution  to  prevent  coagulation  of  the 
blood,  and  then  joined  air-tight. 
The  pressure  of  the  blood  in  the  ventricle  is  thus  transmitted  by  a 
liquid  column  to  the  tambour  and  to  its  attached  lever.  With  such 
a  manometer  a  curve  is  registered  similar  to  that  shown  in  Fig.  136. 
To  obtain  the  absolute  value  of  this  curve  in  millimeters  of  mercury 
it  is  necessary  to  previously  graduate  the  instrument.  An  examination 
of  the  curve  shows  that  previous  to  the  ventricular  contraction  there 
is  a  very  slight  rise  of  pressure  above  that  of  the  atmosphere,  repre- 
sented by  the  line  a  --  b.  This  may  be  due  to  the  inflow  of  blood 
from  the  auricle  during  the  diastole.  At  o  the  pressure  suddenly 
rises,  passes  quickly  to  its  maximum  value,  (2),  which  is  maintained 
with  slight  variations  for  some  time,  and  then  suddenly  (3)  begins 
to  fall,  and  rapidly  reaches  the  line  of  atmospheric  pressure,  or  even 
passes  below  it,  becoming  negative  in  fact  for  a  short  period.  The 
curve  may  also  be  taken  as  a  record  of  the  ventricular  contraction, 
for  there  are  reasons  to  believe  that  the  two  closely  coincide  through- 
out their  entire  course.     A  characteristic  feature  of  this  curve  is  the 


Fig.  136. — V.  Curve  of  the  pres- 
sure in  the  ventricle  of  the  dog. — 
(Hiirthle.)  A.  Curve  of  the  pressure 
in  the  aorta.  The  curves  were  taken 
simultaneously,  s.  Tuning-fork  vi- 
brations, 100  per  second. 


THE  CIRCULATION  OF  THE  BLOOD.  291 

more  or  less  horizontal  portion  comprised  between  the  points  2  and  3, 
marked  by  several  elevations  and  depressions,  which  has  been  termed 
the  systolic  plateau. 

With  other  forms  of  elastic  manometers,  especially  those  in  which 
the  transmission  of  the  intra-ventricular  pressure  is  effected  by  air  or  by 
a  combination  of  air  and  liquid,  this  portion  of  the  curve  is  represented 
by  a  single  peak,  which  is  taken  as  an  indication  that  the  maximum 
pressure  once  reached  is  not  maintained,  but  immediately  begins  to 
fall  to  its  original  level,  notwithstanding  the  continued  contraction  of 
the  ventricle.  Those  who  adhere  to  this  view  attribute  the  plateau 
to  the  closure  of  the  orifice  of  the  catheter  by  the  contracting  and  ap- 
proximating walls  of  the  ventricle.  There  are  reasons  for  believing, 
however,  that  the  former  curve  is  the  more  correct  representation  of  the 
course  of  the  intra-ventricular  pressure.  Bayliss  and  Starling  photo- 
graphed on  a  moving  surface  the  oscillations  of  a  fluid,  a  solution  of 
sodium  sulphate,  in  a  capillary  glass  tube  one  end  of  which  was  closed, 
the  other  end  placed  in  connection  with  an  intra-cardiac  catheter,  the 
oscillations  representing  the  variations  in  pressure.  The  photogram 
thus  obtained  resembles  the  curve  obtained  by  Hurthle's  membrane 
manometer. 

The  Relation  of  the  Intra-ventricular  Pressure  Curve  to  the 
Intra-cardiac  Mechanisms. — By  itself  the  curve  of  the  intra-ventric- 
ular pressure  affords  no  indication  as  to  events  occurring  within  the 
heart:  i.  e.,  as  to  the  times  during  the  systole,  of  the  closure  of  the 
auricolo-ventricular  valves  and  the  opening  of  the  semilunar  valves, 
or  the  times  during  the  diastole,  of  the  closure  of  the  semilunar  valves 
and  the  opening  of  the  auriculo-ventricular  valves. 

By  registering  the  curve  of  pressure  in  the  aorta  simultaneously 
with  the  pressure  in  the  left  ventricle  (Fig.  136),  and  by  comparing 
these  with  the  curve  of  the  successive  differences  of  pressure  in  these 
two  cavities  as  determined  by  the  "differential  manometer,"  it  be- 
comes possible  to  mark  on  the  ventricular  pressure  curve  the  points 
at  which  the  foregoing  events  take  place. 

During  the  systolic  plateau  the  blood  is  passing  from  the  ventricle 
into  the  aorta.  Independent  of  the  slight  elevations  and  depressions 
there  is  an  absolute  fall  of  pressure  between  the  beginning  and  the 
end  of  the  plateau.  There  is  also  a  corresponding  fall  in  the  aortic- 
pressure,  corresponding  to  these  two  points.  The  curve  of  the  dif- 
ference of  pressure  shows,  however,  that  the  ventricular  pressure  is 
slightly  higher  than  the  aortic.  This  fall  in  both  ventricular  and 
aortic  pressures  is  due  to  the  escape  of  blood  from  the  arterial  into 
and  through  the  capillary  system.  At  3,  however,  whether  completely 
emptied  or  not,  the  ventricle  suddenly  relaxes,  and  its  pressure  soon 
falls  below  that  in  the  aorta.  As  soon  as  this  takes  place  the  semi- 
lunar valves  must  close,  if  regurgitation  into  the  ventricular  cavity 
is  to  be  prevented.  A  comparison  of  the  aortic  pressure  curve  shows 
a  slight  notch,  the  "dicrotic  notch,"  just  preceding  a  slight  elevation, 


292  TEXT-BOOK  OF  PHYSIOLOGY. 

the  "dicrotic"  wave.  This  notch  is  taken  as  the  moment  when  the 
semilunar  valves  close.  The  corresponding  point  on  the  ventricular 
pressure  curve  has  been  placed  just  where  the  ordinate  4  cuts  the  de- 
scending portion.  As  yet,  however,  the  pressure  is  higher  in  the  ven- 
tricle than  in  the  auricle,  and  so  continues  until  near  the  line  of  atmos- 
pheric pressure.  At  this  point  the  pressure  in  the  auricle,  due  to  the 
accumulation  of  blood  during  the  ventricular  systole,  now  forces  open 
the  mitral  valve  and  the  blood  flows  into  the  ventricle.  The  opening 
of  the  mitral  valve  occurs  about  the  point  where  the  ordinate  5  cuts  the 
curve. 

The  ventricular  pressure  curve  affords  but  slight,  if  any,  indication 
of  the  auricular  systole.  It  apparently  does  not  give  rise  to  any 
noticeable  increase  in  the  ventricular  pressure.  The  slight  rise  in 
the  pressure  curve,  which  just  precedes  the  abrupt  rise  due  to  the 
ventricular  systole,  may  be  taken  as  an  indication  of  an  increasing 
pressure  due  to  the  inflow  of  blood  from  the  auricle.  As  soon  as  the 
pressure  in  the  ventricle  exceeds  that  in  the  auricle  the  mitral  valve 
closes.  This  is  marked  on  the  curve  where  the  ordinate  cuts  it,  at  o. 
Coincident  with  this,  the  ventricular  systole  begins,  and  as  the  blood  is 
contained  within  a  closed  cavity  the  pressure  abruptly  rises.  A 
comparison  of  the  aortic  curve  shows  that  for  a  short  time  during 
the  ventricular  systole,  the  pressure  is  falling,  but  at  one  point  it  turns  at 
a  sharp  angle  and  rapidly  rises.  This  is  an  indication  that  the  semi- 
lunar valves  are  suddenly  thrown  open  and  the  blood  begins  to  pass 
into  the  aorta.  This  event  occurs  at  a  moment  marked  on  the  ventric- 
ular curve  by  the  ordinate  1.  Beyond  this  point  the  pressure  continues 
to  rise,  for  the  aortic  pressure  must  not  only  be  exceeded,  but  a  certain 
velocity  must  be  imparted  to  the  blood.  Between  the  ordinates  1 
and  4,  the  semilunar  valves  remain  open  and  the  blood  passes  into 
the  aorta. 

In  accordance  with  the  foregoing 
The  ventricular  systole  may  be  subdivided  into  two  periods: 

1.  The  period  of  rising  tension,  from  the  beginning  of  the  systole  to 

the   opening  of  the   semilunar  valves,   occupying  from  0.02   to 
0.04  second. 

2.  The  period  of  ejection,  from  the  opening  of  the  semilunar  valves 

to  the  end  of  the  systole,  occupying  about  0.2  second. 
The  ventricular  diastole  may  also  be  divided  into  two  periods: 

1.  The  period  of   falling  tension  or  relaxation,  from  the  end  of  the 

systole  to  the  time  of  lowest  pressure  in  the  ventricle,  occupying 
about  0.05  second. 

2.  The  period  of  filling,  from  the  opening  of  the  mitral  valve  to  the 

beginning  of  the  systole. 
Negative  Pressure. — As  shown  by  the  ventricular  pressure  curve 
there  is  a  moment   when  the  pressure  falls  below  atmospheric  pres- 
sure, becoming  negative  to  it.     The  extent  to  which  this  takes  place, 
its  duration  and  frequency,  have  never  been  satisfactorily  determined. 


THE  CIRCULATION  OF  THE  BLOOD.  293 

The  cause  of  the  negative  pressure,  its  influence  on  the  opening  of 
the  auriculo-ventricular  valves,  and  on  the  entrance  of  blood  into  the 
ventricles  are  equally  unknown.  The  most  probable  cause  is  an 
expansion  of  the  base  of  the  ventricles  due  to  the  enlargement  of  the 
aorta  and  pulmonary  artery.  That  it  is  not  due  to  the  expansion 
of  the  thorax  is  evident  from  the  fact  that  it  occurs  when  the  thorax 
is  open  and  the  heart  exposed. 

Heart-sounds. — Two  sounds  accompany  each  pulsation  of  the 
heart,  both  of  which  may  be  heard  by  applying  the  ear  or  the  stetho- 
scope to  the  chest-walls,  especially  over  the  region  of  the  heart.  One 
of  these  sounds  is  low  in  pitch,  dull  and  prolonged;  the  other  is  high 
in  pitch,  clear  and  short.  These  sounds  can  be  approximately  repro- 
duced by  pronouncing  the  syllables  lubb-dupp,  lubb-dupp.  The 
long  dull  sound  occurs  with  the  systole,  the  first  phase  of  a  new  cardiac 
cycle,  and  is  therefore  termed  the  -first  sound;  the  short  clear  sound 
occurs  at  the  beginning  of  the  diastole,  with  the  second  phase  of  the 
cardiac  cycle,  and  is  therefore  termed  the  second  sound.  The  first 
sound  is  the  systolic,  the  second  the  diastolic,  sound.  With  the  ear 
it  can  readily  be  determined  that  there  is  a  brief  pause  between  the 
first  and  second  sounds,  and  a  longer  pause  between  the  second  and  the 
first  sounds.  The  duration  of  the  first  sound  is  almost  equal  to  the 
duration  of  the  systole — viz.,  0.3  second;  the  duration  of  the  second 
sound  is  not  more  than  0.1  second.  The  systolic  sound  is  heard 
most  distinctly  over  the  body  of  the  heart;  the  diastolic  sound  is  heard 
most  distinctly  in  the  neighborhood  of  the  third  rib  to  the  right  of  the 
sternum. 

The  causes  of  the  heart-sounds  have  enlisted  the  attention  of 
clinicians  and  physiologists  for  years,  and  many  factors  have  been 
assigned  for  their  production.  At  present  it  is  generally  believed 
that  the  first  sound  is  the  product  of  at  least  two,  possibly  three,  factors : 
viz.,  the  contraction  of  the  muscular  walls  of  the  ventricles,  the  simul- 
taneous closure  and  subsequent  vibration  of  the  tricuspid  and  mitral 
valves,  and  the  sudden  increase  of  pressure  of  the  apex  of  the  heart 
against  the  chest-wall. 

That  the  contraction  of  the  ventricular  muscle  gives  rise  to  a  sound 
is  certain  from  the  fact  that  it  is  perceptible  in  an  excised  heart  when 
the  cavities  are  free  from  blood  and  when  the  valves  are  prevented 
from  closing.  The  explanation  of  this  sound  is  extremely  difficult,  as 
the  contraction,  though  prolonged,  is  not  of  the  nature  of  a  tetanus  and 
therefore  not  characterized  by  rapid  variations  of  tension.  The  apex 
element  may  be  eliminated  by  placing  the  individual  in  the  recumbent 
position. 

The  second  sound  is  the  product  of  the  simultaneous  closure  and 
subsequent  vibration  of  the  aortic  and  pulmonary  valves  which  occurs 
at  the  beginning  of  the  ventricular  diastole  as  the  blood  surges  back 
against  the  closed  valves.  This  has  been  definitely  proved  by  the  fact 
that  the  sound  disappears  when  the  valves  are  destroyed  or  held  back 


294  TEXT-BOOK  OF  PHYSIOLOGY. 

by  hooks  introduced  into  the  aorta  and  pulmonary  artery.  It  is  also 
possible  that  the  vibration  of  the  column  of  blood  produces  an  addi- 
tional tone  which  adds  itself  to  that  produced  by  the  valves. 

The  relation  of  the  sounds  to  the  systole  and  diastole  of  the  heart 
is  represented  in  Figs.  133  and  137. 

The  Blood-supply  to  the  Heart. — The  nutrition  of  the  heart, 
its  contractility,  the  force  and  frequency  of  the  beat,  are  dependent 
on  and  maintained  by  the  introduction  of  arterialized  blood  into  and 
the  removal  of  waste  products  from  its  tissue.  This  is  accomplished 
by  the  coronary  arteries,  on  the  one  hand,  and  the  coronary  veins,  on 
the  other.  The  arteries,  two  in  number,  the  right  and  left,  arise  from 
the  aorta  in  the  pouches  of  Valsalva  just  above  the  right  and  left 
.  semilunar  valves.  Turning  in  opposite  direc- 
tions, they  ultimately  anastomose,  forming  a 
circle  around  the  base  of  the  ventricles.  From 
both  the  right  and  left  artery  branches  are 
given  off  which  run  over  the  walls  of  both 
auricles  and  ventricles,  the  most  important  of 
which  in  man  are  the  anterior  and  posterior 
inter-ventricular.  These  main  vessels  lie  in 
grooves  on  the  surface  of  the  heart  beneath 
Fig.  137.  — Scheme  of  fae  visceral  pericardium,  surrounded  by  con- 
fnne^Sd'  S  Jhat  active  tissue  and  fat.  Small  branches  pene- 
events  occur  in  the  heart,  trate  the  heart-muscle  in  which  they  divide 
and  the  outer,  the  relation     mt0    capillaries.       From  the  capillary  areas 

of  the  sounds  and  snences  n         •  •  i  •   i  •  1        1  1 

to  these  events.  small  veins  arise  which,  passing   backward, 

converge  to  form  the  coronary  veins.  These 
follow  the  course  of  the  arteries  and  finally  terminate  in  the  coronary 
sinus,  located  in  the  auriculo-ventricular  groove  on  the  posterior 
surface  of  the  heart.  This  sinus  opens  into  the  right  auricle  between 
the  opening  of  the  inferior  vena  cava  and  the  auriculo-ventricular 
opening.  Its  orifice  is  guarded  by  a  valve,  which  is  usually  single, 
though  sometimes  double. 

While  by  far  the  larger  portion  of  the  blood  is  returned  by  the 
coronary  veins,  it  is  also  certain  that  some  of  it  is  returned  by  small 
veins  which  open  into  little  pits  or  depressions  on  the  inner  surface  of 
the  heart-walls,  known  as  the  foramina  Thebesii.  It  has,  however,  been 
shown  by  Pratt  that  these  foramina  are  present  not  only  in  the  auricular 
wall,  as  generally  stated,  but  in  the  walls  of  all  the  cavities.  These 
foramina  communicate  through  a  capillary  plexus  with  both  arteries 
and  veins,  and  by  special  large  passages  with  the  veins  alone. 

The  period  of  time  in  the  cardiac  cycle  during  which  the  coronary 
(the  extra-mural)  arteries  are  filled  with  blood,  whether  during  the 
systole  or  the  diastole,  has  been  a  subject  of  much  discussion.  At 
present,  however,  as  the  result  of  many  experiments  it  is  generally 
believed  that  they  are  filled  at  the  time  of  the  systole.  A  comparison 
of  the  tracings  of  the  pulse-wave  taken  simultaneously  in  the  carotid 


THE  CIRCULATION  OF  THE  BLOOD.  295 

and  coronary  arteries  shows  that  the  pressure  rises  and  falls  simul- 
taneously in  both  vessels;  that  there  is  a  complete  agreement  between 
the  two  tracings,  and  as  a  corollary  both  vessels  are  filled  during  the 
systole.  But  because  of  the  pressure  which  the  heart-muscle  must 
exert  upon  the  smaller  arteries  and  veins  within  its  own  substance 
during  systole,  it  is  probable  that  there  is  a  freer  circulation  in  the 
coronary  (the  extra-mural)  vessels,  during  the  period  of  diastolic 
repose. 

During  the  systole  the  intra-mural  vessels  are  compressed  and  the 
blood  driven  out  of  the  capillaries  into  the  veins;  during  the  diastole, 
these  vessels  again  dilate  and  permit  the  blood  to  re-enter  freely  from 
the  extra-mural  arteries.  The  greater  the  force  and  frequency  of 
the  beat,  the  greater  the  volume  of  blood  passing  through  the  coronary 
system. 

As  stated  in  a  foregoing  paragraph  the  nutrition  of  the  heart- 
muscle,  its  irritability  and  contractility,  depend  on  the  blood-supply 
derived  from  the  coronary  vessels.  This  is  shown  by  the  effects  which 
follow  its  withdrawal.  Ligation  of  both  coronary  arteries  in  the  dog  is 
followed  by  a  diminution  in  the  force  and  frequency  of  the  heart-beat, 
and  in  a  few  minutes  by  complete  cessation.  Ligation  of  even  a  single 
branch  of  a  coronary  artery  of  the  dog  heart  provided  it  supply  a 
sufficiently  large  territory — e.  g.,  the  arteria  circumflexa — is  sufficient 
to  cause  arrest  in  at  least  80  per  cent,  of  animals  (Porter).  With  the 
ligation  of  this  vessel  there  occurs  a  gradual  diminution  in  the  force  and 
frequency  of  the  systole.  As  the  power  of  coordinate  contraction  ceases 
the  heart-muscle  frequently  exhibits  a  series  of  independent  contraction 
of  individual  fibers  and  cells  known  as  fibrillary  contraction.  All  the 
results  which  follow  ligation  are  to  be  attributed  in  the  light  of  experi- 
ment to  the  sudden  anemia  which  is  thus  established.  The  removal 
of  the  ligature  and  the  return  of  the  blood  will  restore  the  nutrition  and 
re-establish  coordinate  contractions.  The  excised  heart  of  the  mammal 
which  has  passed  into  the  condition  of  fibrillary  contraction  may  be 
again  made  to  beat  rhythmically  and  vigorously,  by  first  cooling  it 
with  normal  saline,  and  then  perfusing  it  with  warm  defibrinated  blood 
through  the  coronary  vessels  under  a  suitable  pressure.  The  same 
result  can  be  brought  about  by  first  perfusing  it  with  a  r  per  cent, 
solution  of  potassium  chlorid  until  the  heart  comes  to  rest  and  then 
perfusing  it  with  Ringer's  solution. 

In  frogs  and  allied  animals  the  heart  is  nourished  by  blood  flow- 
ing, during  the  diastole,  from  the  interior  of  the  heart  into  a  system 
of  irregular  channels  which  penetrate  the  walls  in  all  directions. 
With  the  systole  the  blood  is  returned  to  the  cavities.  The  excised 
heart  of  the  mammal — e.  g.,  the  cat — may  be  partially  nourished  in 
a  similar  manner  through  the  foramina  Thebesii.  If  the  warm 
defibrinated  blood  of  the  same  animal  be  introduced  into  the  ventricle 
under  a  pressure  of  about  75  mm.  of  blood,  the  heart  will  recommence 
and  continue  to  beat  for  a  period  varying  from  one  to  several  hours. 


296  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Beat  of  the  Excised  Heart.— The  beat  of  the  heart,  its 
frequency  and  regularity,  its  continuance  from  the  early  stages  of 
fetal  development  till  death,  has  long  been  an  interesting  subject 
for  physiologic  investigation.  Though  related  to  the  functional 
activities  of  the  body  at  large,  the  activity  of  the  heart  is  in  a  sense 
independent  of  them,  for  it  will  continue  for  a  variable  length  of  time 
after  they  have  ceased.  The  heart  of  the  frog  and  the  turtle  will 
continue  to  beat  under  appropriate  conditions  for  hours  after  separa- 
tion of  all  its  anatomic  connections  and  removal  from  the  body. 
The  heart  of  the  dog  or  cat  will,  however,  beat  but  for  a  few  minutes. 
The  human  heart  would  in  all  probability  act  in  the  same  way.  Never- 
theless there  are  good  reasons  for  believing  that  though  the  spontaneous 
beat  has  ceased,  the  irritability  yet  endures  though  perhaps  in  lessened 
degree.  For  if,  after  the  heart  has  ceased  to  beat  for  some  time,  warm 
defibrinated  and  oxygenated  blood  be  passed  through  the  coronary 
vessels  the  beat  will  reappear  and  continue  at  its  usual  rate  for  some 
hours. 

The  reason  for  the  longer  continuance  of  the  beat  of  the  excised 
heart  of  the  cold-blooded  animal  beyond  that  of  the  warm-blooded 
animal  lies  probably  in  the  difference  in  the  rate  of  their  respective 
metabolisms.  There  is  reason  to  believe  that  each  cejl  of  the  heart- 
muscle,  in  common  with  other  tissue-cells,  during  life  stores  up  and 
holds  in  reserve  a  larger  quantity  of  nutritive  material  than  is  necessary 
for  its  immediate  needs.  When  separated  from  the  general  blood- 
supply,  the  cells  begin  to  utilize  this  reserved  material.  With  its 
consumption  the  irritability  declines  and  after  a  variable  period  of  time 
disappears.  As  the  metabolism  is  far  more  rapid  in  the  warm-blooded 
than  in  the  cold-blooded  animal,  it  is  probable  that  the  reserved 
nutritive  material  is  utilized  more  quickly  in  the  former  than  in  the 
latter  other  conditions  being  equal.  So  long  as  it  lasts  in  either  class, 
the  irritability  and  contractility  persist. 

Whatever  the  immediate  or  exciting  cause  of  the  heart  contraction 
may  be,  the  fundamental  condition  for  its  manifestation  is  the  main- 
tenance of  the  irritability.  So  long  as  this  persists  at  a  sufficiently 
high  level  the  heart-muscle  will  contract  in  response  to  the  appropriate 
stimulus. 

PROPERTIES  OF  THE  HEART-MUSCLE. 

The  heart-muscle  is  characterized  by  the  following  properties,  viz., 
irritability,  conductivity,  rhythmicity,  tonicity,  and  automaticity. 
r.  Irritability. — The  heart-muscle  in  common  with  other  muscles 
possesses  irritability  by  virtue  of  which  it  responds  by  a  change 
of  form  to  the  action  of  a  stimulus.  Whatever  the  stimulus, 
here,  as  elsewhere,  there  is  a  conversion  of  potential  into  kinetic 
energy — heat,  electricity  and  mechanic  motion.  The  normal 
physiologic    stimulus    has    not  been  positively  determined.       In 


THE  CIRCULATION  OF  THE  BLOOD.  297 

common  with  other  forms  of  muscle-tissue,  the  heart-muscle 
may  be  made  to  contract  by  artificial  stimuli — e.  g.,  mechanic, 
thermic,  chemic,  and  electric. 

For  the  demonstration  of  this  fact  it  is  necessary  to  eliminate  the 
action  of  the  physiologic  stimulus  and  to  bring  the  heart  to  rest 
in  the  condition  of  diastole.  This  can  be  done  with  the  frog 
heart,  by  ligating  the  tissues  between  the  sinu-auricular  junction, 
a  procedure  which  prevents  the  passage  of  the  contraction  wave 
which  originates  in  the  sinus,  over  the  auricles  and  ventricles 
(a  fact  that  will  be  more  fully  alluded  to  later).  With  the  heart 
thus  prepared  and  while  still  in  situ,  the  apex  may  be  connected 
with  a  recording  lever  and  its  provoked  contractions  registered 
on  a  recording  surface.  In  this  condition  it  will  respond  by  a 
contraction  to  any  form  of  an  adequate  stimulus  and  especially 
to  the  induced  electric  current.  Experiment  has  shown  that  the 
irritability  is  most  marked  in  the  neighborhood  of  the  venae  cava? 
terminations.     It  is  least  marked  in  the  ventricles. 

In  its  irritability,  contractility,  and  manner  of  response  to 
stimuli,  the  heart  of  the  mammal  corresponds  in  all  essential 
respects  to  the  heart  of  the  frog  or  turtle.  • 

The  irritability  of  the  heart-muscle  depends  primarily  on  the 
blood-supply  and  secondarily  on  the  maintenance  of  a  normal 
temperature,  and  so  long  as  both  conditions  are  maintained  the 
muscle  will  respond  by  a  contraction  to  any  adequate  stimulus, 
physiologic  or  artificial. 

a.  The  Blood -sup  ply. — The  supply  of  blood  to  the  mammalian 
heart  is  derived  from  the  coronary  arteries  which,  though  filled 
during  the  systole,  deliver  the  blood  to  the  intra-mural  arterioles 
and  capillaries  during  the  diastole.  The  facts  relating  to  the 
blood-supply  have  been  presented  fully  in  a  foregoing  paragraph. 

b.  The  Influence  of  Temperature. — For  the  manifestation  of  the 
irritability  and  contractility  it  is  essential  that  the  heart-muscle 
be  kept  at  a  sufficiently  high  temperature  in  order  that  the  physio- 
logic or  a  given  artificial  stimulus  may  provoke  a  maximal  con- 
traction. This  is  accomplished  by  immersing  the  suspended 
heart  in  a  bath  of  Ringer's  solution  the  temperature  of  which 
can  be  readily  decreased  or  increased  by  appropriate  means. 
The  optimum  temperature  for  the  frog  heart  is  about  2^°  C. 
As  the  temperature  is  lowered  both  rate  and  force  decrease  until 
at  about  from  40  C.  to  o°  C.both  cease.  Beyond  3 5 °  C .  it  also 
ceases  to  contract,  because  of  a  coagulation  of  the  muscle  sub- 
stance. The  mammalian  heart  attains  its  maximum  activity 
at  a  temperature  of  370  C.  It  ceases  to  beat  at  about  470  C. 
on  the  one  hand  and  at  about  170  C.  on  the  other  hand. 

2.  Conductivity. — The  heart-muscle  possesses  conductivity.  The 
excitation  process  and  the  subsequent  contraction  wave,  both  of 
which  take  their  rise  under  physiologic  conditions  near  the  venae 


298  TEXT-BOOK  OF  PHYSIOLOGY. 

cavae  terminations,  are  conducted  over  the  auricles,  thence  to  the 
ventricles  from  base  to  apex.  It  is  now  generally  believed  that 
the  propagation  of  both  processes  is  accomplished  by  muscle- 
tissue  alone,  independently  of  the  nerve  system.  The  conduc- 
tivity, however,  is  not  equally  well  developed  in  every  part  of  the 
heart.  This  is  especially  true  of  the  tissue  at  both  the  sinu- 
auricular  and  the  auriculo-ventricular  junctions.  At  these  points 
in  the  frog  heart  the  contraction  wave  is  delayed  for  an  appreciable 
period,  a  condition  attributed  to  the  embryonic  character  of  the 
muscle-tissue.  In  the  frog's  heart  the  excitation  process  begins 
in  the  sinus  venosus,  from  which  it  passes  to  the  auricles,  thence 
to  the  ventricles.     In  Fig.  138,  which  is  a  graphic  record  of  the 

heart-beat,  the  two  elevations  of  the 
level  on  the  up-stroke,  a  and  b,  repre- 
sent the  contraction  of  the  sinus 
and  the  auricle  respectively,  while 
the  two  depressions,  c  and  d,  indi- 
cate the  delay  in  the  transmission 
of  the  contraction  wave  at  the  two 
junctions.  There  is  here  an  ana- 
tomic obstacle  to  the  conduction  of 
the  contraction  wave.  This  may 
Fig.    138.— Record   of   the      ^      artificially    increased    by  com- 

CONTRACTION       OF       THE       FROG'S  j  \ 

Heart.  pressing  the  heart  between  the  au- 

ricles and  ventricles  with  a  clamp. 
By  carefully  regulating  the  pressure  it  is  possible  to  so  block  the 
wave  that  three  or  four  auricular  contractions  may  occur  before 
the  excitation  process  forces  the  block  and  excites  a  ventricular 
contraction.  (Fig.  139.)  If  the  block  is  complete,  rather  than 
partial,  the  ventricle  will  come  to  rest  and  so  remain.  From 
the  foregoing  facts  it  is  evident  that  the  physiologic  stimulus 
exerts  its  action  in  the  sinus  venosus  and  that  the  auricular  and 
ventricular  beats  are  in  turn  dependent  on  it. 

In  the  mammalian  heart  the  contraction  wave  arising  at  the 
terminations  of,  or  at  a  point  between  the  terminations  of,  the 
venae  cavae,  is  likewise  conducted  over  the  auricles  and  thence  to 
the  ventricles;  between  the  end  of  the  auricular  and  the  beginning 
of  the  ventricular  contraction  there  is  also  a  perceptible  interval 
similar  to  that  observed  in  the  frog  heart.  For  a  long  time  this 
was  attributed  to  an  interference  with  the  passage  of  the  contrac- 
tion wave  across  the  auriculo-ventricular  junction  because  of  the 
extreme  scarcity  of  the  muscle-fibers  in  this  region  or  to  their 
embryonic  character.  In  recent  years,  however,  this  view  has 
been  abandoned  because  the  real  bond  of  union  between  the 
auricular  and  ventricular  tissues,  across  which  the  contraction 
wave  passes,  has  been  found,  as  stated  on  page  280,  in  the  system 
of  muscle-fibers,  described  in  part  by  His,  Retzer  and  Braunig 


THE  CIRCULATION  OF  THE  BLOOD.  299 

and  completed  by  Tawara  and  termed  by  him  the  conduction 
system  of  the  heart.  This  system  it  is  now  believed  constitutes 
the  anatomic  and  physiologic  path  across  which  the  contraction 
wave  passes  from  auricles  to  ventricles.  The  supposition  that 
this  was  the  case  was  demonstrated  by  Hering  and  others  who 
succeeded  in  dividing  the  muscle-bundle  in  the  excised  hearts 
of  rabbits  and  dogs,  kept  actively  beating  by  perfusion  with 
Ringers'  solution.  On  division  of  the  bundle  both  auricles  and 
ventricles  continued  to  beat  though  with  different  rates  and  in- 
dependently of  each  other.  These  and  experiments  of  a  similar 
character  have  demonstrated  beyond  question  that  the  auriculo- 
ventricular  bundle  with  its  widespread  ramifications  is  the  true 


Fig.  139. — Record  of  the  Auricular  and  Ventricular  Contractions  before 
and  after  the  closure  of  the  clamp  at  a. 


conducting  system  between  auricles  and  ventricles.  The  cause 
assigned  by  Tawara,  for  the  interval  between  the  auricular  and 
ventricular  contraction  is  not  so  much  the  embryonic  character 
of  the  fibers  of  the  system,  as  it  is  the  length  of  the  system  as  a 
whole,  which  he  estimates  at  from  4  to  6  centimeters.  Because 
of  this  fact,  the  excitation  process  requires  time  to  pass  from  the 
beginning  to  the  ends  of  the  system  and  to  all  parts  of  the  ventricles, 
which  time  is  represented  by  the  inter-systolic  period  or  interval. 
With  the  mammalian  heart  as  with  the  frog  heart  it  is  possible 
to  increase  the  length  of  the  interval  between  the  auricular  and 
the  ventricular  contraction,  the  inter-systolic  period,  by  com- 
pression of  the  tissues  between  auricles  and  ventricles  including 
presumably  the  central  part  of  the  conducting  system,  the  muscle 
band  of  His.  This  has  been  accomplished  in  the  dog  by  Erlanger 
by  means  of  a  specially  devised  hook  clam]).  When  the  com- 
pression is  brought  about  suddenly  and  completely  the  ventricles 
at  once  cease  beating  though  the  auricles  continue  to  beat  with 
their  customary  rate  and  regularity.  After  a  variable  period  of 
time,  varying  from  a  few  seconds  to  70  seconds,  during  which  the 
ventricles  are  relaxed  and  gradually  filling  with  blood  from  the 


■.  rate  per  minute. 

V 

216 
117.8 
162. 1 

3-7 
2.4 
3-°5 

TEXT-BOOK  OF  PHYSIOLOGY. 

auricles,  the  ventricular  beat  returns,  at  first  slowly  but  with  a 
gradually  increasing  frequency  until  a  definite  but  a  comparatively 
slow  rate  is  attained. 

In  experiments  on  the  dog  heart  performed  by  Erlanger  the 
following  results  were  obtained  when  the  auriculo-ventricular 
bundle  was  completely  crushed. 

Ven.  rate  per  minute. 

Max.  69.8 
Min.  34.8 
Ave.    54.1 

The  reason  assigned  for  the  cessation  of  the  ventricular  contrac- 
tion is  the  non-arrival  of  the  excitation  process  at  the  ventricular 
end  of  the  conducting  system,  because  of  the  blocking  or  com- 
pression. Under  physiologic  conditions  the  ventricular  beat  is 
directly  dependent  on  the  arrival  of  the  excitation  process  from 
the  auricles  and  if  it  fails  to  arrive  the  ventricle  does  not  con- 
tract for  some  seconds.  The  return  of  the  beat  during  complete 
blocking  is  attributed  to  the  development  of  a  hitherto  dormant 
inherent  rhythmicity.  When  this  is  established  both  auricles  and 
ventricles  continue  to  beat  though  with  a  totally  different  rhythm. 

The  effects  which  follow  gradual  compression  of  the  muscle- 
bundle  are  somewhat  different  from  those  which  follow  sudden 
compression.  If  the  clamp  is  accurately  adjusted  and  the  com- 
pression gradually  applied,  the  first  perceptible  effect  is  a  length- 
ening of  the  normal  pause  between  the  auricular  and  the  ventric- 
ular contraction.  With  an  increase  in  the  compression  there 
will  come  a  moment  when  one  of  the  auricular  contraction  waves 
fails  to  reach  the  ventricle,  or  if  it  does,  it  is  so  enfeebled  that 
it  is  incapable  of  exciting  the  ventricle,  which  in  consequence 
fails  to  contract.  This  dropping  out  of  a  ventricular  contraction 
may  occur  once  in  every  10,  9,  8,  7,  6,  etc.,  auricular  beats,  in 
accordance  with  the  degree  of  compression.  With  a  further 
tightening  of  the  clamp,  the  blocking  of  the  excitation  process 
may  be  still  further  increased  so  that  only  every  second,  third  or 
fourth  auricular  beat  will  be  followed  by  a  ventricular  beat,  es- 
tablishing what  has  been  termed  the  2:1,  3:1,  4:1,  rhythms  re- 
spectively; and  finally  when  the  blocking  is  complete  no  excitation 
process  can  reach  the  ventricle. 

Owing  to  the  capability  of  the  mammalian  ventricle  to  develop 
an  independent  rhythm  when  not  stimulated  by  the  auricles  for 
a  few  seconds  or  more,  it  is  not  always  possible  to  state  at  what 
particular  moment  in  the  successive  stages  of  compression  the 
independent  ventricular  rhythm  becomes  manifest.  Usually 
when  the  rhythm  is  of  the  3 : 1  type,  i.  e.,  when  the  third  auricular 
contraction  fails  to  reach  the  ventricle,  it  will  begin  to  beat  of 
itself.  Under  such  circumstances  the  auricles  and  ventricles 
become  dissociated  even  though  the  block  is  not  quite  complete. 


THE  CIRCULATION  OF  THE  BLOOD.  301 

These  experimental  facts  have  afforded  an  explanation  of  the 
altered  rhythm  between  auricles  and  ventricles  in  that  pathologic 
condition  known  as  Stokes-Adams  disease.  In  this  disease  the 
rhythm  may  be  any  one  of  the  rhythms  stated  in  the  foregoing 
paragraph.  In  two  instances  the  following  ratio  of  the  ventricle 
to  the  auricle  was  observed  by  Erlanger. 

Ven.  rate  per  minute.  Aur.  rate  per  minute.  ^ 

22.4  79.6  3.55 

31.0  84.6  2.73 

In  a  few  cases  of  death  from  this  disease  a  postmortem  ex- 
amination showed  a  pathologic  lesion  of  the  auriculo-ventricular 
bundle. 

3.  Rhythmicity. — The   beat  of   the  heart  is  a  uniform  movement, 

occurring  at  regular  intervals.  Each  phase  of  each  beat  occupies  a 
regular  measure  of  time.  The  beat  is  therefore  rhythmic  in  char- 
acter. The  heart-muscle  as  a  whole  varies  in  rhythmic  power  in 
its  different  parts.  It  is  best  developed  in  the  frog  and  tortoise, 
in  the  sinus  venosus,  less  so  in  the  auricles,  least  in  the  ventricles. 
This  may  be  shown  by  division  of  the  tissue  between  sinus  and 
auricles  in  situ.  At  once  the  auriculo-ventricular  portion  ceases 
to  beat,  while  the  sinus  continues  contracting  as  usual.  In  a 
short  time  the  auricles  and  ventricles  begin  to  beat,  though  less 
rapidly  than  formerly.  Separation,  of  the  auricle  from  the  ven- 
tricle is  again  followed  by  rest.  In  due  time  the  auricle  begins 
to  beat,  while  the  ventricle  remains  quiescent.  If  the  ventricle 
be  now  stimulated  in  a  rhythmic  manner,  it  may  resume  rhyth- 
mic activity.  These  facts  are  taken  as  an  indication  that  the 
rhythmic  power  is  developed  in  unequal  degree  in  the  three 
divisions  of  the  heart. 

In  the  warm-blooded  animal,  e.  g.,  dog,  cat,  rabbit,  there  is 
also  a  difference  in  the  rhythmicity  between  the  auricles  and 
ventricles.  This  is  shown  by  the  effects  which  follow  division  of 
the  auriculo-ventricular  bundle,  or  sudden  and  complete  com- 
pression of  the  heart  at  the  auriculo-ventricular  groove.  In 
either  case  the  ventricle  for  a  short  time  remains  at  rest,  though 
the  auricles  continue  to  beat  at  their  usual  rate.  After  a  variable 
number  of  seconds  the  ventricle  develops  a  rhythm  of  its  own, 
though  it  never  attains  that  of  the  auricle. 

4.  Tonicity. — The    heart-muscle,    like    the   vascular    muscle,    main- 

tains continuously  a  certain  degree  of  contraction,  termed  tone, 
upon  which  the  efficiency  of  the  heart  as  a  pumping  organ  is 
largely  dependent.  In  the  physiologic  condition  the  ventricular 
muscle  neither  contracts  nor  relaxes  to  its  fullest  possible  extent, 
but  maintains  an  intermediate  position  between  the  two  extremes 
and  for  this  reason  is  said  to  possess  tonicity.  This  tone  may, 
however,  be  increased  or  decreased  by  the  action  of  various  ex- 


3o2  TEXT-BOOK  OF  PHYSIOLOGY. 

ternal  agents.  Thus  the  passage  of  dilute  solutions  of  various  drugs 
— e.  g.,  alkalies,  digitalis — through  the  cavities  of  the  excised  heart 
will  so  increase  the  tone,  or  the  contractile  power,  that  complete 
relaxation  is  prevented,  until  finally  the  heart  comes  to  a  stand- 
still in  the  condition  of  systole.  The  passage  of  dilute  solutions 
of  lactic  acid,  muscarine,  etc.,  through  the  heart  will,  on  the  con- 
trary, so  decrease  the  tone  or  the  contractile  power  that  the  nor- 
mal contraction  is  not  attained.  The  relaxation  therefore  gradu- 
ally increases  until  the  heart  finally  comes  to  a  standstill  in  the 
condition  of  diastole.  In  the  first  instance  the  tonicity  is  said 
to  be  increased;  in  the  second  instance,  decreased. 

5.  Automaticity. — Inasmuch  as  the  heart  continues  to  contract  in 
a  perfectly  rhythmic  manner  after  removal  from  the  body  and 
apparently  without  the  aid  of  an  external  stimulus,  it  is  said  that 
the  heart-muscle  is  automatic  or  spontaneous  in  action.  Strictly 
speaking,  however,  this  is  not  the  case,  for  the  reason  that  all 
movement,  that  of  the  heart  included,  is  the  resultant  of  the  action 
of  natural  causes  though  their  true  nature  may  be  beyond  the 
reach  of  present  methods  of  investigation, 
The   Nature   of  the  Stimulus.—  As  the  heart  continues  to  beat 

after   removal  from  the  body,  it  is  evident  that  the  stimulus  does  not 

originate  in  the  central  nerve  system  but  in  the  heart  itself.     Two 

views  have  been  held  as  to  its  origin  and  nature: 

1.  That  it  originates  in  the  nerve-cells  found  in  various  parts  of  the 

heart-muscle;  that  it  is  a  nerve  impulse  rhythmically  and  auto- 
matically discharged  by  these  cells  and  transmitted  by  their 
axons  to  the  heart-muscle  cells. 

2.  That  it  originates  in  the  muscle-cells  themselves;  that  it  is  chemic  in 

character  and  due  to  a  reaction  between  the  chemic  constituents, 
organic  and  inorganic,  of  the  muscle-cells  and  those  in  the  lymph 
by  which  they  are  surrounded. 

According  to  the  first  view  the  stimulus  is  neurogenic,  according 
to  the  second  view  myogenic,  in  origin. 

The  presence  of  nerve-cells;  their  relation  to  the  muscle-cells;  the 
pronounced  rhythmic  activity  of  the  sinus  and  auricles  in  which  the 
nerve-cells  are  abundant;  the  feeble  activity  of  the  apex,  in  which  they 
are  wanting — these  and  other  facts  lend  support  to  the  view  that  the 
stimulus  originates  in  the  nerve-cells.  To  them  have  been  attributed 
the  power  of  automatic  activity. 

The  absence  of  nerve-cells  in  portions  of  the  heart-muscle,  which 
nevertheless  exhibit  rhythmic  contractions  for  quite  a  long  period 
of  time;  the  rhythmic  beat  of  the  embryonic  heart  before  the  migration 
of  nerve-cells  to  its  walls  shows,  that  the  stimulus  does  not  necessarily 
originate  in  nerve-cells.  Moreover,  Porter  has  conclusively  shown 
that  the  apex  of  the  dog's  heart,  which  is  generally  believed  to  be 
totally  devoid  of  nerve-cells,  can  be  made  to  beat  for  hours  by  feeding 
it  through  its  nutrient  artery  with  warm  defibrinated  blood.     Unless 


THE  CIRCULATION  OF  THE  BLOOD.  303 

it  be  assumed  that  the  heart-muscle  contracts  automatically,  without  a 
cause,  it  is  a  fair  assumption  that  the  exciting  cause  of  the  contraction 
arises  within  the  muscle-cells  themselves,  and  that  it  is  in  all  proba- 
bility the  outcome  of  a  reaction  between  the  chemic  constituents 
and  more  especially  the  inorganic  constituents  of  the  blood  or  lymph 
on  the  one  hand,  and  the  chemic  constituents  of  the  muscle-cells  on 
the  other.  The  discovery  that  some  of  the  inorganic  salts  of  the  blood 
have  a  specific  physiologic  action  on  the  heart-muscle  was  made  in 
1882  by  Ringer.  Since  then,  many  attempts  have  been  made  to 
isolate  these  constituents,  to  determine  not  only  their  individual,  but 
also  their  collective  action,  when  combined  in  proportions  approxi- 
mating those  in  which  they  exist  in  the  blood. 

The  Action  of  Inorganic  Salts. — 1.  On  the  Frog  and  Terrapin 
Heart. — The  inorganic  salts  which  are  most  directly  concerned  in  ex 
citing  and  sustaining  the  heart-beat  are  sodium  chlorid,  calcium  phos- 
phate or  chlorid,  and  potassium  chlorid.  A  combination  of  these 
salts  in  the  proportions  in  which  they  exist  in  the  blood  was  first  sug- 
gested by  Ringer  and  is  made  by  saturating  a  0.65  per  cent,  solution 
of  sodium  chlorid  with  calcium  phosphate,  and  then  adding  to  each 
100  ex.,  2  c.c.  of  a  1  per  cent,  solution  of  potassium  chlorid.  A  frog's 
heart  immersed  in  this  solution  will  continue  to  beat  for  some  hours. 
A  combination  of  the  chlorids  of  sodium,  calcium,  and  potassium 
in  amounts  which  will  vary  for  different  animals  is  equally  efficient 
in  maintaining  the  heart-beat. 

The  collective  as  well  as  the  individual  actions  of  these  salts  have 
been  strikingly  brought  out  by  the  experiments  of  Profs.  Howell  and 
Greene,  from  whose  published  results  the  following  statements  are 
derived.  Instead  of  employing  the  entire  heart,  they  used  for  various 
reasons  strips  from  the  terminations  of  the  venae  cava?  and  from  the 
ventricle  of  the  terrapin  heart.  The  proportion  of  the  inorganic  salts 
most  favorable  for  the  contraction  of  the  vena  cava  strips  is  the  follow- 
ing: viz.,  sodium  chlorid,  0.7  per  cent.;  calcium  chlorid,  0.026  per 
cent.;  potassium  chlorid,  0.03  per  cent.  When  vena  cava  strips  are 
immersed  in  this  solution,  they  begin  in  a  short  time  to  exhibit  rhythmic 
contractions  which  may  continue  for  several  days.  In  the  same 
strength  of  solution  the  ventricular  strips  remain  inactive  but  if  the 
percentage  of  the  calcium  chlorid  be  raised  from  0.026  per  cent,  to 
0.04,  or  0.05  per  cent.,  spontaneous  contractions  soon  develop  and 
continue  for  several  days  or  more.  In  the  foregoing  solution  when  the 
calcium  chlorid  is  present  only  to  the  extent  of  0.026  per  cent.,  though 
the  ventricular  strip  does  not  contract,  it  is  kept  in  good  condition  for 
contraction,  for  even  after  many  hours  the  raising  of  the  percentage 
of  calcium  chlorid  to  0.04  or  0.05  per  cent,  will  call  forth  after  a  brief 
latent  period,  rapid  and  energetic  contractions.  From  this  fact  it  is 
inferred  that  the  vena?  cava?  region  is  more  sensitive  to  the  combined 
action  of  the  salts  than  is  the  ventricle. 

The  action  of  the  individual  salts  is  also  best  shown  with  ventricular 


304  TEXT-BOOK  OF  PHYSIOLOGY. 

strips.  In  a  0.7  per  cent,  sodium  chlorid  solution  the  strip  beats 
rhythmically  and  energetically,  but  for  a  short  period  and  with  gradually 
diminishing  force,  until  it  entirely  ceases  to  beat.  A  reason  assigned 
for  this  is  the  removal  of  other  salts  necessary  to  the  excitation  of  the 
contraction.  In  a  calcium  chlorid  solution — 0.9  per  cent. — i.  e., 
isotonic  with  the  sodium  chlorid — the  heart  strip  is  thrown  into  strong 
tone,  but  does  not  rhythmically  contract.  If,  however,  the  strip  is 
placed  in  normal  saline,  and  calcium  chlorid  added  in  amounts  equal 
to  that  present  in  the  blood,  it  will  after  a  very  short  latent  period 
begin  to  contract  rapidly  and  energetically  and  for  a  longer  time  than 
when  in  sodium  chlorid  solution  alone.  The  contractions  not  infre- 
quently occur  before  relaxation  is  completed,  so  that  the  strip  passes 
into  the  condition  of  contracture. 

In  potassium  chlorid  solutions  isotonic — 0.9  per  cent. — with 
sodium  chlorid  solution  the  heart  strip  also  fails  to  contract.  This 
is  the  case  also  when  the  potassium  is  added  to  the  sodium  chlorid 
in  amount  practically  equal  to  that  found  in  the  blood. 

2.  On  the  Mammalian  Heart. — The  collective  action  of  the  in- 
organic salts  on  the  isolated  heart  of  all  members  of  this  class  of  animals, 
which  have  been  made  the  subject  of  experimentation,  is  as  marked,  if 
not  more  so,  than  it  is  on  the  heart  of  the  frog  or  terrapin  especially 
when  the  coronary  blood-vessels  are  perfused  with  Ringer's  solution 
or  the  modification  of  it  suggested  by  Locke,  as  follows:  NaCl  0.90 
per  cent.;  CaCl2  0.024  Per  cent.;  KC1  0.042  per  cent.;  NaHC03  0.02 
per  cent.,  dextrose  0.1  per  cent.  The  reviving  and  sustaining  power 
of  this  solution  is  extraordinary.  Locke  and  Rosenheim  were  able  to 
revive  the  isolated  heart  of  a  rabbit  and  to  excite  it  to  active  contrac- 
tion, for  several  hours  at  a  time,  on  four  consecutive  days  by  perfusing 
it  with  this  solution  saturated  with  oxygen  and  at  a  temperature  of 
350  C.  No  special  precautions  were  observed  other  than  keeping  it 
cool  (io°  C.)  and  moist  during  the  intervals  of  experimentation.  The 
duration  of  the  irritability  and  contractility  extended  over  a  period 
of  95  hours.  Kuliabko  revived  the  heart  of  a  rabbit  for  an  hour 
nearly  three  days  after  removal  from  the  body  of  the  animal.  It  was 
then  placed  on  ice,  and  after  four  days  it  was  again  revived  by  per- 
fusing it  with  Ringer's  solution.  Altogether  this  heart  retained  its 
irritability  for  seven  days.  Hering  revived  the  heart  of  a  monkey  on 
three  different  occasions,  the  first,  4^  hours,  the  second,  28  hours,  and 
the  third  54  hours  after  the  death  of  the  animal.  In  the  intervening 
periods  the  heart  was  also  kept  on  ice.  In  this  animal  it  was  even 
possible  to  increase  and  decrease  the  activity  of  the  heart  by  stimula- 
tion of  the  nerves  which  normally  control  the  rate  of  the  beat.  Kuliabko 
was  also  able  to  revive  the  isolated  heart  of  a  child  20  hours  after  death 
from  a  double  pneumonia.  It  was  made  to  beat  rhythmically  at  a 
rate  varying  from  70  to  80  per  minute  when  the  solution  had  a  tem- 
perature of  390  C,  and  at  a  rate  of  98  to  102  per  minute  when  it  had  a 
temperature  of  41  °  C,   though   at  this  temperature  the  beat  became 


THE  CIRCULATION  OF  THE  BLOOD.  305 

arrhythmic.  All  these  instances  demonstrate  the  extreme  longevity 
of  the  irritability  of  the  heart-muscle  under  appropriate  conditions. 

The  action  of  individual  salts  has  been  shown  experimentally  on 
the  hearts  of  rabbits,  cats,  dogs,  monkeys,  by  Gross,  Howell  and  others. 
Thus  it  has  been  found  that  when  an  isolated  heart  is  rhythmically 
beating  in  response  to  the  perfusion  of  Ringer's  or  Locke's  solution, 
the  addition  of  potassium  chlorid  in  small  amounts  is  followed  by  a 
decrease  in  the  rate  and  force  of  the  contraction,  and  in  larger  amounts 
by  a  complete  cessation  of  the  contraction  and  a  standstill  in  diastole. 
On  the  withdrawal  of  the  potassium,  the  former  frequency  and  vigor 
are  regained.  Potassium  exerts  a  depressor  or  an  inhibitor  influence 
on  the  irritability  and  contractility  of  the  heart- muscle. 

Under  the  same  conditions,  the  addition  of  calcium  chlorid  in 
sufficient  amounts  is  followed  by  an  increase  in  the  rate  and  in  the 
vigor  of  the  contractions;  on  its  withdrawal  both  rate  and  force  return 
to  the  previous  condition.  Calcium  exerts  an  accelerator  and  an  aug- 
mentor  influence  on  the  irritability  and  contractility  of  the  heart. 

The  Cause  of  the  Heart-beat. — From  the  foregoing  facts  it  seems 
probable  that  the  heart-beat  is  connected  with  and  dependent  on 
the  presence  and  interaction  of  the  inorganic  salts  present  in  the 
lymph,  though  as  to  the  manner  in  which  they  interact  to  initiate  the 
beat,  there  is  some  obscurity.  A  very  plausible  theory  as  to  the  part 
played  by  the  inorganic  salts  in  initiating  the  contraction  and  one  in 
accordance  with  the  facts  has  been  presented  by  Howell  as  follows: 

The  heart-muscle,  it  is  assumed,  contains  a  stable  organic  energy- 
yielding  compound  of  which  potassium  is  one  of  its  constituents  and 
on  which  its  stability  depends.  This  compound  must  be  present  in 
relatively  large  amounts  as  the  heart  will  continue  to  contract  and 
expend  energy  for  many  hours  after  the  blood-supply  has  been  with- 
drawn. 

During  the  diastole  a  reaction  takes  place  between  this  compound 
and  the  calcium  or  the  calcium  and  the  sodium  salts,  whereby  a  portion 
of  the  organic  compound  is  freed  from  potassium  and  is  then  com- 
bined with  calcium,  or  with  calcium  and  sodium.  In  consequence, 
this  portion  of  the  organic  compound  in  combination  with  the  calcium 
acquires  and  gradually  increases  in  instability,  reaching  its  maximum 
at  the  end  of  the  diastole,  when  it  undergoes  a  dissociation  giving  rise 
to  a  chain  of  events  that  culminate  in  a  contraction.  The  initial 
step,  therefore,  is  a  dissociation  of  a  complex  unstable  molecule  followed 
by  an  oxidation  of  the  dissociated  products.  That  an  active  dissocia- 
tion of  some  character  takes  place  is  evident  from  the  consumption 
of  oxygen,  the  production  of  carbon  dioxid,  the  liberation  of  heat, 
electricity  and  mechanic  motion. 

Inasmuch  as  the  contraction  is  always  maximal  and  as  the  heart 
is  refractory  to  a  stimulus  during  the  systole,  the  probabilities  are 
that  all  of  the  energy-yielding  unstable  compound  is  dissociated  with 
each  contraction.     With  the  relaxation  there  is  a  renewal  of  the  unstable 


306  TEXT-BOOK  OF  PHYSIOLOGY. 

molecules  along  the  lines  stated  above,  which  increase  in  number 
and  instability  until  the  maximum  is  again  attained  when  another 
dissociation  occurs  followed  by  another  contraction.  The  rhyth- 
micitv  of  the  heart's  action,  the  appearance  of  a  refractory  condition 
during  the  systole  and  its  gradual  disappearance  during  the  diastole, 
as  well  as  other  phenomena,  are  readily  explained  by  the  foregoing 
hypothesis. 

The  cause  of  the  dissociation  of  the  energy-yielding  material  is, 
however,  a  subject  of  discussion.  According  to  Howell  it  is  not 
necessary  to  assume  the  presence  of  any  cause  other  than  the  extreme 
instability  of  the  organic  compound  in  question.  According  to 
Engelman,  Langendorff  and  others,  the  dissociation  is  not  spontaneous 
but  is  the  result  of  the  action  of  a  specific  stimulus,  an  "inner  stimu- 
lus," arising  within  the  muscle  elements  themselves  through  metabolic 
processes;  and  so  long  as  these  processes  are  chemically  and  physically 
conditioned  by  blood  or  tissue  fluids  containing  the  inorganic  salts, 
so  long  will  this  stimulus  be  produced.  As  to  the  nature  of  this  stimulus, 
whether  chemic,  electric  or  enzymic,  nothing  definite  can  be  stated  at 
present. 

The  Response  of  the  Heart  to  the  Action  of  a  Stimulus.— 
The  heart  of  the  frog  as  well  as  of  some  other  animals  may  be  brought 
to  a  standstill  by  the  ligation  of  the  tissues  between  the  sinus  veno- 
sus  and  the  auricle,  a  procedure  first  introduced  by  Stannius  and 
now  known  as  the  first  Stannius  ligature.  Under  such  circumstances 
the  heart  may  be  made  to  contract  by  stimulating  it  with  the  single 
induced  current.  With  each  passage  of  the  current  the  heart  con- 
tracts. Contrary  to  what  is  observed  in  other  muscles,  the  heart- 
muscle,  if  it  contracts  at  all,  at  once  reaches  its  maximal  value.  Any 
increase  in  the  strength  of  the  stimulus  above  the  threshold  value  has 
no  greater  effect  on  the  extent  or  force  of  the  contraction  than  the 
minimal  stimulus.  A  conclusion  which  may  be  drawn  from  this 
fact,  according  to  Engelman,  is  as  follows:  By  reason  of  the 
fact  that  the  heart  contracts  at  its  maximum  value  to  the  action  of 
any  strength  of  stimulus,  under  given  conditions,  there  is  always  en- 
sured a  complete  emptying  of  the  ventricular  contents  and  a  uni- 
form discharge  of  blood  into  the  arteries,  which  would  not  be  the 
case  if  the  extent  of  the  contraction  varied  with  the  strength  of  the 
stimulus;  and  there  are  reasons  for  believing  that  the  normal  stimu- 
lus for  the  contraction  varies  within  wide  limits  above  the  threshold 
value  both  in  normal  and  abnormal  conditions  of  the  heart.  The 
changes  in  the  extent  or  force  of  the  contraction  are  the  result,  not  of 
changes  in  the  intensity  of  the  stimulus,  but  of  changes  in  the  heart- 
muscle,  caused  by  variations  in  mechanical  resistances. 

The  periodicity  of  the  heart's  action  or  its  rhythm  may  also  be 
elucidated  by  the  foregoing  fact.  There  are  reasons  for  believing 
that  at  the  time  of  the  contraction  practically  all  of  the  available 
energy-yielding  material  is  completely  utilized,  after  which  the  heart 


THE  CIRCULATION  OF  THE  BLOOD.  '  307 

relaxes  and  remains  at  rest  in  the  diastolic  condition  for  a  given  period; 
and  before  a  second  excitation  wave  can  be  developed  and  pass  from 
the  sinus  over  the  heart  there  must  be  a  re-accumulation  of  energy- 
yielding  material,  and  a  restoration  of  the  irritability.  This  is  accom- 
plished during  the  diastole.  By  virtue  of  this  fact  the  heart  can  not 
act  otherwise  than  in  a  periodic  or  rhythmic  manner. 

Inasmuch  as  there  is  a  conversion  of  all  of  the  potential  energy 
into  kinetic  energy  during  the  systole,  there  is  of  necessity,  a  lowering  of 
the  irritability,  and  to  so  great  an  extent  is  this  the  case  that  the  heart 
will  not  respond  to  the  action  of  a  second  stimulus  either  physiologic 
or  artificial  during  the  systolic  period.  This  non-responsiveness  of 
the  heart  may  be  shown  by  throwing  into  it  a  second  stimulus  at  any 
moment  during  the  systole.  Whatever  the  moment  or  whatever  the 
strength  of  the  stimulus  may  be  the  extent  of  the  contraction  remains 
the  same.  During  the  systolic  period  the  heart  is  said,  therefore,  to 
be  refractory  to  a  second  stimulus.  If, 
however,  a  second  stimulus  of  average 
strength  be  thrown  into  the  ventricle  at 
any  moment  during  the  relaxation,  a 
second  contraction  or  extra  systole  will  be 
developed.  The  extent  of  this  contraction  coJ^^'  Tnd^h/com- 
will  be  proportional  to  the  time  at  which  pensatory  Pause.  The  break 
the  stimulus  is  thrown  into  the  ventricle  as      in  the  horizontal  line  indicates 

,  ,,       ,  ,,  ,      r        the    moment    the    electric    cur- 

it  passes  from  the  beginning  to  the  end  of      rent  passes  through  the  heart. 

its  relaxation.     Whatever  the  extent  of  the 

extra  contraction  it  superposes  itself  on  the  first,  though  its  height  is  no 
greater  than  the  first.  For  this  reason  it  is  believed  a  tetanic  con- 
traction can  not  be  developed.  If  the  stimulus  be  thrown  into  the 
heart  just  as  the  relaxation  is  completed,  the  extra  contraction  attains 
the  same  height  as  the  preceding  contraction.  In  passing  from  the 
beginning  to  the  end  of  the  relaxation  and  into  the  diastolic  or  resting 
period,  it  has  been  found  that  the  extra  contraction  can  be  provoked 
by  a  stimulus  which  is  steadily  decreased  in  intensity.  It  is  evident 
from  this  fact  that  the  restoration  of  the  energy-yielding  material  and 
the  return  of  the  irritability  gradually  increases  from  the  beginning  of 
the  relaxation  to  the  end  of  the  diastole.  For  this  reason  weak  stimuli 
are  more  effective  in  the  later  than  in  the  earlier  period  of  the  relaxa- 
tion and  the  diastole. 

After  the  development  and  disappearance  of  the  extra  contraction 
a  considerable  pause  in  the  heart's  action  occurs  to  which  the  term 
compensatory  pause  has  been  given  (Fig.  140),  on  the  assumption  that 
it  was  necessary  on  the  part  of  the  heart  to  compensate  for  the  dis- 
turbance of  the  rhythm  by  remaining  at  rest  until  the  time  of  the  next 
beat  and  thus  restore  the  rhythm.  This  was  thought  to  be  a  special 
property  of  the  heart-muscle.  This  view,  however,  is  no  longer  enter- 
tained. For  if  an  isolated  ventricle  of  a  frog  heart  be  employed  and 
made  to  contract  rhythmically  by  an  artificial  stimulus,  or  if  a  sponta- 


308  TEXT-BOOK  OF  PHYSIOLOGY. 

neously  beating  portion  of  the  dog's  heart  be  employed  for  experi- 
mentation instead  of  the  whole  heart,  the  results  of  the  same  methods 
of  stimulation  are  different.  Though  an  extra  contraction  is  called 
forth  as  usual,  there  is  no  compensatory  pause;  indeed,  if  anything  the 
pause  is  shorter  than  the  regular  pause.  The  theory  of  a  necessary 
compensation  is  unnecessary. 

The  explanation  assigned  and  generally  accepted  at  present  for 
the  production  of  a  compensatory  pause  is  as  follows:  In  a  sponta- 
neously beating  heart  the  ventricular  contraction  is  provoked  by  the 
arrival  of  an  excitation  process  coming  from  the  auricles.  When  the 
extra  contraction  is  induced  by  an  artificial  stimulus,  the  next  succeed- 
ing excitation  from  the  auricle  falls  into  the  refractory  period  and 
hence  the  ventricle  is  not  stimulated.  It,  therefore,  simply  waits 
for  the  arrival  of  the  third  succeeding  excitation,  when  it  responds  and 
takes  up  the  regular  rhythm. 

If  a  series  of  successive  stimuli  be  thrown  into  the  heart-muscle 
the  effect  will  vary  in  accordance  with  their  time  intervals.  Should 
this  be  less  than  about  three  seconds  there  will  be  a  gradual  increase  in 
the  height  for  some  half  dozen  contractions,  a  result  to  which  the  term 
"staircase"  or  "treppe"  has  been  given.  This  increase  in  the  height 
of  the  contraction  is  attributed  to  an  increase  in  the  irritability  and  con- 
tractility of  the  muscle  the  result  of  the  primary  stimulating  action  of 
fatigue  products. 

THE  NERVE  MECHANISM  OF  THE  HEART. 

By  this  term  is  meant  a  combination  of  nerves  and  nerve-centers 
which  cooperate  to  increase  or  decrease  either  the  rate  or  force — or 
both — of  the  heart's  contraction  in  accordance  with  the  needs  of  the 
system.  That  the  heart  is  normally  influenced  by  the  central  organs 
of  the  nerve  system  in  response  to  the  action  of  nerve  impulses  re- 
flected to  them  from  many  organs  of  the  body  is  a  matter  of  personal 
experience;  that  it  is  abnormally  influenced  by  the  same  or  other  organs 
in  response  to  nerve  impulses  reflected  to  them  in  consequence  of 
pathologic  and  traumatic  processes,  occurring  in  different  regions  of 
the  body,  and  that  both  heart  and  nerves  are  modified  in  different 
ways  by  the  action  of  drugs  introduced  into  the  body,  are  matters  of 
daily  clinical  experience. 

The  nerves  comprising  this  mechanism  and  the  relation  they  bear 
one  to  another  are  represented  in  Fig.  141. 

It  was  stated  in  a  previous  paragraph,  page  296,  that  the  con- 
traction of  the  heart-muscle  is  independent  of  its  connection  with 
the  central  organs  of  the  nerve  system,  and  that  it  will  continue  to 
contract  in  a  rhythmic  manner  for  a  variable  length  of  time  even 
after  its  removal  from  the  body  of  the  animal,  the  length  of  time 
varying  with  the  animal  and  the  conditions  to  which  it  is  subjected; 
that  the  stimulus  is  myogenic  in  origin  and  chemic  in  character,  the 


THE  CIRCULATION  OF  THE  BLOOD. 


3°9 


result  -of  '-.a^ reaction  between  the  chemic  constituents,  organic  and 
inorganic,  of  the  muscle-cells  and  those  in  the  lymph  by  which  they  are 
surrounded.  It  has  also  been  further  shown  that  even  in  the  living 
animal  the  .heart  will  continue  to  beat  and  fulfil  its  functions  after 

Emotional  Centers 

Exhilarating  (blue) 
Depressing  f#Eo) 


Cardio  -Inhibitor  Center. 


Ganglion  Stella  turn 


Intra-CardiacNerue  Cells\ 


Cardio  -Accelerator  Center 


VagasNerve 

Jef    „AExcitutor  (blue) 
fffennt  [Inhibitor  (red) 


Sympath  etic  Nerves 

Accelerator  <£  Aug  men  tor 


Fig.  141. — Diagram  of  the  Nerve  Mechanism  of  the  Heart. — (G.  Bachman.) 


division  of  all  nerves  in  connection  with  it.  A  dog  thus  experimented 
on  lived  for  eleven  months,  and  beyond  the  fact  of  becoming  fatigued 
more  readily  upon  exertion  than  formerly,  exhibited  no  striking  dis- 
turbance  of   his   functions.     Nevertheless   groups   of   nerve-cells   are 


3io  TEXT-BOOK  OF  PHYSIOLOGY. 

present  in  certain  portions  of  the  heart  in  all  classes  of  vertebrate 
animals,  which  bear  an  anatomic  and  physiologic  relation  to  the  heart- 
cells  on  the  one  hand,  and  to  the  nerves  connecting  them  with  the 
central  organs  of  the  nerve  system  on  the  other  hand. 

Intra-cardiac  Nerve-cells. — In  the  frog  heart  a  group  of  nerve- 
cells  is  found  in  the  sinus  at  its  junction  with  the  auricle,  known  as 
the  crescent  or  ganglion  of  Remak;  a  second  group  is  found  at  the 
base  of  the  ventricle  on  its  anterior  aspect,  and  known  as  the  gan- 
glion of  Bidder;  a  third  group  is  found  in  the  auricular  septum,  known 
as  the  septal  ganglion,  or  v.  Bezold's  or  Ludwig's.  The  majority  of 
the  cells  are  situated  on  the  surface  of  the  heart  just  beneath  the 
pericardium.  From  the  cell-body  fine  non-medullated  fibers  pass  into 
the  substance  of  the  heart,  to  become  histologically  and  physiologically 
related  with  the  muscle-fiber. 

In  the  dog  heart  and  in  the  mammalian  heart  generally,  though 
nerve-cells  are  present,  they  are  not  arranged  in  such  definite  groups, 
but  are  more  widely  distributed  in  the  terminations  of  the  venae  cava?, 
pulmonary  veins,  the  walls  of  the  auricles,  and  in  the  neighborhood 
of  the  base  of  the  ventricles. 

Extra-cardiac  Nerves. — The  extra-cardiac  nerves  which  connect 
the  heart  with  the  central  nerve  system  and  through  which  the  activities 
of  the  heart  are  influenced  are  two:  viz.,  the  sympathetic  and  the  vagus 
or  pneumogastric.  Experimental  investigation  has  established  the 
fact  that  the  sympathetic  is  the  motor  nerve  to  the  heart,  the  nerve 
which  accelerates  the  rate  and  augments  the  force  of  the  natural  beat; 
while  the  vagus  is  the  inhibitor  nerve,  the  nerve  which  inhibits  or  con- 
trols the  rate  and  the  force  of  the  beat  in  accordance  with  the  necessi- 
ties of  blood  distribution.  For  this  reason  these  two  nerves  will  be 
considered  in  the  following  order.  The  course  of  the  fibers  compos- 
ing these  nerves,  from  their  origin  to  their  termination,  and  the  relation 
they  bear  to  one  another  and  to  neighboring  structures,  vary  some- 
what in  different  animals. 

The  Origin  and  Distribution  of  the  Sympathetic  Nerves  in 
Mammals. — The  sympathetic  nerve-fibers  which  influence  the  action 
of  the  heart,  are  connected  on  the  one  hand  with  the  heart-muscle 
itself  and  on  the  other  hand  with  nerve-fibers  coming  from  the  central 
nerve  system.  The  former  are  non-medullated  and  post-ganglionic, 
the  latter  medullated  and  pre-ganglionic. 

The  pre-ganglionic  fibers  have  their  origin  in  the  medulla  oblongata 
and  very  probably  from  nerve-cells  in  the  gray  matter  beneath  the 
floor  of  the  fourth  ventricle.  From  this  origin  they  descend  the  spinal 
cord  as  far  as  the  level  of  the  second  and  third  thoracic  nerves.  At 
this  level  they  emerge  from  the  cord  in  company  with  the  nerve-fibers 
composing  the  anterior  roots  of  the  second  and  third  thoracic  nerves. 
After  a  short  course,  they  enter  the  white  rami  communicantes,  enter 
the  sympathetic  chain  and  pass  upward  to  the  ganglion  stellatum, 
and  by  way  of  the  annulus  of  Vieussens  to  the  inferior  cervical  ganglion 


THE  CIRCULATION  OF  THE  BLOOD.  311 

as  well  around  the  nerve-cells  of  which  their  terminal  branches  arborize. 
From  the  nerve-cells  of  both  the  stellate  and  inferior  cervical  ganglia, 
the  sympathetic  nerves  proper  arise  which  after  emerging  from  the 
ganglia  pass  towards  the  heart  and  become  associated  with  the  fibers 
of  the  vagus  and  assist  in  the  formation  of  the  cardiac  plexuses.  On 
reaching  the  heart  they  may  terminate  directly  in  the  muscle-cell 
or  indirectly  through  the  intermediation  of  intra-cardiac  nerve-cells. 
The  former  mode  of  termination  is  the  more  probable.  Experiment 
has  shown  that  both  the  pre-  and  post-ganglionic  fibers  are  efferent 
in  function. 

The  Origin  and  Distribution  of  the  Vagus  Nerve  in  Mammals. 
— The  vagus  nerve-fibers  which  influence  the  heart  are  connected  on 
the  one  hand  with  the  heart  through  the  intermediation  of  the  intra- 
cardiac cells,  and  on  the  other  hand  with  the  central  nerve  system. 
Histologic  investigation  has  shown  that  the  vagus  nerve-trunk  of  man 
and  mammals  generally,  contains  medullated  fibers  of  large  and  small 
size.  Experiment  has  shown  that  the  large  fibers  are  afferent,  the 
small  fibers  efferent  in  function. 

The  large  afferent  fibers  arise  in  the  ganglia  situated  on  the  trunk 
of  the  nerve.  From  their  contained  nerve-cells  a  short  axon  process 
proceeds  which  soon  divides  into  a  central  and  a  peripheral  branch. 
The  central  branch  passes  toward  and  into  the  gray  matter  beneath 
the  floor  of  the  fourth  ventricle  where  its  end-tufts  arborize  around 
nerve-cells;  the  peripheral  branch  passes  toward  the  general  periphery 
to  be  distributed  to  the  mucous  membrane  of  the  lungs,  stomach, 
intestine,  etc.  The  small  efferent  fibers  are  the  peripherally  coursing 
axons  of  nerve-cells  situated  in  the  gray  matter  beneath  the  floor  of 
the  fourth  ventricle  at  the  tip  of  the  calamus  scriptorius.  The  exact 
course  of  these  fibers  from  their  origin  into  the  trunk  of  the  vagus 
is  not  positively  known.  According  to  some  investigators,  they  leave 
the  medulla  by  way  of  the  spinal  accessory  nerve  and  enter  the  trunk 
of  the  vagus  through  its  internal  or  anastomotic  branch;  according  to 
recent  investigations  made  by  Schaternikoff  and  Friedenthal,  they 
leave  the  medulla  along  the  path  by  which  the  afferent  fibers  enter 
and  never  become  associated  with  the  spinal  accessory  nerve  at  its 
origin. 

In  the  neighborhood  of  the  inferior  or  recurrent  laryngeal  nerves, 
branches  containing  efferent  fibers  are  given  off,  which  pass  to  the 
heart  by  way  of  the  cardiac  plexus.  The  terminal  branches  of  these 
fibers  are  not  distributed  directly  to  the  heart-muscle,  but  to  the  intra- 
cardiac nerve-cells,  around  the  bodies  of  which  they  end  in  basket- 
like formations.  The  fibers  in  the  vagus  are  pre-ganglionic;  those 
of  the  nerve-cells  post-ganglionic.     (See  Fig.  142). 

In  the  frog  and  allied  animals  the  relation  of  these  two  sets  of 
nerve-fibers,  viz.,  the  efferent  sympathetic  fibers  and  the  efferent 
vagus  fibers,  is  somewhat  different;  and  because  of  the  fact  that  these 
nerves  in  this  animal  are  largely  employed  for  determining  experi- 


312 


TEXT-BOOK  OF  PHYSIOLOGY. 


thetic  Neuron 


mentally  their  respective  actions  on  the  heart,  this  relation  should  be 
clearly  understood. 

The  sympathetic  nerve-fibers  in  this  animal  are  also  in  connection 
with  the  heart  on  the  one  hand  and  with  nerve-fibers  coming  from 
the  central  nerve  system  on  the  other  hand.  The  pre-ganglionic 
fibers  take  their  origin  very  probably  in  nerve-cells  in  the  medulla 
oblongata.  From  this  origin  they  descend  and  emerge  from  the 
spinal  cord  in  the  anterior  roots  of  the  third  spinal  nerve,  then  pass 
through  the  white  rami  communicantes  to  the  third  sympathetic 
ganglion  around  the  nerve-cells  of  which  their  terminal  fibers  arborize. 
From  the  nerve-cells  of  this  ganglion,  the  sympathetic  nerves  proper, 
the    post-ganglionic,   non-meclullatecl    fibers  arise.     From  this  origin 

they  ascend,  passing  successively  through 
the  second  sympathetic  ganglion,  the 
annulus  of  Vieussens,  the  first  sympa- 
thetic ganglion,  to  the  ganglion  on  the 
trunk  of  the  vagus,  at  which  point  they 
enter  the  sheath  of  the  vagus  fibers  and 
in  company  with  them  pass  to  the  heart. 
For  this  reason  the  common  trunk  is 
generally  spoken  of  as  the  vagosympa- 
thetic nerve. 

The  vagus  nerve  is  connected  with 
the  medulla  oblongata  by  a  series  of  from 
six  to  eight  roots.  A  short  distance  from 
the  medulla,  the  nerve  trunk  passes 
through  a  large  opening  in  the  cranium 
beyond  which  it  presents  an  enlargement,  termed  the  vagus  ganglion. 
The  peripheral  end  of  this  ganglion  gives  off  two  trunks,  one  the 
glossopharyngeal,  the  other  the  vagus  proper. 

The  vagus  nerve  proper  in  the  frog  also  consists  of  both  afferent 
and  efferent  fibers  which  have  practically  the  same  origin,  distribution 
and  termination  as  the  corresponding  fibers  in  the  mammal. 

After  the  union  of  the  sympathetic  fibers  with  the  vagus  fibers,  the 
common  trunk  passes  forward  to  the  angle  of  the  jaw,  winds  around 
the  pharynx  just  beneath  the  border  of  the  petro-hyoid  muscle  and 
in  close  relation  with  the  carotid  artery.  As  the  nerve  approaches  the 
heart  it  divides  into  two  branches,  the  pulmonary  and  the  cardiac. 
At  the  sinus  venosus  some  of  the  fibers  become  related,  histologically 
and  physiologically,  with  the  ganglion  cells,  while  others  plunge  into 
the  heart,  course  along  the  auricular  septum  on  the  left  side  and  finally 
terminate  at  or  near  the  ganglion  cells  of  the  base  of  the  ventricle. 
'flic  mode  of  termination  of  both  the  vagus  and  sympathetic  fibers 
is  similar  to  that  observed  in  the  mammals. 

The  Physiologic  Actions  of  the  Sympathetic  Nerves  in  the 
Frog.  The  information  now  possessed  regarding  the  influence 
which   the  central   nerve  system   exerts  on  the   heart   through   these 


Fig.  142. — Diagram  showing 
the  Relation  of  the  Vagus  to 
the  Heart  Muscle-cell. 


THE  CIRCULATION  OF  THE  BLOOD.  313 

nerves,  has  been  derived  largely  from  experiments  made  on  the  nerves 
of  the  frog,  toad,  and  turtle.  Inasmuch  as  the  sympathetic  and  vagus 
nerves  in  the  frog  and  related  animals  are  bound  up  in  a  common 
sheath,  it  is  necessary  in  order  to  demonstrate  their  respective  functions 
to  first  divide  the  nerves,  before  their  union  at  the  vagus  ganglion,  and 


Fig.  143. — Tracings  Showing  the  Effects  on  the  Heart-beat  of  the  Frog  from 
Stimulation  of  the  Sympathetic  Nerves  Prior  to  Their  Union  with  the  Vagus 
Nerve.  The  upper  tracing  shows  an  increase  in  the  rate  which  before  stimulation  was  15 
per  minute  and  during  stimulation  30  per  minute.  Before  stimulation  the  height  of  the 
ventricular  beat  was  9  mm.  and  during  the  stimulation  it  was  12  mm.  The  lower  tracing 
shows  a  similar  series  of  effects,  the  differences  being  only  of  degree. — (Brodie.) 

then  stimulate  their  peripheral  ends.  The  heart  should  be  exposed 
and  attached  to  a  recording  lever  so  that  its  movements  may  be  taken 
up  and  recorded  on  a  moving  recording  surface. 

Stimulation  of  the  sympathetic  fibers  with  induced  electric  currents, 
prior  to  their  union  with  the  vagus,  is  followed  by  an  increase  in  the 
rate  or  an  augmentation  in  the  force  of  the  heart-beat  or  both  at  the 
same  time.  The  effects  of  such  a  stimulation  with  induced  currents 
of  moderate  intensity  are  graphically  shown  in  Fig.  143.  The  upper 
tracing  shows  that  the  heart  was  first  accelerated,  the  beats  increasing 
from  15  per  minute  before  stimulation,  to  30  per  minute  during  stim- 
ulation. On  the  cessation  of  the  stimulation,  the  heart  slowly  returned 
to  its  former  rate.  Coincidently  with  this  acceleration  of  the  rate  there 
was  an  augmentation  of  the  force  of  the  ventricular  contraction  as 
shown  by  an  increase  in  the  height  of  the  ventricular  contraction  which 
before  stimulation  was  9  mm.,  but  during  stimulation  12  mm. 

In  addition  to  the  foregoing  changes  in  the  heart-beat  there  is  an 
alteration  in  the  sequence  of  the  beat.  The  natural  delay  in  the  con- 
duction of  the  excitation  process  from  the  auricles  to  the  ventricle  is 


3i4  TEXT-BOOK  OF  PHYSIOLOGY. 

increased,  in  consequence  of  which  the  auricle  completely  relaxes  before 
the  ventricular  contraction  begins.  Moreover,  the  auricular  contrac- 
tion again  occurs  before  the  ventricle  has  completely  relaxed.  After 
the  effect  of  the  stimulation  passes  away,  the  acceleration  diminishes, 
the  augmentation  declines  and  a  reverse  change  in  the  sequence  occurs. 
The  lower  tracing  shows  a  similar  series  of  effects.  If  the  stimulus 
be  applied  to  the  pre-ganglionic  sympathetic  nerves,  an  acceleration  or 
augmentation  of  the  heart  follows,  similar  in  all  respects  to  that  which 
follows  stimulation  of  the  post-ganglionic  or  sympathetic  fibers  proper; 
and  the  inference  may  be  drawn  that  if  the  stimulus  could  be  applied 
directly  to  the  nerve-cells  in  the  medulla  oblongata  from  which  the  fibers 
take  their  origin,  the  same  acceleration  or  augmentation  would  follow; 
for  this  reason  this  collection  of  nerve-cells  is  known  as  the  cardio- 
accelerator  or  augmentor  center.  Since  stimulation  of  the  nerve  in  any 
part  of  its  course,  which  in  all  probability  exaggerates  its  normal  func- 
tion, is  followed  by  an  acceleration  or  an  augmentation,  the  sympa- 
thetic is  said  to  have  an  accelerator  or  an  augmentor  influence  on 
the  heart-beat;  with  the  cessation  of  the  stimulation,  and  very  fre- 
quently before,  the  heart  returns  to  its  normal  condition. 

The  Physiologic  Action  of  the  Vagus  Nerve  in  the  Frog. — Stimu- 
lation of  the  intra-cranial  roots  of  the  vagus  with  very  weak  induced  elec- 
tric currents  is  followed  by  a  gradual  diminution  in  the  rate  and  a  dim- 


Fig.  144. — Tracing  showing  the  Effect  on  the  Heart-beat  ot  the  Toad  of 
long  Stimulation  of  the  Intra-cranial  Roots  of  the  Vagus  with  Moderately 
strong  Electric  Currents. — (Caskell.) 

inution  in  the  force  of  the  heart-beat.  If  the  induced  currents  are 
moderate  in  strength,  the  heart  will  at  once  come  to  a  standstill  in 
diastole.  (Fig.  144.)  If  the  stimulus  be  applied  to  the  trunk  or  the 
peripheral  portion  of  the  vagus,  for  example  close  to  the  sinu-auricular 
junction,  an  inhibition  occurs  similar  in  all  respects  to  that  which 
follows  stimulation  of  the  intra-cranial  roots,  and  judging  from  what 
is  known  regarding  the  action  of  nerve  cells,  the  inference  may  be 
drawn  that  if  the  stimulus  could  be  applied  directly  to  the  group  of 
nerve-cells  from  which  the  efferent  fibers  arise,  the  same  inhibition 
would  follow;  for  this  reason  this  collection  of  nerve-cells  is  known 
as  the  car dio -inhibitor  center.  Since  stimulation  of  the  nerve,  either, 
at  its  center,  in  its  course,  or  at  its  periphery,  and  which  in  all  probability 
exaggerates  its  normal  function,  is  followed  by  a  period  of  rest  or 
inactivity,  the  vagus  is  said  to  have  a  retarding  or  an  inhibitor  in- 
fluence on  the  beat  of  the  heart. 

J  Airing    the   continuance  of    the   inhibition,   the   heart-muscle   is 


THE  CIRCULATION  OF  THE  BLOOD. 


3i5 


relaxed,  its    cavities   dilated  and  filled   with  blood.     The  dilatation 
usually  exceeds  that  observed  prior  to  the  vagus  stimulation. 

After  cessation  of  the  stimulation,  the  heart  resumes  its  activity. 
At  first  the  beat  usually  is  slow  and  feeble,  but  with  each  succeeding 
beat  both  rate  and  force  increase,  until  they  attain  or  exceed  that 
observed  prior  to  the  stimulation.  In  some  cases,  however,  the  heart 
begins  to  beat  with  as  much  and  even  more  vigor  than  it  did  prior  to 
the  stimulation.  The  duration  of  the  inhibitor  effect  varies  with  the 
duration  of  the  stimulation.  Thus  during  and  after  a  stimulation  of 
thirty-eight  seconds  the  heart  of  the  toad  remained  at  rest  for  292 
seconds  (Gaskell) ;  the  heart  of  a  snake  for  from  one-half  to  one  hour 


Fig.  14 : 


-Tracing  showing  the  Diminution  in  the  Rate  of  the  Heart-beat 
following  Weak  Tetanization  of  the  Vagus  Trunk. 


(Meyer) ;  the  heart  of  a  turtle  for  four  and  a  half  hours  (Mills) .  The 
period  of  inhibition  will  depend  on  the  strength  of  the  electric  current 
employed,  the  nerve  stimulated,  the  season  of  the  year,  etc. 

The  effects  on  the  heart-beat  which  will  follow  stimulation  of  the 
vago-sympathetic  in  its  course  vary,  however,  because  of  the  antagon- 
istic action  of  the  inhibitor  and  accelerator  nerve  impulses.  Thus 
stimulation  of  the  peripheral  end  of  the  divided  trunk  of  the  vagus  in 


Fig.  146.- 


-Tracing  showing  Complete  Inhibition  following  Strong  Tetan- 
ization of  the  Vagus  Trunk. 


the  frog  or  the  toad  with  weak  tetanizing  induced  electric  currents 
is  followed  by  an  increase  in  the  rate  of  the  heart-beat  because  of  the 
stimulation  of  the  accelerator  fibers  which  apparently  respond  before 
the  inhibitor  fibers;  stimulation  with  somewhat  stronger  currents  is 
followed  by  a  diminution  in  the  rate  of  the  beat  because  of  the  greater 
effect  on  the  inhibitor  nerve-fibers  (Fig.  145).  Stimulation  with 
strong  tetanizing  currents  is  followed  by  complete  inhibition  (Fig.  146). 
The  foreo;oin<r  facts  lead  to  the  inference  that  the  cardio-accelerator 


3i6  TEXT-BOOK  OF  PHYSIOLOGY. 

and  the  cardio-inhibitor  centers  have  as  their  function  the  discharge 
of  nerve  impulses  which  are  conducted  by  their  related  nerves,  the 
efferent  sympathetic  and  vagal  fibers,  to  the  heart,  and  which,  in  an  as 
vet  unexplained  manner,  accelerate  or  augment  or  inhibit,  the  action 
of  the  heart.  The  relation  which  these  two  centers  bear  one  to  the 
other  and  the  manner  in  which  they  are  influenced  in  their  activities 
both  directly  and  reflexly  and  thus  regulate  the  action  of  the  heart  from 
moment  to  moment  will  be  considered  in  a  subsequent  paragraph. 

Changes  in  the  Conductivity  of  the  Heart. — In  addition  to 
the  changes  in  the  rate  and  force  of  the  heart  caused  by  stimulation 
of  the  inhibitor  and  the  augmentor  nerves,  it  is  stated  by  Gaskell 
that  there  is  also  during  the  inhibition  a  decrease  in  the  conductivity 
of  the  heart  at  both  the  sinu-auricular  and  auriculo- ventricular  junc- 
tions, and  an  increase  in  the  conductivity  during  acceleration  of  the 
beat.  The  decrease  in  conductivity  may  be  so  pronounced  that  only 
every  second  or  third  contraction  of  the  auricle  will  be  followed  by  a 
contraction  of  the  ventricle.  In  other  instances  both  auricles  and 
ventricles  remain  at  rest  while  the  sinus  maintains  its  usual  rate. 
The  increase  in  conductivity  is  shown  by  first  artificially  blocking 
the  contraction  wave  at  the  auriculo-ventricular  junction  with  the 
clamp,  until  only  every  second  or  third  auricular  contraction  is  con- 
ducted to  the  ventricle,  and  then  stimulating  the  sympathetic.  At 
once  the  auricular  contraction  forces  the  block,  and  passes  to  the 
ventricle,  calling  forth  a  normal  contraction. 

The  Physiologic  Actions  of  the  Sympathetic  Nerves  in  Mam- 
mals.— In  the  mammal,  stimulation  of  the  sympathetic  nerves  in  any 
part  of  their  course,  either  through  the  rami  communicantes,  the  ven- 
tral portion  of  the  annulus  of  Vieussens,  or  after  their  emergence  from 
the  stellate  or  inferior  cervical  ganglia  is  followed  by  effects  similar  to 
those  observed  in  the  frog:  viz.,  an  acceleration  or  augmentation,  or 
both,  of  the  heart-beat.  The  percentage  increase  in  the  acceleration 
varies  in  different  animals.  In  some  instances  the  increase  varies 
from  58  per  cent,  to  100  per  cent.  (Hunt).  If  the  heart  is  beating 
slowly  before  stimulation,  the  acceleration  is  more  marked  than  if 
it  is  beating  rapidly. 

The  effect  of  the  accelerator  impulses  is  apparently  a  change  in 
the  inner  mechanism  of  the  heart-muscle  itself  and  not  to  a  change  in 
the  peripheral  portion  of  the  inhibitor  apparatus.  This  is  indicated 
by  the  fact  that  acceleration  occurs  after  the  full  physiologic  action  of 
atropin  which  acts  upon,  and  impairs  the  conductivity  of,  the  intra- 
cardiac nerve-cell  terminals. 

A  peculiarity  of  the  sympathetic  nerve  is  that  it  does  not 
respond  to  stimulation  as  rapidly  as  do  many  nerves,  so  that  a  rather 
Jong  latent  period  intervenes  between  the  moment  of  stimulation  and 
the  appearance  of  the  acceleration  as  shown  in  Fig.  147.  A  further  pe- 
culiarity is  that  the  acceleration  sometimes  continues  after  the  stim- 
ulus is  withdrawn,  and  sometimes  ceases  before  it  is  withdrawn. 


THE  CIRCULATION  OF  THE  BLOOD. 


3*7 


Though  an  increase  in  both  the  rate  and  force  frequently  occur 
simultaneously,  there  is  no  necessary  relation  or  connection  between  the 
two  as  they  can  and  do  occur  independently  of  each  other.  For  this 
reason  it  is  generally  assumed  that  the  sympathetic  nerves  contain 
two  groups  of  fibers,  viz.,  accelerators  and  augmentors,  the  functions 
of  which  are  to  respectively  accelerate  the  rate  and  augment  the  force 
of  the  heart-beat.  From  the  fact  that  both  auricles  and  ventricles 
exhibit  these  changes  it  is  assumed  that  the  nerve  impulses  stimulate 
both  chambers.  This  is  rendered  probable  also  from  the  experiments 
of  Erlanger,  who  found  that  after  complete  heart  block,  stimulation 
of  the  sympathetic  caused  independent  acceleration  of  both  auricles 
and  ventricles. 


VW*AMNY\MAi\N\n«v«*«ivi>flj«  /^'v',«^J^J\l^^f\J^J^f\/^^Vl^ 


Fig. 


-Acceleration  of  the  Heart  following  Stimulation  of  the  Cardiac 
Branches  which  come  from  the  Annulus  of  Vieussens. 


The  Physiologic  Action  of  the  Vagus  Nerve  in  Mam- 
mals.— In  the  mammal  the  same  or  similar  effects  result  from 
stimulation  of  the  vagus  as  in  the  frog.  If  the  thorax  of  the 
dog  is  opened  and  artificial  respiration  maintained  the  heart  will 
continue  to  beat  in  a  practically  normal  manner  for  a  long  time. 
Under  such  conditions  if  the  vagus  nerve  on  one  side  be  divided 
and  its  peripheral  end  stimulated  with  induced  electric  currents  of 
moderate  strength  the  heart  will  be  seen  to  come  to  a  standstill  almost 
immediately  in  the  condition  of  diastole,  and  may  be  so  kept  for  a 
variable  period,  from  fifteen  to  thirty  seconds  or  more,  during  which 
its  walls  are  relaxed  and  its  cavities  filled  with  blood.  On  cessation  of 
the  stimulation  the  contractions  return  and  in  a  very  short  time  the 
former  rate  and  force  of  the  beat  are  regained.  If  the  electric  currents 
are  of  feeble  strength,  the  heart  will  come  to  rest  gradually  through  a 
gradual  diminution  in  the  rate  and  force  of  the  contraction.  During 
the  period  of  the  inhibition  the  heart  presents  an  appearance  similar  to 
that  presented  by  the  heart  of  the  cold-blooded  animal.  When  the 
heart  of  an  animal  is  thus  exposed,  the  auricle  and  the  ventricle  of 
one  side  may  be  attached  by  threads  to  writing  levers  and  their  contrac- 
tions registered  on  a  moving  recording  surface.  The  effects  on  both 
auricles  and  ventricles  which  follow  vagus  stimulation  will  then  become 
more  apparent.  Fig.  148  is  a  tracing  thus  obtained.  The  animal 
employed  was  a  rabbit. 

The  inhibitor  effect  of  the  vagus  varies  in  degree  and  duration 
in  different  animals.  In  the  dog  the  effect  of  vagus  stimulation  is 
usually  pronounced,  lasting  from  15  to  30  seconds;  in  the  rabbit  it  is 
perhaps  equally  well  pronounced  but  somewhat  less  in  duration;  in 
the  cat  it  is  almost  wanting.     In  this  latter  animal  a  complete  standstill, 


3i« 


TEXT-BOOK  OF  PHYSIOLOGY. 


even  for  a  few  seconds,  is  very  rarely  seen;  usually  there  is  produced 
merely  a  slight  diminution  in  the  rate  of  the  beat  even  though  the  stim- 
ulus employed  is  quite  strong.  In  all  these  animals,  however,  after 
a  very  short  time  the  nerve  impulses  lose  their  inhibitor  influence  on 


Fig.  14S. 


-Result  of  the  Stimulation  of  the  Peripheral  End  of  the 
Divided  Left  Vagus  in  the  Rabbit. — (Brodie.) 


the  heart-muscle,  and  notwithstanding  continued  stimulation  of  the 
vagus,  the  heart  returns  to  its  former  rate  and  vigor.  This  result  is 
in  marked  contrast  to  that  observed  during  stimulation  of  the  vagus 
in  the  cold-blooded  animals,  in  which  the  heart  may  be  kept  at  rest 
for  relatively  very  long  periods  of  time.     No  satisfactory  explanation 


THE  CIRCULATION  OF  THE  BLOOD.  319 

for  this  loss  of  vagus  control  or  escape  of  the  heart  from  the  vagus 
control  has  as  yet  been  offered. 

Seat  of  Action  of  the  Vagus  Impulses.— In  a  foregoing  experi- 
ment of  which  Fig.  148  is  a  graphic  result,  stimulation  of  the  left  vagus 
with  a  fairly  strong  current  was  followed  by  a  diminution  in  both  the 
rate  and  force  of  the  contraction  of  both  auricles  and  ventricles,  though 
the  effect  was  most  marked  in  the  auricles.  From  this  and  similar 
facts  it  has  come  to  be  the  general  belief  that  the  inhibitor  nerve  im- 
pulses exert  their  influence  mainly,  if  not  exclusively,  on  the  auricle, 
and  that  the  cessation  of  ventricular  action  is  a  secondary  effect  due 
to  the  non-arrival  across  the  conducting  apparatus  of  the  normal 
excitation  process  from  the  auricle.  This  is  the  case  undoubtedly  in 
the  cold-blooded  animals,  and  the  experiments  of  Erlanger  on  the  heart 
of  the  dog  indicate  that  the  same  holds  true  for  the  mammals.  This 
investigator  has  found  that  when  the  auriculo-ventricular  tissues  are 
suddenly  clamped,  including  presumably  the  muscle  band  of  His,  there 
is  for  a  time  a  complete  cessation  of  ventricular  activity,  but  after  a 
variable  period  of  time,  fifty  seconds  or  more,  the  ventricle  develops 
an  independent  rhythm  which  gradually  increases  in  frequency,  but 
seldom  ,  if  ever,  attains  that  of  the  auricles.  Under  such  circumstances 
tetanic  stimulation  of  the  auriculo-ventricular  tissues  by  means  of  the 
clamp  now  transformed  into  stimulating  electrodes,  failed  to  bring 
about  a  stoppage  of  the  ventricles.  Moreover,  if  during  the  time  the 
clamp  is  applied  and  after  the  ventricle  has  developed  a  rhythm  of  its 
own,  the  vagus  is  stimulated,  the  auricles  will  cease  to  beat  as  usual, 
but  the  ventricles  will  continue  to  beat  at  their  usual  rate.  These  and 
similar  facts  lead  to  the  conclusion  that  vagal  inhibitor  action  is  lim- 
ited to  the  auricles. 

From  the  foregoing  facts  it  is  apparent  that  the  accelerator  and 
augmentor  effects  of  the  sympathetic  nerve  impulses,  and  the  inhibitor 
effects  of  the  vagus  nerve  impulses,  closely  resemble  on  the  one  hand 
the  accelerator  and  augmentor  effects  of  increasing  amounts  of  diffusible 
calcium  salts,  and  on  the  other  hand  the  inhibitor  effects  of  increasing 
amounts  of  diffusible  potassium  salts  in  the  blood  or  other  circulating 
fluid;  and  so  closely  do  these  two  sets  of  phenomena  resemble  each 
other,  that  they  are  by  some  observers  regarded  as  identical. 

Some  additional  facts  in  this  connection  have  been  presented  by 
Howell,  viz.,  that  an  increase  (within  limits)  and  a  decrease  in  the 
percentage  of  diffusible  calcium  salts  in  a  circulating  fluid  passing 
through  the  cavities  of  the  mammalian  (cat)  heart,  increases  on  the 
one  hand,  and  decreases  on  the  other  hand,  the  sensitiveness  of  the 
heart  to  sympathetic  acceleration  and  augmentation.  From  this  the 
inference  is  deduced  that  the  acceleration  and  augmentation  of  the 
heart-beat  which  follow  stimulation  of  the  sympathetic  nerves  are 
due  to  the  presence  in  the  heart  tissue  of  a  certain  percentage  of 
diffusible  calcium  salts,  which  have  been  freed  from  combination  with 
organic   matter  by  the  action    of    the    sympathetic    nerve    impulses. 


32o  TEXT-BOOK  OF  PHYSIOLOGY. 

Again,  that  an  increase  (within  limit)  and  a  gradual  decrease  in  the 
percentage  of  diffusible  potassium  salts  in  a  circulating  fluid  passing 
through  the  cavities  of  the  frog  and  the  cat  heart,  increases  on  the  one 
hand  and  decreases  and  finally  abolishes  on  the  other  hand  the  sensi- 
tiveness of  the  heart  to  vagus  inhibition.  From  this  the  inference  is 
deduced  that  the  inhibition  of  the  heart-beat  which  follows  stimulation 
of  the  vagus  nerve  is  due  to  the  presence  in  the  heart  tissue  of  a  certain 
percentage  of  diffusible  potassium  salts,  which  have  been  freed  from 
combination  with  organic  matter  by  the  action  of  the  vagus  nerve 
impulses. 

The  Cardio-accelerator  Center. — The  collection  of  nerve-cells 
from  which  the  pre-ganglionic  fibers  of  the  sympathetic  system  arise 
is  known  as  the  cardio-accelerator  or  augmentor  center.  The  exact 
location  of  this  center  in  the  central  nerve  system  has  not  been  as 
yet  accurately  determined.  It  is"  probably  located  in  the  medulla 
oblongata. 

From  experiments  which  have  been  made  on  the  sympathetic 
nerve  apparatus  in  its  entirety,  it  is  believed  that  the  function  of  this 
center  is  the  discharge  of  nerve  impulses  which,  conducted  to  the  heart 
by  the  pre-ganglionic  and  post-ganglionic  sympathetic  fibers,  cause  an 
acceleration  in  the  rate  or  an  augmentation  in  the  force,  or  both,  of 
the  heart-beat.  It  is  also  generally  believed  since  the  publication 
of  Hunt's  investigations  that  this  center  is  in  a  state  of  tonic  activity. 
This  is  shown  by  the  fact  that  after  the  division  of  the  vagus  nerves 
and  the  removal  of  all  inhibitor  influences,  division  of  the  sympa- 
thetic nerves  or  extirpation  of  the  stellate  or  inferior  cervical  ganglion, 
is  yet  followed  by  a  decrease  in  the  rate  of  the  heart-beat.  After 
division  of  the  sympathetic  nerves  and  the  removal  of  accelerator 
influences  it  is  also  easier  to  bring  about  inhibition  through  vagus 
stimulation. 

The  Factors  which  Determine  the  Activity  of  the  Cardio-ac- 
celerator Center. — The  question  has  been  raised  as  to  whether  the 
tonic  activity  of  this  center  is  maintained  by  central  or  peripheral 
stimuli,  i.  e.,  whether  it  is  maintained  by  causes  within  itself,  the 
result  of  an  interaction  between  the  constituents  of  the  cell  substance 
and  those  of  the  surrounding  lymph,  or  whether  it  is  maintained  by 
nerve  impulses  reflected  to  it  through  various  afferent  or  sensor  nerves. 
Inasmuch  as  there  is  no  way  of  determining  whether  the  causes  are 
central,  except  by  dividing  all  afferent  nerves,  it  is  impossible  to  state 
how  much  influence  is  to  be  attributed  to  this  factor.  On  the  contrary, 
though  it  is  readily  demonstrable  that  stimulation  of  many  afferent 
nerves  will  cause  an  acceleration  of  the  heart  it  can  not  be  stated 
positively  that  this  is  the  result  of  a  reflex  stimulation  of  the  accelerator 
renter.  Though  earlier  investigators  believed  this  to  be  the  correct 
interpretation,  the  more  recent  experiments  of  Hunt  apparently  dis- 
prove it;  for  this  investigator  has  shown  that  if  the  vagus  nerves  are 
divided   it  is  impossible  to  produce  reflex  acceleration  of  the  heart. 


THE  CIRCULATION  OF  THE  BLOOD.  321 

His  conclusion,  confirming  that  of  others,  is  that  cardiac  acceleration 
is  the  result  of  an  inhibition  of  the  cardio-inhibitor  center.  A  freer  play 
to  the  tonic  activity  of  the  accelerator  center  would  thus  be  made  pos- 
sible. 

The  Cardio-inhibitor  Center. — The  collection  of  nerve-cells 
from  which  the  small  efferent  fibers  of  the  vagus  nerve  arise  is  known 
as  the  cardio-inhibitor  center.  It  is  situated  in  the  medulla  oblongata 
and  more  especially  in  the  gray  matter  beneath  the  floor  of  the  fourth 
ventricle  near  the  tip  of  the  calamus  scriptorius.  It  is  in  all  prob- 
ability, a  part  of  the  nucleus  ambiguus. 

From  the  experiments  which  have  been  made  on  the  vagus  inhibitor 
apparatus  in  its  entirety  it  is  believed  that  the  function  of  this  center 
is  the  discharge  of  nerve  impulses,  which  conducted  to  the  heart  by 
the  vagus  fibers  cause  an  inhibition  of  its  beat  of  greater  or  less  extent. 
In  the  dog,  and  probably  in  many  other  mammals,  this  center  exerts 
a  more  or  less  constant  inhibitor  or  restraining  influence  on  the  heart's 
activity.  This  is  indicated  by  the  fact  that  the  rate  of  the  beat  is 
very  much  increased  by  simultaneous  division  of  both  vagi.  The 
degree  of  the  inhibition  which  this  center  exerts  varies  greatly,  however, 
in  different  animals.  In  the  cat  and  in  the  rabbit  the  inhibitor  control 
is  normally  so  slight  that  there  is  but  a  relatively  slight  increase  in  the 
rate  of  the  beat  after  division  of  the  vagi.  The  tone  of  the  vagus  in 
these  animals  is,  therefore,  said  to  be  slight  or  feeble.  In  human 
beings  the  tone  of  the  inhibitor  apparatus  is  poorly  developed  in  earlv 
childhood,  as  shown  by  the  fact  that  the  administration  of  atropim 
which  removes  temporarily  inhibitor  control  is  not  followed  bv  an 
increase  in  the  rate  of  the  beat.  It  develops  steadily  and  reaches 
a  maximum  at  from  the  twenty-fifth  to  the  thirtieth  year.  In  ad- 
vanced years  the  tone  again  declines.  For  these  and  other  reasons  it 
is  believed  that  this  center  is  in  a  state  of  tonic  activity  in  many  if 
not  all  mammals,  discharging  nerve  impulses  which  exert  a  regulative 
influence  on  the  cardiac  mechanism  in  accordance  with  its  needs  and 
especially  in  reference  to  the  variable  resistances  offered  to  the  flow  of 
blood  which  the  heart  must  overcome. 

The  Factors  which  Determine  the  Activity  of  the  Cardio- 
inhibitor  Center. — The  question  has  also  been  raised  as  to  whether  the 
tonic  activity  of  this  center  is  maintained  by  central  or  peripheral 
stimuli,  i.  e.,  whether  it  is  maintained  by  causes  within  itself  the  result 
of  an  interaction  between  the  constituents  of  the  cell  substance  and 
those  of  the  surrounding  lymph,  or  whether  it  is  maintained  bv  nerve 
impulses  reflected  to  it  through  various  afferent  or  sensor  nerves. 
Though  both  factors  play  an  important  part  in  the  maintenance  of  its 
activity,  the  trend  of  evidence  points  to  the  conclusion  that  the  reflected 
impulses  are  by  far  the  more  important  of  the  two.  This  latter  sup- 
position is  supported  by  the  results  of  direct  experimentation  upon 
sensor  nerves  in  almost  any  region  of  the  body.  Thus  stimulation 
of  the  dorsal  roots  of  the  spinal  nerves,  the  trunks  of  the  cranial  sensor 


322  TEXT-BOOK  OF  PHYSIOLOGY. 

nerves,  the  splanchnic  nerves,  the  pulmonary  branches  of  the  vagus, 
etc.,  gives  rise  to  a  more  or  less  pronounced  inhibition  of  the  heart. 
As  a  rule,  stimulation  of  the  peripheral  terminations  of  these  nerves  is 
more  effective  than  stimulation  of  their  trunks,  hence  an  explanation 
is  at  hand  for  the  cardiac  inhibition  which  results  from  sudden  disten- 
tion of  the  stomach  and  intestines,  or  operative  procedures  in  the  nose, 
mouth,  and  larynx. 

Reflex  inhibition  of  the  heart,  even  to. the  stage  of  absolute  and 
permanent  standstill,  eventuating  in  the  death  of  the  individual  is 
a  not  infrequent  result  of  peripherally  acting  causes  of  a  pathologic 
or  operative  character.  From  the  results  of  experimental  procedures 
the  inference  is  drawn  that  normally,  nerve  impulses,  developed  by 
the  action  of  physiologic  causes,  are  reflected  continuously  from  many 
peripheral  regions  of  the  body,  which  falling  into  this  center  gently 
stimulate  and  maintain  it  in  a  condition  of  necessary  tonicity  or  activ- 
ity. 

The  Causes  of  the  Variations  in  the  Heart-beat. — It  has  been 
stated  elsewhere  in  the  text  (page  286),  that  the  rate  of  the  heart-beat 
is  influenced  by  age,  muscle  activity,  the  position  of  the  body,  meals, 
variations  in  blood  pressure,  etc.  In  addition  to  these  factors  there  is 
abundant  evidence  that  other  factors,  e.g.,  the  action  of  peripheral 
stimuli  of  a  physiologic  or  pathologic  character  in  various  regions  of 
the  body,  can  and  do  cause  reflexly  at  one  time  or  in  one  individual 
an  acceleration  of  a  marked  character,  and  at  another  time  or  in 
another  or  the  same  individual  an  inhibition  which  may  be  so  pro- 
nounced as  to  lead  to  a  complete  standstill  in  diastole.  The  records 
of  clinical  medicine  contain  many  instances  which  show  that  gastric, 
intestinal,  uterine  and  other  organic  disorders  as  well  as  various  opera- 
tive procedures  in  different  regions  of  the  body  cause  now  an  accelera- 
tion, now  an  inhibition  of  the  heart. 

The  first  explanation,  that  acceleration  of  the  heart,  the  result  of  a 
peripherally  acting  stimulus,  is  due  to  a  stimulation  of  the  cardio- 
accelerator  center  by  the  arrival  of  nerve  impulses  coming  through 
afferent  nerves,  having  been  made  questionable  and  improbable  by  the 
results  of  Hunt's  experiments,  the  alternate  explanation  must  be,  that 
the  acceleration  is  due  to  an  inhibition  of  the  normal  activity  of  the 
cardio-inhibitor  center,  and  that  inhibition  is  due  to  an  excitation  of 
the  normal  activity  of  the  cardio-inhibitor  center,  and  hence  there  fol- 
lows the  corollary  that  afferent  nerves  contain  two  sets  of  nerve-fibers 
which  are  in  physiologic  relation  with  the  cardio-inhibitor  center,  one 
of  which  when  stimulated  peripherally  inhibits  its  activity,  the  other 
of  which  when  stimulated  excites  or  augments  its  activity. 

The  extent  to  which  both  sets  of  fibers  are  present  in  any  one 
afferent  nerve  is  unknown.  In  the  trigeminus  it  is  believed  the  ex- 
citator  fibers  preponderate  for  the  reason  that  peripheral  stimulation 
of  this  nerve  is  followed  by  inhibition  of  the  heart;  in  the  sciatic,  it  is 
believed  the  inhibitor  nerves  preponderate,  for  the  reason  that  stimu- 


THE  CIRCULATION  OF  THE  BLOOD.  323 

lation  of  the  central  end  of  the  divided  nerve  is  followed  generally  by 
acceleration  of  the  heart. 

It  is  probable  from  the  effects  which  follow  gastro-intestinal  dis- 
orders, that  the  vagus  nerve  contains  both  classes  of  fibers  as  repre- 
sented in  Fig.  141,  inasmuch  as  stimuli  of  a  pathologic  character  in 
one  individual  may  reflexly  excite  or  increase  the  activity  of  the  cardio- 
inhibitor  center,  to  be  followed  by  an  inhibition  of  the  heart;  and  in 
another  individual,  may  reflexly  inhibit  the  activity  of  the  same  center 
and  to  such  an  extent  that  the  cardio-accelerator  center  may  be  enabled 
to  increase  either  the  rate  or  the  force  or  both,  of  the  heart  move- 
ments. Palpitation  of  the  heart  from  gastric  irritation  might  thus  be 
explained. 

The  Influence  of  Psychic  States.— The  cardio-inhibitor  and 
the  cardio-accelerator  centers  may  be  increased  in  activity  also  by 
nerve  impulses  descending  from  the  cerebrum,  the  result  of  emo- 
tional states;  thus  depressing  emotions  according  to  their  intensity 
may  so  increase  the  activity  of  the  cardio-inhibitor  center,  that  the 
heart's  action  may  not  only  be  retarded  but  even  completely  inhibited; 
joyous  emotions,  on  the  contrary,  may  so  increase  the  activity  of  the 
cardio-accelerator  center  or  what  is  more  probable  inhibit  the  activity 
of  the  cardio-inhibitor  center  that  the  heart's  action  will  be  increased 
in  both  its  rate  and  force. 

From  the  results  of  stimulation  of  the  sympathetic  (accelerator)  and 
vagus  (inhibitor)  nerves  under  a  great  variety  of  conditions  it  has  been 
established  that  their  respective  centers  are  mutually  antagonistic; 
that  the  activity  of  the  accelerator  center  at  one  moment  limits  the  ac- 
tivity of  the  inhibitor  and  at  another  moment  is  limited  in  turn  by  it; 
that  the  rate  of  the  heart-beat  at  each  moment  is  the  resultant  of  the 
relative  degree  of  activity  of  the  two  centers. 

The  Depressor  Nerve.— The  vagus  trunk  also  contains  afferent 
fibers  stimulation  of  which  not  only  brings  about  a  reflex  inhibition 
of  the  heart,  but  also  a  dilatation  of  the  peripheral  arteries  and  a  fall 
of  blood-pressure  through  a  depressive  influence  on  the  vaso-motor 
centers.  To  this  nerve  the  term  depressor  has  been  given.  A  con- 
sideration of  the  physiologic  action  of  this  nerve  will  be  found  in  the 
section  devoted  to  the  nerve  mechanisms  concerned  in  the  mainte- 
nance of  the  blood-pressure. 

Modifications  of  the  Nerve  Mechanism  of  the  Heart  due 
to  the  Physiologic  Action  of  Drugs.— The  functions  of  different 
parts  of  the  nerve  mechanism  of  the  heart  may  be  demonstrated  by  an 
analysis  of  the  effects  which  follow  the  administration  of  slightly  toxic 
doses  of  the  alkaloids  of  various  drugs.  The  effects  can  be  shown  to  be 
due  to  a  stimulation  or  to  a  depression  of  the  normal  activity  of  one  or 
more  portions  of  the  mechanism.  The  alkaloid  may  exert  its  specific 
action  on  the  central  portions  in  the  medulla,  or  on  the  peripheral 
portions  in  the  heart,  or  on  both  simultaneously.  The  heart- mus- 
cle  may  at   the   same  time  be  stimulated  or  depressed  in  its  action 


324  TEXT-BOOK  OF  PHYSIOLOGY. 

either  in  the  same  or  in  the  opposite  direction  to  that  of  the  nerve  mech- 
anism. As  a  result  the  heart-beat  may  be  increased  or  decreased  both 
in  rate  and  force. 

The  following  examples  will  illustrate  the  action  of  alkaloids  in 
general. 

Atropin. — After  the  administration  of  atropin  in  sufficient  amounts 
the  heart-beat  increases  in  frequency  in  all  animals  in  which  the  cardio- 
inhibitor  centers  exert  a  steady  inhibitor  influence  over  the  heart. 
This  is  especially  true  in  man  and  the  dog.  In  animals  in  which  the 
inhibitor  control  is  slight,  as  the  rabbit  and  frog,  the  increase  in  fre- 
quency is  not  very  marked.  In  all  animals  thus  far  experimented 
on  after  the  administration  of  atropin,  neither  stimulation  of  the 
trunk  of  the  vagus  nor  stimulation  of  the  intra-cardiac  ganglia  will 
arrest  or  even  retard  the  heart-beat.  The  inference,  therefore,  is  that 
the  alkaloid  exerts  its  action  upon  the  ganglion  cells  and  their  ter- 
minal branches,  impairing  their  chemic  integrity  and  abolishing  their 
normal  function,  that  of  conducting  nerve  impulses  from  the  vagus 
nerve  proper  to  the  heart-muscle.  In  consequence  of  this,  the  influ- 
ence of  the  cardio-inhibitor  center  is  cut  off  and  the  cardio-accelerator 
being  unopposed  in  its  activity,  the  rate  of  the  beat  is  increased. 
After  a  variable  period  the  heart  returns  to  its  normal  rate.  Stimu- 
lation of  the  vagus  is  again  followed  by  the  usual  inhibition.  As 
atropin  is  partly  oxidized,  and  partly  excreted,  it  is  assumed  that  the 
nerve  terminals  have  been  restored  by  nutritive  forces  to  their  normal 
condition  and  their  conductivity  regained.  This  having  been  accom- 
plished the  vagus  nerve  impulses  can  again  reach  the  heart-muscle  and 
the  cardio-inhibitor  center  is  therefore  enabled  to  re-establish  inhibitor 
control  and  antagonize  the  activity  of  the  cardio-accelerator  center. 

Nicotin. — After  the  administration  of  nicotin  in  sufficient  amounts 
the  heart-beat  is  primarily  decreased  in  frequency  even  to  the  point  of 
standstill  in  diastole  for  a  few  seconds,  and  secondarily  increased  both 
in  frequency  and  force  beyond  the  normal.  If  the  vagus  nerves  be 
first  divided  this  primary  decrease  is  not  so  marked  and  the  inference 
is  that  the  alkaloid  primarily  stimulates  the  cardio-inhibitor  center  and 
increases  its  normal  function  and  perhaps  the  terminal  branches  of  the 
vagus  fibers,  the  pre-ganglionic,  as  well.  After  the  secondary  in- 
crease in  the  rate  is  established  stimulation  of  the  vagus  trunk  fails  to 
inhibit  the  heart,  though  stimulation  of  the  intra-cardiac  ganglia  is  at 
once  followed  by  the  usual  inhibitor  phenomenon,  arrest  of  the  heart  in 
diastole.  For  this  reason  it  is  believed  that  nicotin  acts  on  the  periph- 
eral terminations  of  the  pre-ganglionic  fibers  of  the  vagus  as  they 
arborize  around  the  intra-cardiac  ganglia,  depressing  them  and  sus- 
pending their  normal  function,  that  of  conducting  nerve  impulses  from 
the  vagus  to  the  ganglion  cells.  Since  stimulation  of  the  pre-ganglionic 
fibers  of  the  accelerator  apparatus  fails  to  accelerate  the  rate  of  the 
heart-beat,  though  stimulation  of  the  post-ganglionic  fibers  has  the 
usual  accelerating  effect,  the  inference  is  that  nicotin  acts  upon  and 


THE  CIRCULATION  OF  THE  BLOOD.  325 

suspends  the  conductivity  of  their  terminal  branches  in  the  ganglia. 
The  acceleration  of  the  heart  must  therefore  be  attributed  either  to  a 
stimulation  of  the  post-ganglionic  fibers  or  of  the  cardiac  muscle 
itself  (Cushney). 

Pilocarpin  and  Muscarin. — These  alkaloids,  whether  adminis- 
tered internally  or  applied  locally  to  the  heart,  diminish  the  frequency 
and  the  force  of  the  beat  to  such  an  extent  that  it  very  shortly  comes 
to  rest  in  diastole.  For  the  reason  that  the  internal  administration  or 
the  local  application  of  atropin  in  proper  doses,  which  has  a  depressive 
action  on  the  intra-cardiac  cell  terminations,  removes  the  inhibition 
and  restores  the  normal  rhythm,  the  inference  is  drawn  that  both  these 
alkaloids  either  increase  the  irritability  of  the  nerve-cells  or  heighten 
the  conductivity  of  their  terminal  fibers.  The  return  of  the  heart-beat 
is  attributed  to  a  decline  in  irritability  to  the  normal  level  in  conse- 
quence of  the  antagonistic  action  of  the  atropin. 

Digitalin.— -The  administration  of  digitalin  gives  rise  to  effects, 
the  character  and  extent  of  which  vary  in  different  animals.  In  the  frog, 
as  a  rule,  the  only  effect  produced  is  a  gradual  increase  in  the  duration 
and  force  of  the  ventricular  systole,  with  a  corresponding  decrease  in 
the  duration  of  the  diastole,  until  the  heart  comes  to  rest  in  the  systolic 
state.  As  this  effect  is  observed  after  division  of  the  vagus  trunk 
and  also  after  the  suspension  of  the  activity  of  the  intra-cardiac  cell- 
fibers  by  atropin,  it  is  evidently  due  to  a  direct  stimulation  of  the  heart- 
muscle.  In  some  instances,  however,  the  opposite  effect  is  produced, 
viz.,  a  gradual  increase  in  the  length  of  the  diastole,  a  decrease  in  the 
duration  of  the  systole,  until  the  heart  comes  to  rest  in  the  diastolic 
state.  As  this  effect  only  arises  when  the  vagus  nerve  is  intact  it  is 
very  probably  due  to  a  stimulation  of  the  cardio-inhibitor  center  and 
a  consequent  increase  of  its  functional  activity.  Though  either  effect 
may  be  produced  in  the  frog  the  predominant  effect  is  the  increase  in 
the  contraction  of  the  heart-muscle  rather  than  an  inhibition  of  the  beat. 

In  mammals  both  effects  are  observed,  viz.,  a  diminution  in  the 
rate  of  the  beat,  a  lengthening  of  the  diastole  and  an  increase  in  the 
vigor  of  the  systole,  which  are  evidently  due  to  a  simultaneous  stim- 
ulation of  the  cardio-inhibitor  center  and  of  the  cardiac  muscle. 
Digitalin  thus  expends  itself  on  two  opposing  mechanisms;  as  to 
which  gains  the  ascendency  will  depend  on  the  dosage  and  the  character 
of  the  animal. 


CHAPTER  XIV. 
THE  CIRCULATION  OF  THE  BLOOD  (Continued). 

THE  VASCULAR  APPARATUS:  ITS  STRUCTURE  AND  FUNCTIONS. 

The  systemic  vascular  apparatus  consists  of  a  closed  system  of 
vessels  extending  from  the  left  ventricle  to  the  right  auricle,  and  in- 
cludes the  arteries,  capillaries,  and  veins.  Though  serving  as  a  whole 
to  transmit  blood  from  the  one  side  of  the  heart  to  the  other,  each  one 
of  these  three  divisions  has  separate  but  related  functions,  which  are 
dependent  partly  on  differences  in  structure  and  physiologic  properties, 
and  partly  on  their  relation  to  the  heart  and  its  physiologic  activities. 

The  Structure,  Properties  and  Functions  of  the  Arteries. — 
The  arteries  serve  to  transmit  the  blood  ejected  from  the  heart  to  the 
capillaries;  that  this  may  be  accomplished  they  divide  and  subdivide 
and  ultimately  penetrate  each  and  every  area  of  the  body.  Their 
repeated  division  is  attended  by  a  diminution  in  size,  a  decrease  in  the 
thickness  and  a  change  in  the  structure  of  their  walls. 

A  typical  artery  consists  of  three  coats:  an  internal,  the  tunica 
intima;  a  middle,  the  tunica  media;  an  external,  the  tunica  adventitia. 

The  internal  coat  consists  of  a  structureless  elastic  basement  mem- 
brane, on  the  inner  surface  of  which  rests  a  layer  of  elongated  spindle- 
shaped  endothelial  cells.  The  middle  coat  consists  of  several  layers  of 
circularly  disposed,  non-striated  muscle-fibers,  between  which  are 
networks  of  elastic  fibers.  The  external  coat  consists  of  bundles  of 
connective  tissue  of  the  white  fibrous  and  yellow  elastic  varieties. 
Between  the  external  and  middle  coats  there  is  an  additional  elastic 
membrane.  In  the  small  arteries  there  is  but  a  single  layer  of  muscle- 
fibers.  In  the  large  arteries  the  elastic  tissue  is  very  abundant,  ex- 
ceeding largely  in  amount  the  muscle-tissue.  It  is  also  more  closely 
and  compactly  arranged.  The  external  coat  is  well  developed  in  the 
large  arteries  (Figs.  149  and  150). 

In  virtue  of  the  presence  in  their  walls  of  both  elastic  and  con- 
tractile elements,  the  arteries  possess  the  two  properties  of  elasticity 
and  contractility. 

The  elasticity  is  especially  well  developed  in  the  large  arteries,  which 
are  capable,  therefore,  of  both  distention  and  elongation,  and,  when  the 
distending  force  is  withdrawn,  of  returning  to  their  previous  condition. 
The  elasticity  permits  of  a  wide  variation  in  the  amount  of  blood  the 
arterial  system  can  hold  between  its  minimum  and  maximum  disten- 
tion. Thus  the  capacity  of  the  aorta  and  carotid  artery  of  the  rabbit 
can  be  increased  four  times  and  six  times  respectively  by  raising  the  in- 

326 


THE  CIRCULATION  OF  THE  BLOOD. 


327 


b 


tra-arterial  pressure  from  o  to  200  mm.  of  mercury.  The  elasticity 
also  converts  the  intermittent  movement  of  the  blood  imparted  to  it 
by  the  heart  as  it  is  ejected  from  the  ventricle,  into  a  remittent  move- 
ment in  the  arteries  and  finally  into  the  continuous  and  equable 
movement  observed  in  the  capillaries.  This  is  accomplished  in  the 
following  manner:  With  each  contraction  of  the  left  ventricle  more 
blood  is  ejected  into  the  aorta  than  the  arteries  can  discharge 
into  the  capillaries  and  veins  during  the  time  of  the  contraction.  The 
portion  not  so  discharged  exerts  a  lateral 
pressure  against  the  walls  of  the  arteries 
which  at  once  dilate  until  a  condition  of 
equilibrium  is  established  between  the 
pressure  from  within  and  the  elastic  re- 
action of  the  arterial  walls  from  without. 
With  the  cessation  of  the  contraction  the 
elastic  walls  recoil  and  propel  the  blood 
toward  the  capillaries.  The  intermittent 
action  of  the  heart  is  thus  succeeded  by 
the  continuously  reacting  arterial  wall. 

As  the  blood  advances  toward  the 
periphery  of  the  arterial  system  and  larger 
amounts  pass  into  the  capillaries,  both 
the  distention  and  the  elastic  recoil  di- 
minish, and  by  the  time  the  blood  reaches 
the  capillaries  its  intermittency  of  move- 
ment has  been  so  far  obliterated  by  the 
elastic  recoil,  that  as  it  enters  the  capil- 
laries the  movement  becomes  equable 
and    continuous.      The    elasticity    thus 

equalizing   the 
throughout  the 


'« 


■d 


% 

H 


S 


Fig.  149. — Coats  or  a  Small 
Artery,  a.  Endothelium,  b. 
Internal  elastic  lamina,  c.  Cir- 
cular muscular  fibers  of  the  middle 
coat.  d.  The  outer  coat. — (Lan- 
dois  and  Stirling.) 


serves  the  purpose  of 
movement  of  the  blood 
arterial  system. 

In  youth  the  arterial  walls  are  highly 
distensible  and  elastic;  in  advanced  years 
they  are  frequently  relatively  rigid  and 
inelastic;  and  in  consequence  the  flow  of  blood  toward  and  into  the 
capillaries  approximates  in  its  characteristics  the  flow  of  a  fluid  through 
a  rigid  tube  under  the  intermittent  action  of  a  pump;  that  is,  the  inter- 
mittent movement  imparted  by  the  heart  is  not  so  completely  converted 
into  a  continuous  movement,  and  hence  the  blood  flows  through  the 
capillaries  during  the  systole  with  greater  velocity,  and  during  the  dias- 
tole with  less  velocity,  than  is  the  case  when  the  vessel  is  normally 
elastic.  For  these  and  other  reasons  the  tissues  are  not  so  well  nour- 
ished and  hence  their  nutrition  and  functional  activities  decline. 

The  contractility  permits  of  a  variation  in  the  amount  of  blood 
passing  into  a  given  capillary  area  in  a  unit  of  time.  Normally  each 
artery  has  a  certain  average  caliber  due  to  a  given  contraction  of  the 


328  TEXT-BOOK  OF  PHYSIOLOGY. 

muscle  coat.  Beyond  this  average  condition  the  artery  can  pass  in  one 
direction  or  the  other  by  either  a  relaxation  or  increased  contraction 
of  the  muscle  coat.  During  the  functional  activity  of  any  organ  or 
tissue  there  is  need  for  an  increase  in  the  amount  of  blood  beyond 
that  supplied  during  functional  inactivity  or  rest.  This  is  accomplished 
by  a  relaxation  of  the  muscle-fibers.  With  the  cessation  of  activity 
the  muscle-fibers  again  contract  and  reduce  the  amount  of  blood  to 
that  required  for  nutritive  purposes  only.  An  increased  contraction 
of  the  muscle-fibers  beyond  the  average,  diminishes  the  outflow  of 
blood,  and  if  sufficiently  great  may  give  rise  to  anemia  and  pallor. 
The  contractile  elements  at  the  periphery  of  the  arterial  system,  in  the 

so-called  arteriole  re- 
gion, therefore  regu- 
late the  supply  of 
blood  to  the  tissues  in 
accordance  with  their 
|  functional  needs. 

||b  Moreover,  as  will 

be    stated    in    subse- 
quent paragraphs  the 
-Trai  sverse  Section  of  Part  of  the        de  of   contraction 

Wall   of  the   Posterior   Tibial  Artery   (Man).  ° 

—{Schajer.)  a.  Endothelium  lining  the  vessel,  ap-  01  the  arteriole  mus- 
pearing  thicker  than  natural  from  the  contraction  of  cle  influences  verv 
the  outer  coats,  b.  The  elastic  layer  of  the  intima.  rnnrl-prll-w  tVip  Hpotpp 
c.  Middle  coat  composed  of  muscle-fibers  and  elastic  ]r .  .     y      me    utbice 

tissue,  d.  Outer  coat  consisting  chiefly  of  white  of  friction  which  the 
fibrous  tissue.— (From   Yeo's  "Physiology.")  blood  has  to  overcome 

in  passing  from  the 
arteries  into  the  capillaries.  If  the  muscle  contracts  vigorously  the 
caliber  of  the  arteriole  is  diminished  and  the  friction  increases;  if  the 
muscle  relaxes,  the  caliber  of  the  arteriole  is  augmented  and  the  friction 
decreases.  By  virtue  of  its  tonic  activity,  the  arteriole  muscle  at  the 
periphery  of  the  arterial  system  offers  considerable  resistance  to  the 
outflow  of  the  blood  and  which  is  therefore  spoken  of  generally  as  the 
peripheral  resistance,  though  there  is  included  under  this  term  the 
resistance,  offered  by  the  small  caliber  of  the  capillary  blood-vessel 
as  well.     This  latter  factor  is  constant,  the  former  variable. 

The  Structure,  Properties  and  Functions  of  the  Capillaries. — 
The  capillaries  are  small  vessels  that  connect  the  arteries  with  the  veins. 
Though  different  in  structure  from  a  small  artery  or  vein,  there  is  no 
sharp  boundary  between  them,  as  their  structures  pass  imperceptibly 
one  into  the  other.  A  true  capillary,  however,  is  of  uniform  size  in 
any  given  tissue  and  does  not  undergo  any  noticeable  decrease  in  size 
from  repeated  branchings.  The  diameter  varies  in  different  tissues 
from  0.0045  mm-  to  0.0075  mm.,  just  sufficiently  large  to  permit  the 
easy  passage  of  a  single  red  corpuscle.  The  length  varies  from  0.5  mm. 
to  1  mm.  The  wall  of  the  capillary  (Fig.  151)  is  composed  of  a  single 
layer  of  nucleated  endothelial  cells  with  serrated  edges  united  by  a 


THE  CIRCULATION  OF  THE  BLOOD. 


329 


cement  material.  Though  extremely  short,  the  capillaries  divide  and 
subdivide  a  number  of  times,  forming  meshes  or  networks,  the  close- 
ness and  general  arrangement  of  which  very  in  different  localities. 

As  the  endothelial  cells  are  living  structures  and  characterized  by 
irritability,  contractility  and  tonicity,  it  may  be  assumed  that  the  cap- 
illary wall  as  a  whole  is  characterized  by  the  same  properties.  Upon 
the  possession  of  these  properties,  the  functions  of  the  capillary  depend. 

The  junction  of  the  capillary  wall  is  to  permit  of  a  passage  of  the 
nutritive  materials  of  the  blood  into  the  surrounding  tissue  spaces 
and  of  waste  products  from 
the  tissue  spaces  into  the 
blood.  The  structure  of 
the  capillary  wall  is  well 
adapted  for  this  purpose. 
Composed  as  it  is  of  but  a 
single  layer  of  endothelial 
cells,  the  thickness  of  which 
defies  accurate  measure- 
ment, it  readily  permits, 
under  certain  conditions, 
of  the  necessary  exchange 
of  materials  between  the 
blood  and  the  tissues.  The 
forces  which  are  concerned 
in  the  passage  of  materials 
across  the  capillary  wall  are 
embraced  under  the  terms 
diffusion,  osmosis,  and  filtra- 
tion. As  a  result  of  the  in- 
terchange of  materials  the  tissues  are  provided  with  nourishment  and 
relieved  of  the  presence  of  waste  products.  The  blood  at  the  same  time 
changes  to  a  variable  extent  in  chemic  composition;  because  of  the  loss 
of  oxygen  and  the  gain  of  carbon  dioxid  it  also  changes  in  color  from 
red  to  bluish-red. 

In  order  that  the  nutritive  materials  may  pass  through  the  cap- 
illary wall  in  amounts  sufficient  to  maintain  the  necessary  supply  of 
lymph  in  the  lymph  or  tissue  spaces,  it  is  essential  that  the  blood  shall 
flow  into  and  out  of  the  capillary  vessels  constantly  and  equably,  in 
volumes  varying  with  the  activities  of  the  tissues,  under  a  given  pres- 
sure and  with  a  definite  velocity.  These  conditions  are  made  possible 
by  the  cooperation  of  the  physical  properties  and  physiologic  functions 
of  the  heart  and  vascular  apparatus,  the  nature  of  which  will  be  ex- 
plained in  subsequent  pages. 

The  Structure,  Properties  and  Functions  of  the  Veins.— The 
veins  serve  to  collect  the  blood  from  the  capillary  areas  and  return 
it  to  the  right  side  of  the  heart.  As  they  emerge  from  the  capillary 
areas  the  veins,  which  in  these  regions  are  termed  venules,  are  quite 


Fig.  151. — Capillaries.  The  Outlines  of 
the  Nucleated  Endothelial  Cells  with  the 
Cement  Blackened  by  the  Action  of  Silver 
Nitrate. — (Landois  and  Stirling.) 


330  TEXT-BOOK  OF  PHYSIOLOGY. 

small.  By  their  convergence  and  union  the  veins  gradually  increase 
in  size  in  passing  from  the  periphery  toward  the  heart.  Their  walls 
at  the  same  time  correspondingly  increase  in  thickness.  The  veins 
from  the  lower  extremities,  the  trunk,  and  abdominal  organs  finally 
terminate  in  the  inferior  vena  cava.  The  veins  from  the  head  and 
upper  extremities  terminate  in  the  superior  vena  cava.  Both  vense 
cavse  empty  into  the  right  auricle. 

A  typical  vein  consists  of  the  same  three  coats  as  the  artery:  viz., 
the  tunica  intima,  the  tunica  media,  and  the  tunica  adventitia.  The 
media,  however,  does  not  possess  as  much  of  either  the 
elastic  or  muscle  tissue  as  the  artery,  but  a  larger 
amount  of  the  fibrous  tissue.  Hence  they  readily 
collapse  when  empty.  In  virtue  of  their  structure  the 
veins  also  possess  both  elasticity  and  contractility , 
though  in  a  far  less  degree  than  the  arteries.  These 
properties  come  into  play  and  are  of  value  in  further- 
ing the  movement  of  the  blood  toward  the  heart,  es- 
pecially after  a  temporary  obstruction.       • 

Veins  are  distinguished  by  the  presence  of  valves 
Valves  °  of  a  throughout  their  course.  These  are  arranged  in  pairs 
Vein.  v.  Semi-  and  formed  by  a  reduplication  of  the  internal  coat, 
lunar  valve,  i.  strengthened  by  fibrous  tissue.  They  are  always 
valve.6—  \r>avid-  directed  toward  the  heart  and  in  close  relation  to  the 
son.)  walls  of  the  veins,  so  long  as  the  blood  is  flowing  for- 

ward (Fig.  152).  An  obstruction  to  the  flow  causes 
the  valves  to  turn  backward  until  they  meet  in  the  middle  line,  when 
they  act  as  a  barrier  to  regurgitation.  Under  these  circumstances  the 
elastic  tissue  permits  the  veins  to  distend  and  accommodate  the 
blood.  With  the  removal  of  the  obstruction  the  recoil  of  the  elastic 
tissue,  and  perhaps  the  contraction  of  the  muscle-tissue,  forces  the 
blood  quickly  onward. 

HYDRODYNAMIC  CONSIDERATIONS. 

The  blood  flows  through  the  arteries,  capillaries  and  veins  in  ac- 
cordance with  definite  laws.  During  its  transit  certain  phenomena 
are  presented  by  each  of  these  three  divisions  of  the  vascular  appara- 
tus. Since  these  phenomena,  as  well  as  the  laws  which  govern  them 
are  similar  to,  though  more  complex  than  the  phenomena  presented 
by  relatively  simple  tubes  with  rigid  or  elastic  walls,  while  liquids  are 
flowing  through  them  under  a  steadily  acting  or  an  intermittently 
acting  pressure,  it  will  be  conducive  to  clearness  of  conception  of  the 
mechanics  of  the  vascular  apparatus,  if  there  be  considered : 

1.  The  flow  of  a  liquid  through  a  horizontal  tube  with  rigid 
walls  and  of  uniform  or  variable  diameter  under  a  steadily  acting 
pressure,  and 

2.  The  flow  of  a  liquid  through  a  tube  with  elastic  walls  under 
an  intermittently  acting  pressure. 


THE  CIRCULATION  OF  THE  BLOOD. 


33i 


THE  FLOW  OF    A  LIQUID  THROUGH  A  HORIZONTAL  TUBE 
WITH  RIGID  WALLS. 

The  phenomena  and  the  laws  which  govern  them,  that  attend  the  flow 
of  a  liquid  through  a  rigid  tube  of  uniform  diameter  under  a  steadily  acting 
pressure  may  be  readily  observed  in  an  apparatus  similar  to  that  represented 
in  Fig.  153,  which  consists  of  a  reservoir  or  pressure  vessel,  P,  provided  with  a 
horizontal  tube  into  which  is  inserted  at  equal  distances  a  series  of  vertical 
tubes.  If  the  reservoir  be  filled  with  a  liquid,  water  for  example,  the  latter 
under  certain  conditions  will  exert  a  downward  pressure  and  act  as  a  pro- 
pelling or  driving  power,  the  degree  of  which  will  depend  on  the  height  of 
the  column  and  may  be  represented  by  H.  If  the  stopcock  at  O  be 
opened  the  column  of  water,  which  has  heretofore  been  exerting  an  equal 
pressure  in  all  directions,  will  now  exert  a  downward  pressure  only,  and 


Fig.  153. — A  Pressure  Vessel,  P,  with  an   outflow  horizontal  tube,  O-n,  into 
which  vertical  tubes  or  manometers  are  inserted. 


in  consequence  it  will  be  driven  into  and  through  the  horizontal  tube  and 
discharged  from  its  free  extremity  with  a  definite  velocity.  The  velocity 
can  be  determined  by  measuring  the  quantity,   q,   discharged  in  a  given 

time,  t,  (1  sec.)  and  dividing  it  by  the  area  of  the  tube,   jt»«;  e.g.,  v= — - 

Moreover  in  a  tube  of  uniform  diameter  the  velocity  through  each  cross- 
section  will  be  the  same. 

As  the  water  flows  through  the  horizontal  tube  it  meets  with  resistance, 
namely,  the  cohesion  and  friction  of  its  molecules,  and  which  must  be 
overcome  if  the  flow  is  to  continue.  Because  of  the  fact  that  water  will 
moisten  most  surfaces  with  which  it  comes  in  contact  there  will  be  an  adhe- 
sion between  the  walls  of  the  tube  and  the  outer  layer  of  the  column  of 
water,  in  consequence  of  which  it  will  become  more  or  less  stationary. 
Between  the  outer  stationary  layer  and  the  axis  of  the  stream,  there  is  an 
infinite  number  of  layers  of  molecules,  the  cohesion  of  which  one  for  the 
other  steadily  diminishes  in  passing  from  the  periphery  to  the  center  of  the 
stream.     The  extent  of  this  cohesion  will  increase  as  the  tube  is  lengthened. 


332  TEXT-BOOK  OF  PHYSIOLOGY. 

and  decrease  as  it  is  widened.  In  tubes  of  large  diameter  the  axial  cohe- 
sion is  slight  and  hence  the  friction  of  the  molecules,  the  resistance  to  the 
flow,  is  readily  overcome;  in  the  tubes  of  small  diameter  the  axial  cohesion 
is  relatively  great  and  the  resistance  less  easily  overcome. 

As  a  result  of  the  resistance  the  forward  movement  of  the  water  under 
the  pressure  in  P  is  somewhat  retarded,  and  as  a  consequence  it  will  exert 
a  lateral  or  radial  pressure  against  the  walls  of  the  tube,  as  shown  by  the 
rise  of  the  water  in  the  vertical  tubes.  The  amount  of  the  lateral  pressure 
at  any  given  point  is  indicated  and  measured  by  the  height  to  which  the 
water  rises.  For  this  reason  these  tubes  are  termed  pressure  tubes  or 
piezometers. 

Since  the  resistance  in  a  tube  of  uniform  diameter  is  proportional  to  its 
length  the  lateral  pressure  will  gradually  but  progressively  decrease  from 
the  reservoir  to  the  outlet.  Therefore  the  pressure  at  any  given  point  is 
proportional  to  the  resistance  yet  to  be  overcome  and  conversely  the  resist- 
ance to  be  overcome  is  indicated  by  the  height  of  the  pressure.  (In  the 
conduct  of  an  experiment  the  propelling  power  should  be  kept  constant  by 
permitting  fluid  to  flow  into  the  reservoir  as  rapidly  as  it  flows  out  of  the 
horizontal  tube.) 

The  power,  or  force  which  overcomes  the  resistance  in  the  horizontal 
tube  and  imparts  velocity  to  the  fluid  is  the  downward  pressure  of  the 
water  in  the  reservoir,  represented  by  H.  The  amount  of  this  power  util- 
ized in  overcoming  the  resistance  is  approximately  indicated  by  the  height 
of  the  fluid,  y,  at  which  point  the  line  uniting  the  upper  limits  of  the  water 
in  the  vertical  tubes  intersects  it.  The  height  of  the  fluid  at  this  point  is  a 
measure,  therefore,  not  only  of  the  resistance  but  also  an  indication  of  the 
relative  amount  of  the  pressure  used  in  overcoming  it  and  is  therefore 
known  as  the  pressure  height. 

The  amount  of  the  pressure  consumed  in  imparting  the  observed  ve- 
locity is  determined  by  ascertaining  the  height  from  which  a  particle  must 
fall  in  empty  space  to  acquire  this  velocity.  This  is  obtained  by  dividing 
the  square  of  the  velocity  by  twice  the  accelerating  force  of  gravity  as  ex- 
pressed in  the  formula,  ~;  the  quotient  is  the  height  and  is  known  as  the 
velocity  height.  Conversely  if  the  moving  fluid  were  discharged  into  empty 
space  through  an  opening  in  the  tube  at  n,  it  would  ascend  an  equal  dis- 
tance. If  now  this  height  is  represented  by  F,  and  a  line  be  drawn  from 
it,  parallel  to  the  line  of  pressure  until  it  meets  the  reservoir  at  x,  it  will  be 
seen  what  percentage,  x  y,  of  the  primary  propelling  power  is  consumed  in 
imparting  the  observed  velocity. 

Of  the  total  pressure  a  small  portion  is  left  over  which  is  utilized  in 
forcing  into,  and  overcoming  the  resistance  offered  by,  the  orifice  of  the 
horizontal  tube.  The  initial  pressure  in  P  therefore  divides  itself  into  two 
portions;  one,  the  larger  by  far,  is  utilized  in  overcoming  the  resistance  to 
the  flow  of  the  water;  the  other,  the  smaller,  in  imparting  velocity. 

Thus  the  two  phenomena  presented  by  the  flow  of  a  liquid  through  a 
tube  with  rigid  walls  and  of  uniform  diameter  are  velocity  and  pressure,  of 
which  the  former  is  the  same  for  each  cross-section,  and  the  latter  at  any 
point  directly  proportional  to  the  resistance  to  be  overcome. 

If,  instead  of  a  horizontal  tube  of  uniform  diameter,  there  be  substi- 
tuted a  tube,  the  middle  third  of  which  is  enlarged,  the  conditions  will  be 
the  same  as  in  the  previous  case  until  the  fluid  flows  into  the  enlarged  por- 
tion, when  the  velocity  will  diminish  and  will  become  inversely  proportional 


THE  CIRCULATION  OF  THE  BLOOD. 


333 


to  the  area  of  the  cross-section.  The  resistance  will  be  also  diminished 
and  as  a  result  the  pressure  rises  or  at  least  less  of  the  initial  pressure  is 
utilized  in  this  than  in  the  first  section  of  the  tube.  When  the  liquid  flows 
into  the  narrow  or  third  section,  the  primary  velocity  returns.  Though  the 
resistance  again  increases  the  amount  to  be  overcome  is  small,  and  hence 
there  is  a  rapid  and  steady  fall  of  pressure. 

On  the  contrary,  if  a  tube  be  substituted,  the  middle  third  of  which  is 
narrowed,  the  conditions  will  be  the  same  as  in  the  previous  cases  until  the 
liquid  flows  into  the  narrowed  section,  when  at  once  the  velocity  increases 
and  becomes  inversely  proportional  to  the  area  of  the  cross-section;  the  re- 
sistance being  increased  at  the  same  time,  there  will  be  a  rapid  consump- 
tion and  a  steep  fall  of  pressure.  On  flowing  into  the  third  section,  the 
velocity  again  diminishes  and  the  pressure  falls  though  less  slowly  to  the 
end  of  the  tube. 

THE    FLOW  OF  A  LIQUID  THROUGH  A  SERIES  OF  BRANCHING 
AND  AGAIN  RE-UNITING  TUBES  WITH  RIGID  WALLS. 

In  a  system  of  this  character,  such  as  represented  in  Fig.  154,  there  may 
be  assumed  as  a  result  of  the  repeated  branchings,  a  progressive  increase 
in  the  total  sectional  area  of  the  collective  tubes  coincident  with  a  progres- 
sive decrease  in  the  sectional  area  of  individual  tubes  in  the  section  B  c, 


Fig.  154. — A  Series  of  Branching  and  again  Re-uniting  Tubes. 


while  in  the  section  c  d,  there  is  a  progressive  decrease  in  the  total  sectional 
area  of  the  collective  tubes  coincident  with  a  progressive  increase  in  the 
sectional  area  of  individual  tubes. 

Assuming  the  system  to  be  connected  with  a  pressure  vessel,  as  in  the 
preceding  instance,  but  which  has  been  omitted  from  the  figure,  and  the 
stop  cock  to  be  suddenly  opened,  the  column  of  water  will  t  now  exert  a 
downward  pressure,  and  in  consequence  the  water  will  be  driven  into  and 
through  the  system  with  a  definite  velocity  and  pressure. 

The  velocity  of  a  fluid  through  such  a  system  should  theoretically 
be  decreased  from  b  to  c  in  a  ratio  inversely  proportional  to  the  total  area 
of  each  cross-section.  This,  however,  will  not  be  the  case  because  of  the 
presence  of  the  angles  formed  by  the  repeated  branchings  which,  as  de- 
termined experimentally,  simultaneously  increase  the  velocity.  The  extent 
to  which  the  initial  velocity  will  be  changed  will  therefore  be  proportional 


334  TEXT-BOOK  OF  PHYSIOLOGY. 

to  the  ratio  between  these  two  factors.  If  they  balance  each  other  there 
will  be  no  change;  according  as  the  one  or  the  other  preponderates,  there 
will  be  a  corresponding  decrease  or  increase  in  velocity. 

In  flowing  from  c  to  d  the  velocity  will  be  changed  in  the  opposite  di- 
rection and  for  the  reverse  reasons  and  in  E  it  will  have  regained  the  value 
it  had  in  A,  if  the  entrance  and  exit  tubes  are  of  the  same  diameter. 

The  lateral  pressure  should  theoretically  be  increased  from  B  to  c  be- 
cause of  the  increase  in  the  total  sectional  area  and  the  consequent  diminu- 
tion of  friction,  but  because  of  the  increase  in  resistance  due  to  the  decrease 
in  sectional  area  of  individual  tubes  as  well  as  by  the  angles  formed  by  the 
branching  of  the  tubes  it  will  be  simultaneously  decreased.  The  extent  to 
which  the  pressure  will  be  changed  will  also  be  proportional  to  the  ratio 
between  these  two  factors.  If  they  balance  each  other,  there  will  be  no 
change;  according  as  one  or  the  other  preponderates  will  there  be  an  in- 
crease or  decrease  in  pressure.  In  passing  from  c  to  D  the  pressure  will  be 
changed  in  the  same  direction  but  for  the  reverse  reasons. 

In  a  general  way  it  may  be  said  that  in  a  system  in  which  the  succes- 
sive branchings  are  accompanied  by  a  progressive  decrease  in  diameter,  the 
influence  of  an  increase  in  the  total  sectional  area  on  velocity  will  prepon- 
derate over  that  due  to  the  production  of  angles,  and  hence  the  initial 
velocity  will  be  decreased  from  B  to  c,  and  increased  from  c  to  D,  for  the 
reverse  reasons;  and  if  the  tubes  in  the  center  of  the  system  are  capillary 
in  character,  the  resistance  offered  by  them  and  hence  the  consumption  of 
the  propelling  power  will  preponderate  over  the  decrease  in  resistance  due 
to  the  widening  of  the  stream  bed,  and  hence  as  the  liquid  flows  from  b  to  c 
there  will  be  a  fall  in  pressure,  and  in  flowing  from  c  to  D  there  will  be  an 
additional  fall  of  pressure,  owing  to  the  narrowing  of  the  stream  bed  and 
the  increase  in  resistance,  which  is  however  slight  in  amount. 

The  pressure  throughout  this  system  is  the  result  of  the  resistance  to 
the  flow  of  the  water  and  the  extent  of  the  pressure  in  any  one  section 
will  be  proportional  to  the  resistance  yet  to  be  overcome.  It  will 
naturally  be  high  in  the  section  a-b  and  low  in  the  section  d-e.  The 
value  of  these  two  pressures  in  these  sections  and  their  relation  to  each 
other  may  be  varied  temporarily  or  permanently  by  the  introduction  along 
the  course  of  the  tubes  between  B  and  c  of  a  series  of  stopcocks.  If  the 
lumen  of  each  stopcock  has  a  certain  average  value,  so  as  to  permit  of  a 
certain  outflow  of  water,  the  pressure  will  have  a  certain  value  in  both 
a-b  and  d-e.  But  if  the  lumen  of  each  stopcock  is  decreased,  there  will 
be  an  increase  in  the  resistance  and  hence  a  rise  of  pressure  in  a-b  and 
a  fall  of  pressure  in  d-e.  If,  on  the  contrary,  the  lumen  of  each  stopcock 
is  increased,  there  will  be  a  decrease  in  the  resistance  and  hence  a  fall  of 
pressure  in  a-b  and  a  rise  of  pressure  in  d-e.  The  stopcocks  may  be 
spoken  of  as  a  variable  peripheral  resistance. 

In  the  foregoing  exposition  it  has  been  assumed  that  in  all  instances  the 
pressure  in  the  pressure  vessel  was  steadily  acting.  If,  however,  the  pres- 
sure be  made  to  act  intermittently  as  it  can  be  by  alternately  opening  and 
closing  the  stopcock,  both  the  velocity  and  the  pressure  will  be  alternately 
increased  and  decreased.  The  outflow  of  the  fluid  during  the  moment  the 
pressure  is  acting  will  be  rapid,  and  during  the  moment  the  pressure  is  not 
acting  the  outflow  will  cease.  It  becomes  therefore  intermittent.  Coinci- 
dently  there  is  an  alternate  temporary  increase  and  decrease  of  the  lateral 
pressure. 


THE  CIRCULATION  OF  THE  BLOOD.  335 

THE  FLOW  OF  A  FLUID  THROUGH  A  TUBE  WITH  ELASTIC 

WALLS. 

When  a  tube  with  elastic  walls  is  connected  with  a  pressure  vessel,  the 
conditions  which  are  established  on  opening  the  stopcock  and  the  conse- 
quent flow  of  water,  will  soon  approximate  those  observed  in  a  tube  with 
rigid  walls.  As  the  water  moves  forward,  it  encounters  friction,  exerts  a 
lateral  pressure  and  causes  a  distention  of  the  tube.  This  latter  effect  con- 
tinues until  the  elastic  recoil  of  the  walls  of  the  tube  exactly  counterbal- 
ances the  pressure  of  the  water  from  within.  When  this  condition  is  es- 
tablished the  tube  becomes  practically  a  tube  with  rigid  walls,  and  hence 
so  long  as  the  primary  pressure  is  uniform,  the  velocity  and  lateral  pressure 
will  obey  the  laws  which  hold  true  for  rigid  tubes. 

If,  however,  the  primary  pressure  be  intermittently  applied  or  alter- 
nately increased  or  decreased,  and  the  water  forced  into  the  tube,  previously 
filled  with  water  but  under  no  particular  pressure,  it  will  be  forced  out  of 
the  peripheral  end  of  the  tube  more  or  less  rapidly  during  the  period  of  the 
increase  of  pressure  and  less  rapidly  during  the  period  of  the  decrease  of 
pressure  or  it  may  cease  entirely.  The  extent  to  which  the  outflow  be- 
comes merely  remittent,  or  entirely  intermittent,  will  depend  on  the  amount 
of  resistance,  whether  this  be  due  to  length  of  tube  or  a  narrowed  outlet, 
and  the  degree  of  elasticity. 

When  these  factors  have  a  negative  value  the  outflow  will  be  intermit- 
tent. But  if  they  are  made  to  gradually  change,  and  this  is  especially  the 
case  with  the  resistance,  from  a  negative  to  a  positive  value,  the  outflow 
gradually  changes  from  an  intermittent  to  a  remittent  and  finally  to  a  con- 
tinuous outflow  and  for  the  following  reasons: 

With  a  given  resistance  and  elasticity,  the  fluid  which  is  driven  into  the 
tube  by  the  action  of  the  primary  pressure  exerts  more  or  less  lateral  pres- 
sure, gives  rise  to  a  distention  of  the  tube  and  acquires  a  certain  velocitv 
of  outflow.  In  consequence  of  the  distention,  a  portion  of  the  fluid  accumu- 
lates. With  the  cessation  in  the  action  of  the  primary  pressure,  the  elastic 
walls  recoil  and  force  the  accumulated  fluid  forward  and  so  maintain  more 
or  less  effectively  the  same  velocity  of  outflow  until  there  is  a  return  of  the 
pressure.  If  the  resistance  and  elasticity  have  a  negative  value  this  is  im- 
possible and  the  outflow  will  be  entirely  intermittent.  But  if  they  are 
made  to  increase  in  value,  the  proportionate  amount  of  the  fluid  which  ac- 
cumulates during  the  action  of  the  primary  pressure  will  also  increase  in 
amount  and  hence  there  will  be  an  increase  in  the  distention  of  the  tube. 
The  elastic  recoil  will  therefore  be  greater  in  amount  and  longer  in  dura- 
tion, and  hence  the  outflow  will  change  to  a  remittent  and  finallv  to  a  con- 
tinuous outflow. 

Coincident  with  the  action  and  cessation  of  action  of  the  primary  pres- 
sure there  is  a  corresponding  increase  and  decrease  of  the  lateral  pressure 
and  when  the  intermittency  in  their  action  is  sufficiently  rapid,  the  excess 
of  fluid  entering  the  tube  over  that  discharged  becomes  sufficiently  great  to 
maintain  a  certain  average  or  mean  pressure,  which,  however,  undergoes 
an  alternate  increase  and  decrease  with  each  variation  in  the  primary 
pressure. 

The  temporary  increase  and  decrease  of  the  pressure  and  the  consequent 
expansion  and  recoil  of  the  tube  in  the  neighborhood  of  the  pressure  vessel, 
gives  rise  to  a  wave  on  the  surface  of  the  fluid  which  is  propagated  with 


336 


TEXT-BOOK  OF  PHYSIOLOGY. 


more  or  less  rapidity — though  with  decreasing  size  from  the  beginning  to 
the  end  of  the  tube  and  causing  in  each  section  a  corresponding  expansion 
and  recoil,  and  known  as  the  expansion  wave. 

THE  APPLICATION   OF  THE  FOREGOING  FACTS  TO  THE 
VASCULAR  APPARATUS. 

The  systemic  vascular  apparatus  may  be  conceived  of  as  a  system 
of  tubes  which  have  symmetrically  divided  and  subdivided  and  after- 
wards again  united  and  reunited  in  a  corresponding  manner.  The 
arteries,  arterioles,  capillaries,  venules  and  veins  may  therefore  be  sche- 
matically arranged  (Fig.  155)  in  a  manner  identical  with  the  schematic 
arrangement  of  tubes  represented  on  page  333.  The  heart,  with  which 
they  are  in  connection,  when  filled  with  blood  may  be  compared  with 
the' reservoir  filled  with  water,  and  the  intra-ventricular  pressure  devel- 

CflPILLARIES 


ARTERIES 


VEINS 


Fig.  155. — Schematic  Arrangement  of  the  Vascular  Apparatus. 


oped  during  the  contraction,  to  the  downward  pressure  of  the  water 
when  the  stopcock  at  O  is  opened  (See  Fig.  153). 

The  Stream-bed. — The  stream-bed,  the  path  along  which  the 
blood  flows,  varies  widely  in  its  total  sectional  area  in  different  parts 
of  its  course,  being  least  in  the  aorta  and  venae  cavae,  and  greatest  in 
the  capillaries.  In  passing  from  the  base  of  the  aorta  toward  the  capil- 
laries the  sectional  area  of  individual  arteries,  in  consequence  of  re- 
peated branching,  diminishes,  though  their  total  sectional  area  in- 
creases and  in  direct  proportion  to  their  distance  from  the  heart. 
In  the  capillary  system  the  sectional  area  of  an  individual  capillary 
attains  its  minimal  value,  though  the  total  sectional  area  attains  its 
maximal  value.  Comparing  one  with  the  other,  it  has  been  estimated 
that  the  total  sectional  area  of  the  aortic  bed  is  to  the  total  sectional 
area  of  the  capillary  bed  as  1  is  to  600  or  800.  In  passing  from  the 
capillary  into  the  venous  system  the  sectional  area  of  individual  veins 
increases,  though  the  total  sectional  area  decreases  and  in  direct  pro- 
portion to  their  distance  from  the  capillaries. 

The  stream-bed  in  the  aorta  is  relatively  narrow,  but  widens  grad- 


THE  CIRCULATION  OF  THE  BLOOD. 


337 


ually  as  it  approaches  the  capillaries,  where  it  attains  its  maximum 
width;  it  again  narrows  gradually  as  it  passes  into  the  veins,  until  in 
the  venae  cavae  it  becomes  almost  as  narrow  as  in  the  aorta.  As  the 
combined  sectional  areas  of  the  venae  cavae  are  greater  than  the  sectional 
area  of  the  aorta,  the  stream-bed  of  the  former  never  becomes  as  narrow 
as  that  of  the  latter. 

The  gradual  increase  in  the  width  of  the  stream-bed  from  the 
beginning  of  the  aorta  to  the  middle  of  the  capillary  system,  and  the 
gradual  decrease  in  the  width  of  the  stream-bed  from  the  middle  of 
the  capillary  system  to  the  terminations  of  the  venae  cavae,  which 


Fig.  156. — Diagram  Designed  to  Give  an  Idea  of  the  Aggregate  Sectional 
Area  of  the  Different  Parts  of  the  Vascular  System.  A.  Aorta.  C.  Capillaries. 
V.  Veins.  The  transverse  measurement  of  the  shaded  part  may  be  taken  as  the  width  of 
the  various  kinds  of  vessels,  supposing  them  fused  together. — (Yeo.) 


results  from  the  repeated  branching  and  subsequent  reuniting,  as 
well  as  its  relative  width  in  the  arteries,  capillaries  and  veins,  is  shown 
graphically  in  Fig.  156. 

When  the  heart  contracts  and  when  the  intra- ventricular  pressure 
rises  above  the  pressure  in  the  aorta,  the  aortic  valves  are  forced 
open  and  the  blood  is  driven  into  and  through  the  arteries,  capillaries  and 
veins  and  empties  into  the  right  side  of  the  heart  with  a  definite  velocity 
and  pressure.  Because  of  the  fact  that  the  stream-bed  progressively 
increases  in  width  from  the  aorta  to  the  middle  of  the  capillar}-  system 
the  velocity  through  each  cross-section  should  theoretically  be  inversely 
proportional  to  its  total  sectional  area.  This,  however,  strictly  speak- 
ing, is  not  the  case,  owing  to  the  accelerating  influence  of  the  angles 
formed  by  the  repeated  branchings.  But  for  the  reason  that  the  in- 
fluence of  the  former  factor  so  largely  preponderates  over  the  influence 


338  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  latter  factor,  it  may  be  accepted  that  the  initial  velocity  of  the 
blood  in  the  aorta  gradually  decreases  inversely  as  the  sectional  area 
increases,  until  it  attains  its  minimal  value  in  the  capillary  system. 
The  actual  velocity  is  the  resultant  of  these  two  opposing  forces. 
For  the  reverse  reason,  viz.,  the  narrowing  of  the  stream-bed  and  the 
retarding  influence  of  the  angles,  the  velocity  of  the  blood  gradually 
increases  from  the  middle  of  the  capillary  system  to  the  end  of  the  vense 
cavse  though  it  will  never  attain  its  initial  velocity  in  these  vessels 
because  their  combined  sectional  areas  are  greater  than  that  of  the 
aorta.  The  accelerating  effect  of  the  narrowing  of  the  stream-bed 
preponderates  over  the  retarding  effects  of  the  angles  and  the  velocity 
will  be  the  resultant  again  of  these  two  opposing  forces.  The  same 
facts  of  course  hold  true  for  the  pulmonic  vascular  apparatus. 

In  its  passage  through  the  vessels  the  blood  meets  with  resistance, 
viz.,  the  cohesion  and  friction  of  its  molecules  which  must  be  over- 
come if  the  flow  or  the  movement  of  the  blood  is  to  continue.  The 
statements  made  in  previous  paragraphs  regarding  the  causes  and  the 
results  of  friction  in  tubes  with  either  rigid  or  elastic  walls  hold  true 
for  the  most  part  also  for  the  blood-vessels.  As  a  result  of  friction 
the  forward  movement  of  the  blood  is  somewhat  retarded  and  in 
consequence  will  exert  a  lateral  or  radial  pressure  against  the  walls  of 
the  vessels,  and  from  the  facts  previously  stated,  this  pressure  should 
diminish  in  a  more  or  less  progressive  manner  from  the  origin  of  the 
aorta  to  the  ends  of  the  venae  cavse.  That  this  is  the  case  can  be 
demonstrated  for  the  arteries  and  veins  at  least  by  the  insertion  of  pres- 
sure tubes  and  into  which  the  blood  can  ascend.  The  height  of  the 
pressure  at  any  given  point  in  the  system  will  be  proportional  to  the 
resistance  yet  to  be  overcome. 

Inasmuch  as  the  stream-bed  is  not  uniform  in  diameter  the  fall 
in  pressure  will  not  be  uniform.  As  it  enlarges  in  its  middle  portion 
the  resistance  lessens  and  theoretically  there  should  be  a  rise  of  pressure 
in  passing  towards  the  capillary  system,  but  as  the  repeated  branching 
of  the  vessels  is  attended  by  an  enormous  diminution  in  the  diameter  of 
individual  vessels,  the  resistance  is  proportionately  increased  and  for 
this  reason  there  should  be  a  great  consumption  of  the  propelling  power 
and  a  marked  and  rapid  fall  of  pressure  at  the  periphery  of  the  arterial 
and  throughout  the  capillary  systems.  That  this  is  the  case,  though 
not  to  the  extent  it  might  be,  is  because  the  effect  of  the  increase  in 
resistance  so  largely  preponderates  over  the  effect  of  the  increase  in 
the  width  of  the  stream-bed.  The  actual  fall  of  pressure  is  therefore 
the  resultant  of  these  two  opposing  factors.  For  the  reverse  reasons, 
however,  viz.,  the  narrowing  of  the  stream-bed  and  the  increase  in 
the  sectional  area  of  individual  tubes,  there  will  be  a  further  consump- 
tion of  the  propelling  power  and  a  further  fall  of  pressure  to  the  ends 
of  the  venae  cavae.  The  effect  of  the  increase  in  resistance  of  the  rapidly 
narrowing  stream-bed  preponderates  over  the  effect  of  the  enlarge- 
ment of  diameter  of  individual  vessels.     The  fall  of  pressure  from 


THE  CIRCULATION  OF  THE  BLOOD.  339 

the  middle  of  the  capillary  system  to  the  ends  of  the  venae  cavae  will 
be  again  the  resultant  of  these  two  factors. 

The  pressure  throughout  the  vascular  apparatus  is,  of  course,  the 
result  of  the  resistance  offered  to  the  flow  of  the  blood  and  therefore 
it  will  be  high  in  the  aorta  and  its  main  branches  and  low  in  the  large 
veins.  The  amount  and  the  relation  of  these  two  pressures  in  these 
two  sections  of  the  vascular  apparatus  can  be  temporarily  or  permanently 
changed  in  one  direction  or  another  by  an  increase  or  decrease  in  the 
resistance  offered  to  the  flow  of  blood  from  the  arteries  through  the 
capillaries  into  the  veins.  This  variation  in  the  resistance  is  brought 
about  by  an  increase  or  a  decrease  in  the  degree  of  the  contraction  of 
the  arteriole  muscles.  Thus,  if  the  muscle  contraction  increases,  the 
resistance  is  increased  and  the  pressure  in  the  arteries  rises;  if,  on  the 
contrary,  the  muscle  contraction  decreases,  the  resistance  diminishes 
and  the  pressure  falls  in  the  arteries  and  rises  in  the  veins.  The 
contraction  of  the  arteriole  muscle  is  spoken  of  as  the  peripheral 
resistance. 

The  Distribution  of  the  Intra-ventricular  Pressure. — The 
pressure  developed  during  the  ventricular  contraction  is  thus  expended 
in  imparting  velocity  to  the  blood  and  overcoming  the  cohesion  and 
friction  of  its  molecules.  The  percentage  of  the  pressure  utilized  in 
overcoming  the  resistance  could  be  approximately  determined  from 
the  pressure  in  the  aorta  if  this  were  accurately  known;  the  percent- 
age of  the  pressure  utilized  in  imparting  velocity  could  be  determined 
with  the  formula  ^,  if  the  actual  velocity  of  the  blood  in  the  aorta 
could  be  experimentally  determined.  On  account  of  the  difficulty  in 
obtaining  this  latter  factor  at  least,  the  results  must  only  be  approxi- 
mative. 

An  idea  of  the  ratio  between  the  velocity  pressure  and  the  resistance 
pressure,  however,  may  be  obtained  from  the  distribution  of  the  aortic 
pressure  in  the  dog  in  reference  to  the  carotid  artery.  Thus,  if  it  be 
assumed  that  the  average  velocity  of  the  blood  is  35  cm.,  the  velocitv 
pressure  is  equal  to  -^  or  0.62  centimeters  of  blood  or  0.046  centi- 
meters of  mercury,  and  if  the  average  aortic  pressure  is  150  mm.  of 
mercury,  the  ratio  of  the  velocity  pressure  to  the  resistance  pressure  is 
as  1  to  326. 

The  phenomena  which  for  the  most  part  characterize  the  flow  of 
blood  through  the  blood-vessels  are  velocity  and  pressure,  combined 
with  an  alternate  expansion  and  recoil  of  the  arterial  vessels  due  to  the 
intermittent  character  of  the  heart-beat.  For  special  reasons  it  is 
convenient  to  consider  the  pressure  first. 

BLOOD-PRESSURE. 

From  theoretic  considerations  alone  it  may  be  inferred  that  the 
blood,  as  it  flows  through  the  vascular  apparatus,  exerts  a  pressure 
against  the  walls  of  the  vessels,  and  that  this  pressure  is  greatest  at  the 


34o  TEXT-BOOK  OF  PHYSIOLOGY. 

beginning  of  the  aorta,  and  least  at  the  ends  of  the  vense  cavae.  The 
fact  that  the  blood  flows  from  the  aorta  to  the  vense  cavae  indicates 
that  there  is  a  higher  pressure  in  the  former  than  in  the  latter.  The 
same  holds  true  for  the  pulmonary  artery  and  veins.  So  long  as  this 
is  the  case,  the  blood  must  flow  from  the  point  of  high  to  the  point  of 
low  pressure. 

To  this  pressure  the  term  blood-pressure  is  given,  and  may  be 
defined  as  the  pressure  exerted  radially  or  laterally  by  the  moving 
blood-stream  against  the  sides  of  the  vessels.  That  there  is  such  a 
pressure  within  the  arteries,  capillaries,  and  veins,  different  in  amount 
in  each  of  these  three  divisions  of  the  vascular  apparatus,  is  evident 
from  the  results  which  follow  division  of  an  artery  or  a  vein  of  corre- 
sponding size.  When  an  artery  is  divided,  the  blood  spurts  from  the 
opening  for  a  considerable  distance  and  with  a  certain  velocity.  The 
reason  for  this  lies  in  the  fact  that  the  vessel  has  been  distended  by  the 
pressure  from  within  and  its  walls  thrown  into  a  condition  of  elastic 
tension,  so  that  at  the  moment  there  is  an  outlet,  the  vessel  suddenly 
recoils  and  forces  the  blood  out  with  a  velocity  proportional  to  the 
distention.  When  a  vein  is  divided,  the  blood  as  a  rule  merely  wells 
out  of  the  opening  with  but  slight  momentum,  and  for  the  reason 
that  the  vessel  has  been  but  slightly,  if  at  all  distended  by  the  pres- 
sure. These  results  indicate  that  the  blood  in  the  arteries  stands 
under  a  pressure  considerably  higher  than  that  of  the  atmosphere, 
while  that  in  the  veins  stands  under  a  pressure  perhaps  but  slightly 
above  that  of  the  atmosphere.  Especially  true  is  this  of  the  larger 
veins. 

The  same  facts  may  be  demonstrated  in  another  and  more  striking 
way.  A  dog  or  cat  is  anesthetized  and  securely  fastened  in  an  appro- 
priate holder.  The  carotid  artery  on  the  right  side  and  the  jugular 
vein  on  the  left  side  are  freely  exposed  and  clamped.  Into  the  artery 
there  is  inserted  on  the  distal  side  of  the  clamp  and  in  the  direction  of 
the  heart  a  cannula  to  which  is  connected  a  tall  glass  tube,  200  cm. 
high  and  of  about  4  mm.  internal  diameter.  Into  the  vein  there  is 
passed  on  the  proximal  side  of  the  clamp  and  in  the  direction  of  the 
capillaries  a  second  cannula,  to  which  is  connected  a  similar  tube, 
though  of  less  height.  If  the  two  clamps  are  removed  at  the  same 
time,  the  blood  will  mount  in  both  tubes  simultaneously.  In  the 
arterial  tube  the  blood  will  ascend  by  leaps  corresponding  to  the  heart- 
beats until  a  certain  height  is  reached,  when  the  column  becomes 
relatively  stationary,  being  kept  in  equilibrium  by  the  blood-pressure 
within  the  vessel  and  the  atmospheric  pressure  without.  Though 
stationary  in  a  general  sense,  nevertheless,  the  blood-column  oscillates, 
rising  and  falling  with  each  contraction  and  relaxation  of  the  heart. 
Not  infrequently  larger  excursions  of  the  column  are  seen  which  corre- 
spond in  a  general  way  to  the  respiratory  movements.  This  experi- 
ment was  originally  performed  on  the  horse,  by  the  Rev.  Stephen 
Hales  ('1732). 


THE  CIRCULATION  OF  THE  BLOOD. 


34i 


In  the  venous  tube  the  blood  also  rises  to  a  certain  height,  after 
which  it  remains  quite  stationary,  as  the  effect  of  the  cardiac  con- 
traction is  not  propagated  under  normal  conditions  beyond  the  arterial 
system.  The  height  to  which  it  rises  is  but  slight  as  compared  with 
that  in  the  arterial  tube.  The  pressure  in  both  vessels  is  thus  recorded 
in  millimeters  of  blood.  Strictly  speaking  the  pressure  thus  obtained 
does  not  represent  the  lateral  pressure  in  the  carotid  artery  but  in  the 
vessel  from  which  it  arises.  The  central  end  of  the  carotid  is,  under 
the  circumstances,  but  a  continuation  of  the  cannula  and  the  pressure 
thus  obtained  is  the  lateral  pressure  of  either  the  innominate  artery 


B.  P  TRACING 


^v-V 


P^P\ 


W 


Fig.  157. — Diagram  to  show  the  Relation  of  the  Mercurial  Manometer  to 
the  Artery,  on  One  Hand,  and  to  the  Recording  Cylinder,  on  the  Other  Hand, 
when  Arranged  for  Recording  Blood-pressure. 


or  the  aorta  as  the  case  may  be.  In  order  to  obtain  the  lateral  pressure 
in  the  carotid  or  any  other  artery  it  is  only  necessary  to  take  the  end 
pressure  of  any  one  of  its  branches  or  what  amounts  to  the  same  thing, 
to  divide  the  vessel  and  insert  the  horizontal  portion  of  a  T-shaped 
tube  into  the  central  and  distal  ends  and  through  which  the  blood  can 
continue  to  flow,  and  to  connect  the  vertical  portion  with  a  vertical 
pressure  tube  or  with  a  mercurial  manometer.  The  absolute  pressure 
on  any  given  unit  of  vessel  surface — e.  g.,  1  sq.  mm. — is  obtained 
by  multiplying  the  height  of  the  column,  expressed  in  millimeters, 
by  the  unit  of  surface,  and  then  determining  the  weight  of  this  mass 
of  blood.  Thus  if  the  height  of  the  column  of  blood  in  the  carotid 
artery  tube  is  2000  mm.,  then  the  pressure  on  r  sq.  mm.  is  2000  mm. 
of^blood.     The  weight  of  2000  c.mm.  of  blood  is  equal  to  2.1  grams. 


342 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Arterial  Blood-pressure. — For  accurate  and  long-continued 
observation  the  arterial  blood-pressure  is  more  conveniently  studied 
by  means  of  a  U-shaped  tube  (a  manometer)  partially  filled  with 
mercury.  One  limb  of  the  manometer  is  connected  by  means  of  a 
tube  and  a  cannula  with  an  artery  (Fig.  157).  For  the  purpose  of 
retarding  coagulation  of  the  blood  and  for  preventing  the  escape  of  a 
large  volume  of  blood  from  the  vessels,  the  system  is  filled  with  a  solu- 
tion of  carbonate  of  soda  of  sp.  gr.  1060,  5.58  grams  per  1000  c.c, 
and  under  a  pressure  approximately  equal  to  that  in  the  vessel  of  the 


r6o 

/v 

f*5 

f4o 

r35 

~3° 

f25 

~TZ6 

■  1 

p5 

fio  Time  Record  in  Seconds 

rsi    1    1    |    1    1    |    1    |    1    1    l    1    1    III   

Line  of  Atmospkeric  Pressure 

Fig.  158. — A  Portion  of  a  Blood-pressure  Tracing  Obtained  from  the  Carotid 
Artery  of  the  Rabbit  with  a  Mercurial  Manometer.  The  small  oscillations  are 
due  to  the  heart-beat,  the  large  oscillations  are  due  to  the  respiratory  movements. 

animal  as  determined  in  previous  experiments.  When  commun- 
ication is  established  between  the  vessel  and  the  cannula,  the  mercurial 
column  adjusts  itself  to  the  pressure  in  the  artery  and  at  once  exhibits 
the  same  cardiac  oscillations  and  respiratory  undulations  as  did  the 
column  of  blood  in  the  previous  experiment. 

The  height  of  the  mercurial  column  kept  in  equilibrium  by  the 
pressure  of  the  blood  within,  and  the  pressure  of  air  without  the  vessel 
is  that  between  the  lower  level  of  the  mercury  in  the  proximal,  and 
the  higher  level  in  the  distal  limb  of  the  manometer,  both  of  which 
can  be  read  off  on  a  scale  placed  between  the  two  limbs. 

The  height  of  the  mercury  as  well  as  its  oscillations  in  the  distal 
limb  may  be  recorded  by  placing  on  the  top  of  the  mercury  a  light 


THE  CIRCULATION  OF  THE  BLOOD. 


343 


max  valve 


to  manometer 


mm  valve 


float,  the  upper  end  of  which  carries  a  writing  point.  When  the 
latter  is  placed  in  contact  with  the  moving  blackened  surface  of  a 
recording  cylinder  or  kymograph,  the  height  and  the  oscillations  are 
recorded  in  the  form  of  a  tracing  similar  to  that  shown  in  Figs.  157 
and  158,  in  which  the  smaller  oscillations  represent  the  changes  in 
pressure  due  to  the  systole  and  diastole  of  the  heart  and  the  larger 
oscillations  to  variations  in  the  average  pressure  due  to  the  respiratory 
movements.  The  height  of  the  mercurial  column  kept  in  equilibrium 
at  any  particular  moment  is  determined  by  measuring  the  distance 
between  a  base-line  or  abscissa,  which  represents  the  position  of  the 
mercury  at  atmospheric  pressure,  and  any  given  point  on  the  trace 
above,  and  multiplying  it  by  2,  for  the  reason  that  the  mercury  sinks 
in  the  proximal  limb  as  high  as  it  rises 
in  the  distal  limb  of  the  manometer 
and  hence  the  column  of  mercury  sup- 
ported is  that  observed  between  the 
upper  and  lower  level  of  the  mercury 
in  the  distal  and  proximal  limbs  of 
the  manometer. 

The  blood-pressure  as  revealed  by 
the  tracing  may  be  resolved  into  two 
components:  viz.,  (1)  a  constant  ele- 
ment and  represented  by  the  pressure 
in  the  arteries  during  the  period  of  the 
cardiac  diastole,  which  is  termed  the 
diastolic  or  minimum  pressure;  and 
(2)  a  variable  element  and  represented 
by  that  additional  pressure  occurring 
at;the  time  of  the  cardiac  systole,  which 
is  termed  the  systolic  or  maximum 
pressure.  The  diastolic  pressure  is 
represented  by  the  distance  between 

the  base-line  and  the  points  of  the  curve  corresponding  to  the  diastolic 
rest;  the  systolic  pressure,  by  the  distance  between  the  base-line  and 
the  apices  of  the  curves  following  the  cardiac  systole.  The  relation 
of  these  two  components  varies  in  different  animals  and  in  the  same 
animal  at  different  times.  If  the  diastolic  pressure  is  low,  the  systolic 
increase  may  be  considerable;  if  the  former  is  high,  the  latter  may  be 
slight.  The  relation,  however,  of  these  components  is  not  so  accurately 
shown  by  the  mercurial  manometer,  owing  to  the  inertia  of  the  mercury, 
as  by  one  of  the  various  forms  of  quickly  responsive  spring  man- 
ometers used  in  determining  the  rapid  variations  of  intra-cardiac 
pressure.  These  instruments  show  a  much  larger  rise  of  pressure 
during  the  systole,  often  amounting  to  as  much  as  one-third  or  one- 
fourth  of  the  diastolic  pressure. 

For  the  purpose  of  obtaining  the  maximum  systolic  and  the  mini- 
mum diastolic  pressures,  it  is  best,  however,  to  insert  between  the 


to  heart 

Fig.  159. — v.  Frank's  Valve. 
This  is  placed  in  the  course  of  the 
tube  between  heart  and  manometer, 
so  that  the  latter  may  be  used  as  a 
maximum,  minimum,  or  ordinary 
manometer  according  to  the  tap  which 
is  left  open. — (Starling.) 


344  TEXT-BOOK  OF  PHYSIOLOGY. 

cannula  and  the  manometer  a  maximum  and  a  minimum  valve 
similar  in  principle  to  that  shown  in  Fig.  159.  By  permitting  the  blood 
to  exert  its  pressure  first  through  the  maximum  valve  and  then  per- 
mitting the  mercurial  column  to  exert  its  pressure  through  the  mini- 
mum valve  in  the  reverse  direction  for  a  certain  length  of  time,  or  by 
permitting  each  to  exert  its  pressure  alternately  with  each  heart-beat, 
the  maximum  systolic  and  the  minimum  diastolic  pressures  will  be  re- 
corded. By  this  method  Dawson  found  an  average  maximum  pres- 
sure in  the  carotid  artery  of  the  dog  of  162,  and  a  minimum  pressure 
of  103  mm.  of  mercury,  a  difference  of  59  mm.  Hg.  The  difference 
between  these  two  pressures  is  known  as  the  pulse  pressure.  (A  dia- 
gram showing  the  relation  of  these  different  pressures  one  to  another 
will  be  found  on  page  346.) 

In  a  series  of  experiments  it  will  be  found  that  the  blood-pressure 
in  the  arteries,  though  rising  and  falling  a  certain  number  of  milli- 
meters, yet  retains  a  fairly  constant  general  average,  the  result  of  an 
adjustment  between  the  number  of  heart-beats  per  minute  and  the 
amount  of  the  resistance  offered  to  the  escape  of  blood  into  the  capil- 
laries and  veins.  Between  the  two  extremes  there  is,  of  course,  a 
certain  average  or  mean  pressure  which  represents  the  power  driving 
the  blood  through  the  vessels.  It  is  frequently  stated  that  in  a  tracing 
in  which  the  respiratory  undulations  are  absent,  that  the  mean 
pressure  is  the  arithmetic  mean  of  the  systolic  and  diastolic  pressures. 
This  is,  however,  not  strictly  correct,  as  can  be  demonstrated  experi- 
mentally. Thus,  if  at  some  one  point  between  the  artery  and  the 
manometer,  the  lumen  of  the  connecting  tube  be  largely  obliterated 
by  a  constriction,  the  variations  in  the  pressure  following  the  systole  and 
diastole  of  the  heart  will  be  largely,  if  not  entirely  excluded,  and  the 
mercury,  instead  of  rising  rapidly  in  the  manometer  and  fluctuating 
with  each  heart-beat,  will  rise  slowly  to  a  certain  level  and  then  remain 
at  rest.  The  number  of  millimeters  of  mercury  thus  supported 
represents  the  mean  or  absolute  pressure.  The  same  result  can  be 
obtained  by  employing  the  compensatory  manometer  of  Marey  which 
presents  a  constriction  of  this  character.  From  many  experiments 
made  by  Dawson  it  has  been  learned  that  the  mean  pressure  lies 
nearer  to"  the  diastolic  than  to  the  systolic  pressure  and  may  be  ex- 
pressed numerically  by  the  statement  that  it  is  equal  in  millimeters 
of  mercury  to  the  diastolic  pressure  plus  one-third  of  the  pulse  pres- 
sure. In  a  tracing  in  which  the  respiratory  undulations  are  present 
the  mean  pressure  can  be  calculated.  The  method  by  which  this  is 
done,  however,  is  rather  complicated  and  need  not  be  detailed  here. 
In  a  general  way  the  mean  pressure  in  such  a  tracing  may  be  repre- 
sented by  a  line  drawn  horizontally  across  the  tracing  midway  between 
the  apex  and  trough  of  the  undulation. 

Estimates  of  the  Mean  Arterial  Pressure. — Because  of  the 
difficulty  in  obtaining  the  pressure  in  small  arteries,  the  experimental 
determinations  have  for  the  most  part  been  confined  to  large  arteries 


THE  CIRCULATION  OF  THE  BLOOD. 


345 


such  as  the  carotid,  brachial  and  femoral,  and  hence  the  results  which 
have  been  obtained  have  reference  to  the  lateral  pressure  in  the  aorta 
or  in  the  large  vessels  which  immediately  arise  from  it.  The  pressure 
obtained  in  the  usual  way  at  the  central  end  of  a  divided  carotid  is 
generally  known  as. the  "end  pressure"  and  represents  the  mean 
lateral  pressure  in  the  aorta  or  in  the  innominate  artery.  Among  the 
results  thus  obtained  in  different  experiments  from  the  carotid  arterv 
of  different  animals  are  the  following:  In  the  horse,  from  122  to  214 
mm.  Hg.;  in  the  dog,  from  140  to  160  mm.;  in  the  cat,  150  mm.;  in  the 
rabbit,  from  90  to  100  mm.;  in  the  sheep,  170  mm.;  in  the  calf,  from  133 
to  165  mm.  In  two  observations  made  on  human  beings  previous  to 
the  amputation  of  a  limb,  the  pressure  was  found  in  the  brachial 
artery  of  one  patient  to  vary  from  no  mm.  to  120  mm.  Hg.,  and  in  the 
anterior  tibial  artery  of  the  other  patient  from  no  mm.  to  160  mm.  Hg. 

The  investigations  made  in  different  parts  of  the  arterial  system 
indicate  that  the  mean  pressure  is  remarkably  constant  and  uniform 
and  does  not  show  any  noticeable  falling  off  until  near  the  arteriole 
region  where  the  resistance  suddenly  and  rapidly  increases.  Thus 
Volkman  found  simultaneously  in  the  carotid  artery  and  in  the  meta- 
tarsal artery  of  the  sheep  a  mean  pressure  of  165  and  146  mm.  Hg. 
respectively  and  this  for  the  reason  that  the  resistance  throughout 
the  arterial  system  does  not  markedly  increase,  until  the  arteriole 
region  is  reached.  The  careful  investigations  of  Dawson  show  that 
in  the  large  blood-vessels  of  the  dog  the  diastolic  pressure  is  as  con- 
stant as  the  mean  pressure  though  it  undergoes  slight  variations  in 
different  regions;  but  that  the  systolic  pressure,  as  shown  by  taking 
the  end  pressure  in  the  thyroid  and  similar  sized  arteries  in  different 
parts  of  the  arterial  tree,  undergoes  a  considerable  falling  off,  though 
it,  too,  remains  high  in  large  arteries. 

The  numerical  expressions  of  these  various  pressures  in  different 
parts  of  the  arterial  system  are  shown  in  the  following  table  abstracted 
from  the  more  extensive  tables  of  Dawson.  The  results  were  obtained 
from  experiments  made  on  dogs.  The  figures  represent  in  milli- 
meters of  mercury  certain  average  end  pressures  in  the  arteries  named. 


Artery- 

Systolic. 

Mean. 

Diastolic. 

Pulse  Pressure. 

Brachio-cephalic 

Right  carotid.  .  .  . 

163 
160 

121 

ttS 

i°3 
no 

IOi 

105 
no 
103 
102 
97 

60 

Left  carotid 

160                           123 
168                           123 
160                        11S 
165                        123 
152                        118 

4 

59 
63 

Left  subclavian 

Left  brachial 

Left  renal 

b-> 

Deep  femoral 

Thyroid 

5° 
43 

. 

The  Capillary  Pressure. — The  small  size  of  the  capillaries  pre- 
cludes an  investigation  of  their  pressure  by  manometric  methods. 
It  may  be  stated,  however,  to  be  approximately  equal  to  the  pressure 


346 


TEXT-BOOK  OF  PHYSIOLOGY. 


required  to  obliterate  their  lumen  and  to  whiten  the  skin.  The 
apparatus  of  v.  Kries  is  based  on  this  theory.  A  small  glass  plate, 
from  2.5  to  5  sq.  mm.,  is  fastened  to  the  under  surface  of  a  support 
of  suitable  size  carrying  a  small  scale  pan.  The  glass  plate  is  placed 
on  the  skin  near  the  root  of  a  finger-nail  and  the  scale  pari  gradually 
weighted  until  the  vessels  are  obliterated,  as  shown  by  the  blanching 
of  the  skin.     From  results   obtained   with   this   apparatus  v.    Kries 


Fig.  160. — A  Diagram  Designed  to  Show  the  Amount  and  the  Relation  of 
the  Blood-pressure  in  the  three  Divisions  of  the  Vascular  Apparatus,  as  well 
as  the  Relation  of  the  Diastolic.  The  mean  and  the  systolic  pressures  in  the  arterial 
system.  Based  on  experiments  made  on  dogs.  H.  Heart.  A.  Arteries.  C.  Capillaries. 
V.  Large  veins.  O,  O,  being  the  zero  line  (=  atmospheric  pressure),  the  pressure  is 
indicated  by  the  height  of  the  curve  The  numbers  on  the  left  give  the  pressure  (approxi- 
mately) in  millimeters  of  mercury,  h.  Pressure  in  heart,  a.  Arteriole  region  showing 
sudden  fall  of  pressure,  c.  The  fall  of  pressure  in  the  capillaries,  v.  The  negative  pres- 
sure in  the  large  veins. 


estimated  the  pressure  in  the  capillaries  of  the  hand  at  37  mm.  Hg. 
and  in  the  ear  at  20  mm. 

The  Venous  Pressure. — In  passing  from  the  capillaries  to  the 
heart  the  pressure  continues  to  fall.  The  increasing  size  of  the  veins 
permits  again  of  manometric  observations  in  different  regions.  In  the 
crural  vein  the  pressure  has  been  found  to  be  equal  to  14  mm.  Hg.,  and 
in  the  brachial  vein  9  mm.  of  Hg.  In  the  jugular  and  subclavian  and 
other  vessels  near  the  heart  it  is  zero  or  even  negative;  that  is,  less  than 
atmospheric  pressure  to  the  extent  of  from  1  to  10  mm.  of  mercury. 

The  amount  and  relation  of  the  different  pressures  in  the  three 
divisions  of  the  systemic  vascular  apparatus  are  approximately  shown 
in  Fig.  160. 


THE  CIRCULATION  OF  THE  BLOOD.  347 

RESUME   OF  THE  FACTS   OF   THE   BLOOD-PRESSURE   AND   OF 
THE  FACTORS  WHICH   CAUSE  IT. 

From  a  consideration  of  the  foregoing  facts  and  statements  the 
following  resume  may  be  made:  1.  The  blood  during  its  flow 
exerts  a  pressure  against  the  sides  of  the  blood-vessels.  2 .  This  pressure 
is  the  resultant  on  the  one  hand  of  the  intra-ventricular  pressure 
developed  at  the  time  of  the  contraction,  and  on  the  other  hand  of  the 
resistance  to  the  forward  movement  of  the  blood.  3.  The  resistance 
is  to  be  sought  for  in  the  cohesion  and  friction  of  the  molecules  of  the 
blood.  4.  The  resistance  is  proportional  to  the  diameter  of  the 
vessel  and  is  therefore  least  in  the  large  arteries  and  veins  and  great- 
est in  the  arterioles  and  capillaries.  5.  The  pressure  is  highest  in  the 
aorta  where  it  may  amount  in  man  to  150  mm.  of  mercury  above  that 
of  the  atmosphere,  and  lowest  at  the  ends  of  the  vena?  cavae  where  it 
may  be  no  greater  than  that  of  the  atmosphere  or  may  be  even  10 
mm.  Hg.  below  it.  6.  The  pressure  falls  from  the  beginning  to  the 
end  of  the  vascular  apparatus,  though  not  progressively,  for  throughout 
the  large  vessels  of  the  arterial  system  it  continues  relatively  high.  7. 
The  high  pressure  in  the  aorta  is  due  to  the  total  resistance  of  the 
vascular  apparatus  and  the  pressure  at  any  given  point  of  the  appa- 
ratus represents  the  resistance  yet  to  be  overcome.  8.  The  high 
pressure  in  the  arterial  system  and  its  marked  fall  at  its  periphery  is 
more  especially  the  result  of  the  very  great  resistance  at  this  point, 
known  as  the  peripheral  resistance,  the  result  of  a  rapid  diminution 
in  the  diameter  of  the  arterioles  and  the  capillary  vessels,  modified 
by  the  tonic  contraction  of  the  arteriole  muscles.  9.  The  pressure 
in  the  arterial  system  undergoes  considerable  variation  both  above 
and  below  the  mean  pressure  during  the  systole  and  diastole  of  the 
heart. 

The  Heart. — The  primary  factor  in  the  production  of  the  pressure 
is  the  pumping  action  of  the  heart.  Should  there  be  any  cessation 
in  its  activity,  the  elastic  walls  of  the  arteries  would  recoil  and  force 
the  blood  into  the  veins.  There  would  be  coincidently  a  fall  of  pres- 
sure equal  to  that  of  the  atmosphere.  Even  under  normal  circum- 
stances this  condition  is  approximated  during  the  diastole.  The 
recoil  of  the  arterial  wall  by  which  the  forward  movement  of  the  blood 
is  maintained  is  attended  by  a  fall  in  pressure.  But  before  this  reaches 
any  considerable  extent,  the  heart  again  contracts  and  forces  its  con- 
tained volume  of  blood  into  the  arteries. 

That  this  may  be  accomplished  it  is  essential  that  the  cardiac 
energy  be  sufficient  not  only  to  drive  a  portion  of  the  blood  through 
the  capillaries  into  the  veins,  but  to  oppose  the  recoiling  arteries,  and 
to  distend  them  to  their  previous  extent,  so  that  the  incoming  volume 
of  blood  may  be  accommodated.  This  at  once  reestablishes  the 
pressure  at  its  former  level. 

During  the  contraction  of  the  heart  the  kinetic  enerew  is  trans- 


348  TEXT-BOOK  OF  PHYSIOLOGY. 

formed  into  potential  energy,  represented  by  the  tense  distended  walls 
of  the  arteries.  With  the  relaxation  of  the  heart  and  the  closure  of 
the  semilunar  valves  the  potential  energy  of  the  arteries  is  again  trans- 
formed into  kinetic  energy,  represented  by  the  moving  blood.  The 
artery  thus  continues  the  work  of  the  heart  during  its  period  of  in- 
activity. The  rapidity  with  which  the  cardiac  contractions  succeed 
each  other  prevents  the  pressure  from  sinking  below  a  certain  average 
level. 

The  Resistance.- — The  secondary  factor  is  the  resistance  to  the 
flow  of  blood  through  the  vessels,  the  nature  of  which  has  been  pre- 
viously stated.  So  long  as. the  resistance,  and  especially  that  variable 
element  of  it  at  the  periphery  of  the  arterial  system,  maintains  a 
certain  average  value,  so  long  will  the  pressure  in  each  division  of  the 
vascular  apparatus  maintain  an  average  or  a  physiologic  value. 
Should  the  resistance  at  the  periphery  of  the  arterial  system  vary  in 
either  direction,  the  result  of  an  increase  or  a  decrease  in  the  degree 
of  the  contraction  of  the  arteriole  muscle,  there  will  arise  a  change  in 
the  relative  degree  of  pressure  in  each  of  the  three  divisions  of  the 
vascular  apparatus. 

The  Elasticity  of  the  Vessel  Walls. — A  tertiary  factor  is  the 
elasticity  of  the  arterial  wall.  While  it  can  hardly  be  said  that  the 
elasticity  is  a  cause  of  the  pressure,  there  can  be  attributed  to  it  the 
capability  of  modifying  and  assisting  in  the  maintenance  of  the 
pressure  at  a  more  or  less  constant  level;  for  were  it  not  for  this  property 
of  the  vessel  wall  the  variations  in  pressure  during  and  after  the  systole 
would  be  far  more  extensive  than  they  are,  and  would  approximate 
the  variations  observed  in  tubes  with  rigid  walls.  The  elasticity, 
moreover,  assists  in  the  equalization  of  the  blood-stream,  converting 
the  intermittent  and  remittent  flow  characteristic  of  the  large  arteries 
into  the  continuous  equable  stream  characteristic  of  the  capillaries. 
It  also  permits  of  wide  variations  in  the  amount  of  blood  the  arteries 
can  contain  between  their  minimum  and  maximum  distention. 

VARIATIONS  IN  THE  BLOOD-PRESSURE. 

A.  In  the  Arterial  Pressure. — It  is  evident  from  the  preceding 
statements  that  the  arterial  blood-pressure  as  a  whole  may  be  increased 
by: 

i.  An  increase  in  the  rate  or  force  of  the  heart's  contraction. 

2.  An  increase  in  the  peripheral  resistance. 

3.  An  increase  in  both  the  force  of  the  heart  and  the  peripheral 
resistance. 

And  that  it  may  be  decreased  by: 

1.  A  decrease  in  the  rate  and  force  of  the  heart's,  contraction. 

2.  A  decrease  in  the  peripheral  resistance. 

3.  A  decrease  in  both  the  force  of  the  heart  and  the  peripheral 
resistance. 


THE  CIRCULATION  OF  THE  BLOOD.  349 

If  when  the  arterial  pressure  is  in  a  condition  of  equilibrium  the 
heart  ejects  into  the  arteries  in  a  given  period  of  time  an  increased 
quantity  of  blood  as  a  result  of  an  increased  rate  of  contraction,  there 
will  be  an  accumulation  of  blood  temporarily  in  the  arteries  and  a  rise 
of  pressure  (the  peripheral  resistance  remaining  the  same),  for  the 
reason  that  the  pressure  is  only  sufficient  to  force  into  the  capillaries 
a  given  volume,  in  the  same  period  of  time.  As  the  pressure  rises  the 
velocity  and  the  outflow  will  be  increased  until  equilibrium  is  restored 
though  at  a  somewhat  higher  level.  A  rise  of  pressure  from  an 
increase  in  the  rate  of  the  beat  alone  has  been  questioned,  for  it  has 
apparently  been  demonstrated  that  there  is  a  definite  relation  between 
the  normal  rate  and  the  volume  discharged  from  the  ventricle,  and 
that  when  the  rate  is  increased,  the  volume  discharged  diminishes 


Fig.  161. — A  Tracing  showing  an  Increase  in  the  Blood-pressure  in  the 
Carotid  Artery  of  a  Rabbit  due  to  an  Increase  in  the  Peripheral  Resistance 
from  a  Contraction  of  the  Arterioles  Caused  by  Reflex  Stimulation  of  the 
Vaso-motor  Center.  The  nerve  stimulated  was  the  sciatic.  Stimulation  began  at  s. 
The  rate  of  the  heart-beat  is  unchanged.  With  the  cessation  of  the  stimulation  the  blood- 
pressure  falls  for  the  reverse  reasons. 


and  hence  the  pressure  remains  normal  or  even  falls  below  the  normal" 
An  increase  in  the  pressure  is  readily  brought  about  by  an  increase 
in  the  force  or  power  of  the  contraction,  the  frequency  remaining  the 
same.  An  increase  in  the  volume  of  blood  ejected  at  each  contraction 
will  necessarily  lead  to  an  accumulation.  With  the  accumulation 
there  goes  an  increased  distention  of  the  artery  and  a  corresponding 
increase  of  pressure.  In  a  short  time,  therefore,  the  increased  pressure 
will  force  out  of  the  arteries  at  a  higher  rate  of  speed  this  excess  of 
blood  until  the  outflow  again  equals  the  inflow.  This  restores  the 
equilibrium  but  establishes  the  mean  pressure  at  a  higher  level. 

If  the  peripheral  resistance  is  increased  by  a  contraction  of  the 
muscle  walls  of  the  arterioles,  the  frequency  and  force  of  the  heart 
remaining  the  same,  there  will  also  be  an  accumulation  of  blood  in  the 
arteries,  an  increased  distention  and  consequent  rise  of  pressure  (Fig. 
161).  The  outflow  of  blood  will  at  the  same  time  be  diminished.  A  rise 
of  pressure  from  this  cause  much  beyond  the  normal  is  to  a  large  extent 
prevented  by  a  simultaneous  decrease  in  the  rate  and  force  of  the 
heart-beat.     This  is  due  to  a  stimulation  of  the  peripheral  ends  of  the 


35° 


TEXT-BOOK  OF  PHYSIOLOGY. 


depressor  nerve,  and  a  consequent  reflex  stimulation  of  the  cardio- 
inhibitor  center,  and  not  to  a  direct  action  on  the  heart -muscle,  inas- 
much as  the  effect  is  not  observed  after  division  of  the  vagi.  When 
both  the  force  of  the  heart  and  the  peripheral  resistance  are  simul- 
taneously increased  there  is  a  rapid  increase  in  pressure ;  the  former 
factor  tends  to  increase,  the  latter  factor,  to  decrease,  the  velocity  of 
the  outflow.  According  as  the  one  or  the  other  preponderates,  will 
there  be  an  increase  or  decrease  in  velocity.     If  they  balance  each 

other,  there  will  be  no  change. 
A  rise  of  pressure  from  a  combi- 
nation of  these  factors  is  rather 
a  pathologic  than  a  physiologic 
condition  and  is  observed  in  cer- 
tain diseases  of  the  vascular  ap- 
paratus. 

The  converse  of  these  state- 
ments also  holds  true.  If  when 
the  general  arterial  pressure  is 
in  a  condition  of  equilibrium 
the  heart  ejects  into  the  arteries 
in  a  given  period  of  time  a  less- 
ened quantity  of  blood,  either  as 
a  result  of  a  decrease  in  the  rate 
or  force,  there  will  soon  be  a 
diminution  of  the  arterial  disten- 
tion and  a  consequent  fall  in 
pressure  (Fig.  162).  The  velocity 
at  the  same  time  diminishes. 
This  continues  until  the  outflow 
no  longer  exceeds  the  inflow. 
Equilibrium  will  again  be  estab- 
lished, but  the  pressure  will  be  at 
a  lower  level. 

If   the   peripheral    resistance 
is  diminished  by  a  dilatation  of 
the    arterioles,  the  heart's   con- 
tractions remaining  the  same,  the  existing  pressure  soon  diminishes. 
The  outflow  of  blood  at  once  increases  in  velocity. 

As  a  rule  a  diminution  in  peripheral  resistance  is  attended  by  an 
increase  in  the  rate  or  force  of  the  heart,  and  this  is  especially  the  case 
if  the  pressure  has  been  above  the  normal. 

When  both  the  force  of  the  heart  and  the  peripheral  resistance  are 
simultaneously  diminished,  there  will  be  a  rapid  fall  in  pressure.  The 
former  factor  tends  to  decrease,  the  latter  factor  to  increase  the  velocity 
of  outflow.  According  as  the  one  or  the  other  preponderates  will 
there  be  a  decrease  or  an  increase  in  velocity.  If  they  balance  each 
other  there  will  be  no  change.     This  condition  is  also  a  pathologic 


Abscissa 


Fig.  162. — A  Tracing  of  the  Blood- 
pressure  in  the  Carotid  Artery  of  a 
Rabbit,  showing  a  sudden  decrease  in 
the  pressure  due  to  an  arrest  in  the  rate 
and  force  of  the  heart -beat  the  result  of 
stimulating  the  vagus  nerve  from  "  on " 
to  "  off."  With  the  cessation  of  the  stimu- 
lation the  pressure  began  to  rise  as  the 
rate  and  the  force  of  the  heart-beat  re- 
turned. (The  abscissa  should  be  20  mm. 
lower.) 


THE  CIRCULATION  OF  THE  BLOOD.  351 

rather  than  a  physiologic  condition  and  observed  in  states  of  profound 
depression  due  to  serious  injuries. 

Local  Variations  in  the  Arterial  Blood-supply. — The  varia- 
tions in  pressure  and  velocity  from  variations  either  in  the  activity  of 
the  heart  or  in  the  peripheral  resistance  recorded  in  preceding  para- 
graphs, have  reference  to  the  arterial  system  in  its  entirety;  but  it  is 
evident  from  many  facts  that  similar  variations  take  place  in  special  re- 
gions or  organs  of  the  body.  Thus,  it  is  a  well-known  fact  that  for  the 
exhibition  of  the  functional  activity  of  every  organ  there  must  be  an  in- 
crease in  the  volume  of  blood  supplied  to  it  in  each  unit  of  time.  This 
is  accomplished  by  an  active  dilatation  of  the  arterioles  of  the  artery 
of  supply,  and  unless  the  area  or  organ  supplied  is  large,  as  the 
splanchnic  area  for  example,  there  will  be  no  necessary  diminution 
in  either  the  general  blood-pressure  or  the  average  velocity.  With  the 
cessation  of  functional  activity,  there  is  no  longer  any  need  for  so 
large  a  blood-supply  and  hence  the  arterioles  contract,  diminish  the 
outflow,  and  raise  the  pressure.  If,  on  the  other  hand,  the  area  to  be 
supplied  be  large,  as  the  splanchnic  area,  the  dilatation  of  the 
intestinal  arteries  will  be  attended  by  such  a  large  inflow  of  blood  that 
not  only  will  there  be  a  fall  of  pressure  in  these  vessels,  but  a  fall  of 
pressure  in  other  arteries  as  well,  combined  with  a  diminution  in 
velocity  through  them.  With  the  contraction  of  the  intestinal  arteries 
the  reverse  conditions  at  once  arise.  By  constant  variations  in  the 
peripheral  resistance  of  individual  arteries  in  each  and  every  region 
of  the  body,  and  in  association  with  variations  in  the  rate  or  force  of  the 
heart,  the  blood  is  shunted  now  into  this,  now  into  that  organ  in  ac- 
cordance with  its  functional  needs.  All  variations  in  peripheral  re- 
sistance are  largely  brought  about  reflexly  by  the  vaso-motor  nerves, 
the  origin,  distribution  and  mode  of  action  of  which  will  be  considered 
in  subsequent  paragraphs. 

B.  In  Capillary  Pressure. — The  pressure  in  the  capillaries, 
though  for  the  most  part  possessing  a  permanent  value,  is  subject 
to  variations  in  accordance  with  variations  in  the  pressure  in  either 
the  arterial  or  venous  systems  or  both.  The  marked  difference  in  the 
pressure  in  the  large  arteries  and  the  capillaries  is  partly  due  to  the 
resistance  offered  by  the  narrow  arterioles.  If  the  latter  dilate  in  any 
given  area,  the  capillary  pressure  increases  because  of  the  propaga- 
tion into  them  of  the  arterial  pressure.  The  reverse  condition  would 
decrease  the  pressure.  On  the  other  hand,  any  interference  with 
the  outflow  from  any  given  area,  due  to  venous  compression,  would 
likewise  increase  the  pressure;  any  factor  which  would,  on  the  con- 
trary, favor  the  outflow  would  decrease  the  pressure.  Independent 
of  any  change  in  the  arteriole  resistance,  it  is  evident  that  a  rise  in 
arterial  pressure  alone  would  increase  the  capillary  pressure.  If 
both  arterial  and  venous  pressures  rise,  the  capillary  pressure  increases; 
if  both  fall,  it  decreases. 

C.  In  Venous    Pressure. — Independent  of    any  change    in   the 


352  TEXT-BOOK  OF  PHYSIOLOGY. 

venous  pressure  in  a  given  area  from  local  or  temporarily  acting 
causes — e.  g.,  aspiration  of  the  thorax  or  heart,  muscle  contractions, 
change  of  position,  etc. — the  general  venous  pressure  will  be  increased 
by  a  decrease  in  the  value  of  those  factors  which  produce  the  difference 
of  pressure  between  the  arteries  and  veins.  An  increase  in  the  value 
of  these  factors  would  necessarily  decrease  the  pressure. 

Variations  in  the  Relation  of  the  Arterial  and  Venous  Pres- 
sures.^— So  long  as  the  heart  maintains  a  given  rate  and  force  and  the 
resistance  at  the  periphery  of  the  arterial  system  (due  to  the  contraction 
of  the  arteriole  muscle)  a  given  value,  will  the  usual  physiologic 
difference  between  the  pressure  in  the  arteries  and  veins  remain  un- 
changed. If,  however,  either  factor  changes  in  one  direction  or  another, 
there  will  arise  a  change  in  the  relative  degree  of  pressure  in  the  dif- 
ferent divisions  of  the  vascular  apparatus.  Thus  if  the  heart  force 
increases  and  a  larger  volume  of  blood  is  discharged  into  the  arteries 
in  a  unit  of  time,  the  amount  of  blood  in  the  venous  system  diminishes, 
and  the  result  is  a  rise  of  the  arterial  and  a  fall  of  the  venous  pressures. 
If,  on  the  contrary,  the  heart  force  decreases  or  the  mitral  valve  permits 
of  a  regurgitation,  a  smaller  volume  of  blood  is  ejected  into  the  arteries 
in  a  unit  of  time,  the  amount  of  blood  in  the  venous  system  increases, 
and  the  result  is  a  fall  of  the  arterial  and  a  rise  of  the  venous  pressure. 

Again  if  the  arteriole  muscle  relaxes  and  a  larger  volume  of  blood 
flows  from  the  arteries  into  the  veins  in  a  unit  of  time,  the  result  will 
be  a  fall  of  arterial  and  a  rise  of  venous  pressure.  If,  on  the  contrary, 
the  arterial  muscle  contracts  and  a  smaller  volume  of  blood  flows  into 
the  veins,  the  reverse  change  of  pressure  obtains. 

The  Determination  of  the  Arterial  Blood-pressure  in  Man.— 
Inasmuch  as  the  blood-pressure  undergoes  considerable  variation  in 
both  physiologic  and  pathologic  conditions  as  well  as  in  response  to 
the  action  of  drugs,  it  seemed  desirable  to  possess  some  means  by  which 
an  accurate  knowledge  of  the  pressure  under  a  variety  of  conditions 
could  be  obtained  both  for  diagnostic  and  therapeutic  purposes. 
The  foregoing  method  of  obtaining  the  blood-pressure  not  being  of 
general  application  to  human  beings  for  obvious  reasons,  special 
instruments  have  been  devised  by  which  the  pressures  may  be  deter- 
mined at  least  approximately  without  resorting  to  any  surgical  proce- 
dure. These  instruments  are  turned  sphygmomanometers.  Some 
of  the  many  forms  of  this  instrument  are  adapted  for  obtaining  the 
systolic  pressure  only,  while  others  are  adapted  for  obtaining  either 
the  systolic  or  the  diastolic  pressure,  or  both. 

The  principle  involved  in  the  first  group  is  the  application  of  an 
elastic  pressure  to  an  artery,  e.  g.,  the  temporal,  radial,  etc.,  until  the 
lumen  is  completely  obliterated  as  indicated  by  the  disappearance  of 
the  pulse  beyond  the  point  of  compression,  and  at  the  same  time  reg- 
ister the  pressure  applied,  by  means  of  a  mercurial  or  spring  man- 
ometer. The  pressure  just  sufficient  to  obliterate  the  pulse  or  to  allow 
it  to  reappear  after  obliteration,  is  taken  as  the  systolic  pressure. 


THE  CIRCULATION  OF  THE  BLOOD. 


353 


The  principle  involved  in  the  second  group  is  based  on  a  suggestion 
of  Marev,  that  the  maximum  pulsation  of  the  artery  or  the  maximum 
distention  and  recoil  following  a  heart-beat  would  be  most  likely  to 
take  place  when  an  elastic  pressure  applied  to  the  outside  of  an  artery 
is  just  sufficient  to  equalize  the  diastolic  pressure  within.  Inasmuch 
as  these  pulsations  can  be  transmitted  to,  taken  up  and  reproduced  by 
a  mercurial  column  in  connection  with  the  pressure  appliances,  it 
becomes  possible,  when  the  maximum  oscillation  of  the  mercurial, 
column  is  attained,  to  read  off  the  diastolic  pressure. 

With  either  form  of  apparatus  it  became  necessary  to  devise  a 
suitable  elastic  sac  or  tube  enclosed  bv  non-elastic  or  rigid  walls  and 


Fig.  i 6v — The  Sphygmomanometer  of  Mosso. 


which  could  be  made  to  encircle  a  finger  or  an  arm,  and  which  could 
in  turn  be  connected  with  a  pressure  apparatus,  and  with  a  manometer 
by  which  any  given  pressure  could  be  registered. 

One  of  the  best  known  of  the  sphygmomanometers  in  that  of  Mosso 
represented  in  Fig.  163.  It  consists  essentially  of  rubber  capsules, 
contained  within  metallic  tubes  and  into  which  two  fingers  of  each 
hand  can  be  inserted.  This  system  is  connected,  on  the  one  hand, 
with  a  pressure  apparatus,  and,  on  the  other,  with  a  manometer  pro- 
vided with  a  scale.  A  float  and  writing-pen  record  the  movements 
of  the  mercurial  column  on  a  moving  blackened  surface.  In  using 
this  apparatus  the  pressure  is  adjusted  to  the  point  at  which  the  mer- 
curial column  exhibits  the  greatest  oscillations. 

Mosso's  interpretation  of  the  results  obtained  with  this  apparatus 

was  that  when  the  greatest  oscillations  of  the  mercurial  column  were 

taking  place,  the  external  pressure  was  just  equal  to  the  mean  arterial 

pressure,  the  latter  being  the  mean  between  the  maximum  pressure 

23 


354  TEXT-BOOK  OF  PHYSIOLOGY. 

during  the  systole  and  the  minimum  pressure  during  the  diastole  of 
the  heart.  It  was  only  necessary,  therefore,  to  take  the  readings  cor- 
responding to  the  excursions  of  the  mercurial  column  and  to  determine 
from  them  the  mean  arterial  pressure. 

It  has  been  experimentally  demonstrated,   however,  by  Howell  and 
Brush  that  this  interpretation,  either  for  this  or  any  similar  form  of 
apparatus,  is  not  correct,  but  that  the  maximum  oscillations  take  place 
when  the  pressure  applied  to  the  exterior  of  the  artery  is  just  equal 
to  the  pressure  within  the  artery  at  the  end  of  the  cardiac  diastole; 
or  in  other  words,  the  pressure  in  the  manometer  from  which  the 
greatest  oscillation  takes  place  indicates  diastolic  pressure.     These 
experimenters  connected  the  right  carotid  artery  of  a  dog  with  a  mercur- 
ial manometer,  interposing  along  the  course  of  the  connecting  tube 
a  maximum  and  a  minimum  valve.     The  left  carotid  artery  was  sur- 
rounded by  a  plethysmograph  which  was  connected,  on  the  one  hand, 
with  both  a  mercurial  and  a  spring  manometer,  the  former  for  the 
purpose  of   indicating  the  pressure  necessary  to  obtain  the   greatest 
oscillation,  the  latter  for  the  purpose  of  magnifying  and  recording  the 
pulsation.     When    the    observations    were    simultaneously    made    it 
was  found  that  the  diastolic  pressure  in  the  right  carotid  measured  by 
the  minimum  manometer  was  almost  exactly  equal  to  the  pressure 
measured  by  the  manometer  in  connection  with  the  sphygmomanometer 
surrounding  the  left  carotid  artery,  when  it  was  exhibiting  its  maxi- 
mum  excursions.     The   difference   in   the   results   of   the   two   sides 
scarcely   exceeded   more   than   one   or  two   millimeters   of   mercury. 
It   was,    therefore,   established   that   the   greatest   oscillations   record 
diastolic   pressure.     It   was   also   shown   by   the   same   investigators 
that  Mosso's  apparatus  is  not  adapted  for  obtaining  systolic  pressure. 
Among  the  many  forms  of  sphygmomanometers  adapted  for  clinic 
purposes  and  with  which  both  systolic  and  diastolic  pressures  may  be 
obtained  is  that  devised  by  Stanton*  (Fig.  164).     The  pressure  is  ap- 
plied to  the  arm  by  the  rubber  armlet  h,  which  is  3 1  inches  wide. 
This  is  the  widest  armlet  that  can  be  adjusted  to  the  average-sized 
arm  and  presents  distinct  advantages  over  the  narrow  armlet  hitherto 
employed.     This  armlet  is  prevented  from  expanding  outward  by  a 
cuff,  f,  of  double  thick  canvas  with  inserted  strips  of  tin,  which  is 
held  in  place  by  two  straps  which  completely  encircle  the  cuff.     On 
the  rigidity  of  this  depends  to   a  large   extent  the  transmission  of 
pulsation.     The  rubber  armlet  is.  connected  by  glass  with  a  stiff- walled 
rubber  tube,  g,  which  in  turn  connects  with  the  manometer.     The 
manometer  is  perhaps  the  most  important  part  of  the  apparatus.     It 
is  constructed  entirely  of  metal  except  for  the  glass  tube  containing 
the   mercury  column.     The  chamber  c  communicates  by  means  of  a 
metal  tube  with  the  glass  column  d,  which  is  connected  by  a  screw- 
thread  at  3,  the  caliber  of  c  being  approximately  100  times  that  of  d. 

*The   following   description  of  this  apparatus  is  abstracted  from  the  Univ.  of  Pa. 
Medical  Bulletin,  Feb.,  1903. 


THE  CIRCULATION  OF  THE  BLOOD. 


355 


The  cap  of  the  chamber,  which  screws  on,  is  provided  with  a  metal  T 
which  is  connected  at  2  with  the  rubber  armlet  and  at  1  with  the  bulb, 
used  as  an  air-pump.  At  a  is  a  stopcock  shutting  the  rubber  bulb 
completely  from  the  rest  of  the  apparatus  while  at  b  is  a  screw-valve 
which  allows  the  air  to  escape  from  the  closed  system.  When  de- 
sired, the  manometer  can  be  made  portable  (without  removing  the 
mercury)  by  screwing  the  caps  1  and  2  into  either  end  of  the  T  at 
1  and  2.  The  manometer  is  then  tilted  away  from  the  glass  column 
d  until  all  the  mercury  has  run  into  the  chamber,  the   glass  is  then 


Fig.  164. — Stanton's  Sphygmomanometer. 


unscrewed  and  cap  3  screwed  in.  Before  removing  cap  3  the  man- 
ometer must  always  be  tilted,  else  the  mercury  will  be  lost. 

The  rubber  bulb  is  similar  to  those  found  on  atomizers. 

In  using  this  apparatus  the  pressure  is  raised  by  the  air-bulb 
forcing  air  into  the  closed  system — distending  the  rubber  armlet  and 
with  the  same  degree  of  force  displacing  the  mercury  in  c,  driving 
it  up  the  glass  column  d.  When  the  pulse  is  no  longer  felt,  the  bulb 
still  being  compressed,  the  arm  of  valve  a  is  turned  until  it  is  at  right 
angles  with  the  thumb  and  finger.  The  valve  b  is  now  slowly  unscrewed 
until  the  mercury  column  begins  to  fall.  With  the  eye  on  the  scale 
the  point  at  which  the  pulse  reappears  is  mentally  noted  as  the  systolic 
pressure.     Often  considerably  before  the  reappearance  of  the  pulse 


356 


TEXT-BOOK  OF  PHYSIOLOGY. 


to  palpation,  a  pulsation  is  seen  in  the  mercury  column.  As  the 
column  slowly  falls  this  increases  up  to  its  greatest  oscillation  and  then 
diminishes.  The  lowest  point  of  the  greatest  pulsation  is  noted  as 
the  diastolic  pressure. 


Fig.  165. — Erlanger's  Sphygmomanometer. 

Erlanger's  sphygmomanometer  is,  also,  a  most  valuable  instrument 
for  obtaining  both  systolic  and  diastolic  pressure.  It  possesses,  an 
advantage  in  that  it  is  provided,  in  addition  to  the  mercurial  man- 
ometer, with  a  tambour  and  lever  by  which  the  changes  in  pressure  can 


THE  CIRCULATION  OF  THE  BLOOD.  357 

also  be  recorded  on  a  revolving  cylinder  (Fig.  165).  A  complete  de- 
scription of  this  apparatus,  the  manner  of  using  it  and  the  results  that 
can  be  obtained  with  it  will  be  found  in  the  Johns  Hopkins  Hospital 
Reports,  Vol.  XII. 

Any  statement  as  to  the  numerical  values  of  the  different  pressures 
is  somewhat  difficult  to  make  inasmuch  as  they  will  vary  within  phys- 
iological limits  in  accordance  with  the  position  of  the  body,  exercise, 
character  of  psychic  states,  digestion,  temperature  and  other  conditions. 
For  comparative  investigations  it  is  necessary,  therefore,  to  place  the 
subject  of  the  investigation  in  one  and  the  same  position,  to  apply  the 
cuff  to  the  corresponding  arm,  to  use  always  a  uniform  width  of  cuff 
and  to  select  the  same  time  of  day  with  reference  to  meals,  etc. 

It  may  be  stated,  however,  that  in  adult  life  the  systolic  pressure 
in  the  brachial  artery  ranges  from  no  to  135  millimeters  of  Hg.  in 
men  and  about  10  mm.  less  in  women;  the  diastolic  pressure  ranges 
from  6s  to  no  mm.  Hg. ;  the  pulse  pressure  ranges  from  25  to  40  mm. 

The  conclusions  of  Erlanger  regarding  the  results  of  his  investi- 
gations with  this  apparatus  may  be  partially  summed  up  in  the  follow- 
ing statements,  and  as  they  hold  true  for  other  forms  of  apparatus 
which  determine  both  systolic  and  diastolic  pressures,  they  are  here 
appended:  "The  pressure  that  is  determined  by  occluding  an  artery 
is  probably  the  maximum  end  pressure  of  the  artery  occluded.  The 
pressure  determined  by  the  method  of  maximal  oscillations  is  the 
minimum  lateral  pressure  of  the  artery  compressed,  and,  therefore, 
as  the  minimum  lateral  pressure  is  the  same  in  all  of  the  larger  ar- 
teries, the  pulse  pressure,  determined  when  the  pressures  in  the  brachial 
artery  are  observed,  tends  to  approximate  the  lateral  pulse  pressure 
in  the  aorta." 

THE  VELOCITY  OF  THE  BLOOD. 

From  the  number  of  heart-beats  per  minute,  72,  and  the  amount 
of  blood  discharged  from  the  left  ventricle  at  each  beat,  180  c.c,  it  is 
evident  that  the  blood  must  be  flowing  through  the  vascular  appa- 
ratus with  a  certain  velocity,  for  during  the  minute  the  entire  volume 
of  blood,  5769  grams,  must  have  passed  twice  through  the  heart. 
Direct  observation  of  the  escape  of  blood  from  the  central  end  of  a 
divided  artery,  and  from  the  peripheral  end  of  a  divided  vein,  as  well 
as  of  the  flow  through  the  capillaries  as  seen  with  the  microscope, 
shows  that  the  velocity  of  the  flow  varies  in  different  parts  of  the  vas- 
cular apparatus.  In  the  arteries,  moreover,  the  flow  is  not  quite 
uniform,  but  experiences  alternate  acceleration  and  retardation  with 
each  heart-beat.  In  the  capillaries  and  veins  the  flow  is  continuous 
and  uniform,  as  the  conditions  of  the  arterial  walls  are  such  as  to 
completely  overcome  the  intermittency. 

If  the  systemic  vascular  apparatus  be  conceived  of  as  a  system 


358  TEXT-BOOK  OF  PHYSIOLOGY. 

of  tubes  which  have  symmetrically  divided  and  subdivided,  and  have 
again  united  and  reunited  in  a  corresponding  manner,  it  is  clear  that 
the  total  sectional  area  will  steadily  increase  from  the  beginning  to 
the  middle  of  the  system,  and  then  as  steadily  decrease  from  the  middle 
to  the  end  of  the  system.  In  such  a  system  the  same  volume  of  blood 
must  pass  through  any  given  section  in  a  unit  of  time  if  the  balance 
of  the  circulation  is  to  be  maintained.  As  the  velocity  of  a  fluid  is 
inversely  as  the  sectional  area  of  the  tubes  through  which  it  flows,  it 
follows  that  the  initial  mean  velocity  of  the  blood  in  the  aorta  will 
steadily  decrease  as  it  flows  into  the  steadily  enlarging  stream- bed 
until  it  reaches  a  minimal  value  in  the  middle  of  the  capillary  system; 
and  that  it  will  again  steadily  increase  as  it  flows  into  the  narrowing 
stream-bed  until  it  reaches  the  heart.  The  initial  mean  velocity  of 
the  blood  in  the  aorta  will  not  be  attained  in  the  venae  cavae,  for  the 
reason  that  the  total  sectional  area  of  the  latter  is  somewhat  greater 
than  that  of  the  former.  The  same  facts  hold  true  for  the  pulmonic 
vascular  system. 

The  Mean  Velocity  in  the  Aorta. — From  the  well-known  fact 
that  the  velocity  with  which  a  fluid  is  flowing  through  a  tube  may  be 
determined  by  dividing  its  sectional  area  into  the  quantity  discharged 
in  a  unit  of  time,  attempts  have  been  made  to  determine  the  mean 
velocity  of  the  blood  at  the  beginning  of  the  aorta.  If  it  be  assumed 
that  the  volume  discharged  at  each  contraction  is  180  c.c,  as  stated  by 
Vierordt,  and  the  number  of  heart-beats  per  minute  at  72,  the  total 
volume  discharged  per  minute  would  be  12,960  c.c,  or  215  c.c.  per 
second.  The  sectional  area  of  the  aorta  at  its  origin  is  6.15  sq.  cm. 
On  the  principle  above  stated,  these  two  factors  would  show  a  velocity 
of  350  mm.  per  second.  An  objection  to  this  estimate  is  that  the  amount 
of  blood  discharged — i.  e.,  the  contraction  volume— is  much  larger 
than  recent  investigations  warrant.  Different  observers  have  estimated 
that  in  man  the  contraction  volume  is  considerably  less,  probably  not 
more  than  80  c.c. 

The  Mean  Velocity  in  the  Arteries. — The  mean  velocity  of  the 
blood  in  the  larger  and  more  superficially  lying  arteries  has  been  de- 
termined by  Volkmann  with  the  hemodromometer,  by  Ludwig  and 
Dogiel  with  the  stromuhr,  and  by  other  investigators  with  different 
forms  of  apparatus. 

Since  neither  the  blood  nor  any  particle  placed  in  it  can  be  seen 
through  the  walls  of  the  artery,  it  occurred  to  Volkmann  to  inter- 
calate along  the  course  of  a  vessel  a  U-shaped  glass  tube  about  one 
meter  in  length  with  a  lumen  the  diameter  of  that  of  the  selected 
vessel,  into  and  through  which  the  blood  could  be  made  to  flow. 
The  mechanic  construction  of  the  apparatus  is  such  (Fig.  166)  that 
the  blood  can  be  made  to  flow  directly  into  the  distal  portion  of  the 
artery  across  the  base  or  indirectly  by  way  of  the  glass  tube.  Pre- 
vious to  the  intercalation  of  the  tube  it  is  filled  with  serum  or  normal 
saline  solution.     With  the  turning  of  the  cocks  at  B  the  blood  enters 


THE  CIRCULATION  OF  THE  BLOOD. 


359 


the  glass  tube  and  drives  the  serum  ahead  of  it  into  the  arterial  system. 
From  the  difference  in  time  between  the  moment  the  blood  enters  and 
the  moment  it  leaves  the  tube  and  from  the  length  of  the  tube  the  veloc- 
ity is  determined. 

The    stromuhr    or    rheometer    of  )( 

Ludwig  (Fig.  167)  is  constructed  on 
the  same  principle,  but  instead  of  the 
glass  tube  having  the  same  diameter 
it  is  considerably  enlarged  on  its  two 
sides.  The  bulbs  are  fastened  to  a 
metallic  disk  which  rotates  around  an 
axis  in  the  metallic  base  which  carries 
the  tubes  to  be  inserted  into  the  arteries. 


Fig.  166. — Volkmann's  Hemodromom- 
eter.     C,  C.  Arterial  cannulas. 


Fig.  167. — Ludwig  and 
Dogiel's  Rheometer.  X,  Y. 
Axis  of  rotation.  A,  B.  Glass 
bulbs,  h,  k.  Cannulas  inserted 
in  the  divided  artery,  e,  eIt 
rotates  on  g,  f.     c,  d.  Tubes. 


With  this  device  it  is  possible  to  place  either  bulb  in  connection  with  the 
proximal  end  of  the  artery.  Previous  to  the  experiment  the  proximal 
bulb  is  filled  with  oil,  the  distal  bulb  with  serum  or  normal  saline. 
On  removing  the  clips  on  the  artery  the  blood  flows  into  the  proximal 


360 


TEXT-BOOK  OF  PHYSIOLOGY 


bulb  and  drives  the  oil  into  the  distal  bulb.  As  soon  as  the  former  is 
filled  with  blood  the  bulbs  are  reversed  and  the  same  relative  condi- 
tions are  attained.  This  is  repeated  a  number  of  times.  Knowing 
the  capacity  of  the  bulbs,  and  the  number  of  times  they  are  filled  in  a 
given  period,  the  total  quantity  of  blood  discharged  is  obtained.  This 
divided  by  the  sectional  area  of  the  artery  gives  the  velocity.  The 
following  values  have  thus  been  obtained :  For  the  carotid  of  the  dog, 
205  to  357  mm.  per  second;  for  the  carotid  of  the  horse,  306  mm.; 
for  the  metatarsal  artery  of  the  horse,  56  mm.  (Volkmann).  For  the 
carotid  of  rabbits,  94  to  226  mm.;  for  the  carotid  of  the  dog,  349  to  733 
mm.  (Dogiel). 

The  variations  in  the  velocity  of  the  blood  in  the  arteries  during 

the  different  phases  of  the 
cardiac  cycle  have  been  de- 
termined by  Chauveau  and 
Lortet  with  the  hematach- 
ometer  (Fig.  168).  This  con- 
sists of  a  metallic  tube  carry- 
ing a  graduated  disk.  At  one 
point  the  tube  is  perforated 
but  covered  with  a  rubber 
band  through  which  passes  an 
index.  When  the  tube  is  in- 
serted into  the  divided  ends  of 
an  artery,  the  current  of  blood 
strikes  the  short  arm  of  the 
index  and  gives  to  the  outer 
long  arm  a  movement  in  the 
opposite  direction.  The  ex- 
tent of  the  excursion  indicates 
the  velocity.  The  apparatus 
is  first  graduated  with  currents 
of  water  of  known  velocity. 
With  this  instrument  Chau- 
veau found  that  in  the  horse  the  velocity  during  the  systole  was  520 
mm.  per  second,  at  the  beginning  of  the  diastole  220  mm.  per  second, 
and  during  the  pause  150  mm.  per  second. 

The  Velocity  in  the  Capillaries. — The  rate  of  flow  in  the  capil- 
lary vessels  can  not  be  experimentally  determined.  It  has  been  esti- 
mated by  Vierordt  at  0.5  mm.  per  second  in  his  own  retinal  capillaries; 
by  Weber  at  0.8  mm.  In  frogs  the  velocity  can  be  fairly  well  deter- 
mined by  observing  the  time  required  for  a  corpuscle  to  pass  over  one 
or  more  divisions  of  an  ocular  micrometer.  Weber  calculated  in  this 
way  that  the  velocity  is  0.5  mm.  per  second. 

As  the  velocity  varies  inversely  with  the  sectional  area,  it  becomes 
possible  to  approximately  determine  the  relation  of  the  sectional  area 
of  the  capillary  system  to  that  of  the  aorta  from  the  above-mentioned 


Fig.  168. — The  Hematachometek  of  Chau- 
veau and  Lortet.  A,  B.  Tube  inserted  in 
artery.  C.  Lateral  tube  connected  with  a  manom- 
eter, b.  Index  moving  in  a  caoutchouc  mem- 
brane, a.     G.  Handle. 


THE  CIRCULATION  OF  THE  BLOOD. 


?6i 


velocities.  If  it  be  assumed  that  the  velocity  in  the  aorta  averages 
300  mm.  and  in  the  capillaries  0.5  mm.  per  second,  then  the  sectional 
area  of  the  capillaries  is  to  that  of  the  aorta  as  600  to  1. 

The  Velocity  in  the  Veins. — In  the  venous  system  the  velocity 
increases  in  proportion  as  the  sectional  area  decreases.  In  the  jugu- 
lar vein  Volkmann  found  the  velocity  225  mm.  per  second,  which 
was  about  one-half  that  in  the  aorta  of  the  same  animal.  The  reason 
for  the  slow  rate  of  movement  in  the  jugular  vein  is  to  be  found  in  the 
fact  that  the  sectional  area  of  the  combined  venae  cavas  is  about  twice 
that  of  the  aorta;  hence  the  relation  of  the  sectional  area  of  the  cap- 
illary system  to  the  sectional  area  of  the  venae  cavae  is  about  300  to  1. 


Arteries.                                     Capillaries.  Veins. 

Fig.  169. ,  Blood-pressure.     ,  Velocity.     — o — o — o — o,  Sectional  area. 


The  blood-pressure,  the  velocity  of  the  blood,  the  sectional  area 
of  the  vascular  apparatus,  and  their  relation  one  to  the  other  are 
shown  in  Fig.  169. 

The  Relations  of  Blood-pressure  and  Velocity. — Though  the 
pressure  of  the  blood  bears  a  definite  relation  to  the  velocity  it  must  be 
kept  in  mind  that  it  is  rather  the  difference  in  pressure  between  the 
beginning  and  the  termination  of  the  arterial  system,  rather  than  the 
mean  pressure  that  influences  the  velocity.  Thus,  with  a  given  force 
of  the  heart  and  a  given  peripheral  resistance,  the  velocity  will  have 
a  given  value,  and  so  long  as  these  factors  remain  constant  will  the 
velocity  remain  constant,  even  though  the  mean  pressure  should  fall, 
as  from  a  hemorrhage,  or  should  rise,  as  from  an  injection  of  some 
indifferent  fluid. 

If,  however,  the  primary  factors,  viz.,  the  cardiac  force  or  the 
peripheral  resistance,  change  their  values  in  either  the  same  or  opposite 
directions,  there  will  be  a  change  at  once  in  the  velocity.  The  varia- 
tions in  pressure  and  velocity,  both  in  the  same  and  opposite  directions, 
which  are  theoretically  possible  from  a  change  in  the  force  of  the  heart. 


362 


TEXT-BOOK  OF  PHYSIOLOGY. 


or  in  the  peripheral  resistance,  or  both,  are  shown  in  the  following 
table  arranged  by  Waller.  The  plus  sign  indicates  increase,  the 
minus  sign,  decrease,  in  effect. 


No. 

Heart 

Arterioles 

Blood-pressure 

Blood-flow 

i 

2 

/  Force  constant  .... 
\  Force  constant  .... 

Resistance  increased 
Resistance  diminished 

+ 

+ 

3 
4 

/  Force  increased 
\  Force  diminished  . 

Resistance  constant 
Resistance  constant 

+ 

+ 

5 
6 

/  Force  increased  .... 
\  Force  diminished  .  . 

Resistance  diminished 
Resistance  increased 

+      - 

-      + 

+      + 

7 
8 

f  Force  increased .... 
\  Force  diminished  .  . 

Resistance  increased 
Resistance  diminished 

+      + 

+      - 
-        + 

The  statements  herein  embodied  have  been  established  by  Marey 
with  an  artificial  schema  of  the  circulatory  apparatus,  and  by  Chau- 
veau  and  Lortet  by  experiments  on  animals  with  the  hemodromo- 
graph,  a  specially  devised  apparatus  for  this  purpose. 

Though  all  the  relations  between  pressure  and  velocity  in  the  table 
are  possible,  those  which  are  most  physiological  are  probably  5  and 
6,  for  in  both  instances  there  is  a  minimum  alteration  in  pressure,  but 
a  maximum  alteration  in  blood  flow  or  velocity.  The  first  instance 
is  the  condition  most  favorable  for  the  functional  activity  of  organs, 
for  the  reason  that  the  volume  of  blood  which  the  organ  receives  in  a 
unit  of  time  is  increased  without  any  change  in  pressure;  and  it  is  an 
established  fact  that  within  physiological  limits  it  is  the  volume  of 
blood  which  an  organ  receives  rather  than  the  pressure  under  which  it 
is  received,  that  determines  its  activity.  In  the  second  instance,  on 
the  cessation  of  activity  the  velocity  is  decreased  and  the  normal 
condition  restored  without  any  appreciable  change  in  pressure. 


THE  PULSE. 

The  pulse  may  be  defined  as  a  periodic  expansion  and  recoil  of 
the  arterial  system.  The  expansion  is  caused  by  the  discharge  from 
the  heart  into  the  arteries  of  a  volume  of  blood  during  the  systole;  the 
recoil  is  due  to  the  elastic  reaction  of  the  arterial  walls  on  the  blood, 
driving  it  forward,  into,  and  through  the  capillaries,  during  the  dias- 
tole. 

At  the  close  of  the  cardiac  diastole  the  arterial  system  is  full  of 
blood  and  considerably  distended.  During  the  occurrence  of  the 
succeeding  systole,  a  definite  volume  of  blood  is  again  discharged  into 
the  aorta.  The  incoming  volume  of  blood  is  now  accommodated  by  the 
discharge  of  a  portion  of  the  general  blood  volume  into  the  capillaries 
and  by  the  expansion  of  the  arteries.  The  expansion  naturally  begins 
at  the  root  of  the  aorta  and  at  the  beginning  of  the  systole.  As  the 
blood  continues  to  be  discharged  from  the  heart,  adjoining  segments 


THE  CIRCULATION  OF  THE  BLOOD.  363 

of  the  aorta  and  its  branches  expand  in  quick  succession,  and  by  the 
time  the  systole  is  completed  the  expansion  has  traveled  over  the  entire 
arterial  system  as  far  as  the  capillaries.  With  the  cessation  of  the 
systole  and  perhaps  even  before,  the  recoil  of  the  arterial  walls  at  once 
occurs,  beginning  at  the  root  of  the  aorta  and  rapidly  passing  over  the 
arteries  to  the  capillaries. 

This  expansion  movement  which  thus  passes  from  the  beginning 
to  the  end  of  the  arterial  system  in  the  form  of  a  wave  is  known  as  the 
pulse-wave  or  pulse.  Preceding  and  causing  the  expansion  and  recoil 
of  the  arterial  system  there  is  an  alternate  increase  and  decrease  of  the 
general  blood-pressure,  as  shown  by  the  small  curves  on  a  blood-pressure 
tracing,  and  for  this  reason  the  pressure  which  causes  the  expansion 
and  recoil  is  termed  the  pulse  pressure.  It  is  defined  as  the  rhythmic 
change  in  pressure  at  any  given  point  of  the  arterial  system;  and  in 
amount,  is  the  difference  between  the  diastolic  and  the  systolic  pres- 
sures, at  the  corresponding  points.  The  volume  of  blood  ejected 
from  the  ventricle  is  frequently  termed  the  pulse  volume. 

The  pulse-wave  which  thus  spreads  itself  over  the  entire  arterial 
system  with  each  systole  of  the  heart  can  be  perceived  in  certain  locali- 
ties by  the  eye,  by  the  sense  of  touch,  and  investigated  with  various 
forms  of  apparatus  or  instrumental  means.  The  pulse-wave,  or  at 
least  the  elevation  of  the  soft  tissues  overlying  it,  can  be  seen  in  the 
radial  artery,  where  it  passes  across  the  wrist-joint,  in  the  carotid 
artery,  in  the  temporal  artery,  in  the  arteries  of  the  retina  under  certain 
conditions,  with  the  ophthalmoscope.  If  the  ends  of  the  fingers  are 
firmly  placed  over  the  radial  artery,  not  only  the  increase  and  decrease 
of  pressure,  but  also  many  of  the  peculiarities  of  the  pulse-wave,  may 
be  perceived.  Without  much  difficulty  it  may  be  perceived  that  the  ex- 
pansion takes  place  quickly,  the  recoil  relatively  slowly;  that  the  waves 
succeed  one  another  with  a  certain  frequency,  corresponding  to  the 
heart-beat;  that  the  pulsations  are  rhythmic  in  character,  etc.  Inas- 
much as  the  individuality  of  the  pulse-wave  varies  at  different  periods 
of  life  and  under  different  physiologic  and  pathologic  conditions,  vari- 
ous terms  more  or  less  expressive,  have  been  suggested  for  its  varying 
peculiarities.  Thus  the  pulse  is  said  to  be  frequent  or  infrequent  accord- 
ing as  it  exceeds  or  falls  short  of  a  certain  average  number — 72  per 
minute;  quick  or  slow,  according  to  the  suddenness  with  which  the  ex- 
pansion takes  place  or  strikes  the  fingers;  hard  or  soft,  tense  or  easily 
compressible,  according  to  the  resistance  which  the  vessel  offers  to  its 
compression  by  the  fingers;  large,  full,  or  small,  according  to  the  vol-, 
ume  of  blood  ejected  into  the  aorta,  or,  in  other  words,  the  degree  of 
fullness  of  the  arterial  system. 

Frequency  of  the  Pulse. — As  the  pulse  or  the  arterial  expansion 
and  recoil  is  the  direct  result  of  the  heart's  action,  its  frequency  must, 
under  physiologic  conditions,  coincide  with  that  of  the  heart.  All 
conditions  which  modify  the  rate  of  the  heart  will  modify  at  the  same 
time  the  rate  of  the  pulse. 


364 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Velocity  of  Propagation  of  the  Pulse-wave.— The  propag- 
ation of  the  pulse-wave  from  its  origin  at  the  root  of  the  aorta  to  any 
given  point  of  the  arterial  system  occupies  an  appreciable  period  of 
time.  The  difference  in  time  between  the  systole  and  the  appearance 
of  the  pulse-wave  at  the  dorsal  artery  of  the  foot  can  be  appreciated 
by  the  sense  of  touch.  The  absolute  time  occupied  by  the  wave  in 
reaching  this  point  was  determined  by  Czermak  to  be  0.193  second. 
The  rate  at  which  the  wave  is  propagated  over  the  vessels  of  the  lower 
extremity  has  been  estimated  by  the  same  observer  at  11. 16  meters 
per  second,  and  for  the  upper  extremities  at  but  6.7  meters  per  second. 
Other  experimenters  have  obtained  for  the  lower  extremities  some- 


Fig.  170. — Von  Frey's  Sphygmograph.  G.  S.  Metal  framework.  P.  Button  at 
tached  to  spring.  F.  Vertical  rod.  U.  Clock-work  which  turns  the  recording  cylinder. 
VI.  Time  markei. 


what  different  results,  varying  from  6.5  to  11  meters  per  second. 
Weber's  original  estimate  was  from  7.92  to  9.24  meters  per  second. 
The  slower  rate  of  movement  in  the  vessels  of  the  upper  extremities 
has  been  attributed  to  a  greater  distensibility  of  their  walls,  a  condi- 
tion unfavorable  to  rapid  propagation.  For  this  reason  a  low  arterial 
tension  will  occasion  a  delay  in  the  appearance  of  the  pulse-wave  in 
any  portion  of  the  body;  a  high  arterial  tension  will  of  course  have  the 
opposite  effect.  The  difference  in  the  speed  of  the  pulse-wave  and 
the  blood-current  shows  that  they  are  not  identical  and  must  not  be 
confounded  with  each  other. 

The  Sphygmograph. — The  sphygmograph  is  an  apparatus  de- 
signed to  take  up,  reproduce,  and  record  the  alternate  expansion  and 
recoil  of  an  artery  caused  by  the  temporary  increase  and  decrease  of 
pressure  following  each  heart-beat.  The  tracing  or  record  obtained 
with  it  is  termed  the  pulse-curve  or  the  sphygmogram.  Different  forms 
of  this  apparatus  have  been  devised  by  Marey,  Dudgeon,  v.  Frey,  and 


THE  CIRCULATION  OF  THE  BLOOD.  365 

many  others.  The  instrument  of  v.  Frey  is  shown  in  Fig.  170.  This 
consists  first  of  a  metal  framework  by  which  the  apparatus  is  fastened 
to  the  arm  and  support  given  to  the  lever,  recording  surface,  etc.  The 
essential  part  is  the  spring  carrying  a  button  which  is  placed  over  the 
artery,  usually  the  radial,  before  it  crosses  the  wrist-joint.  A  vertical 
rod  transmits  the  movement  of  the  spring  to  the  recording  lever;  the 
movements  of  the  latter  are  recorded  on  a  small  cylinder  inclined 
slightly  so  that  the  upstroke  may  be  vertical.  A  small  electro-magnet 
serves  to  record  the  time  relations  of  the  changes  in  the  blood-pressure. 
An  average  tracing  taken  from  the  radial  artery  is  shown  in  Fig.  171. 
This,  however,  is  not  a  tracing  of  the  pulse-wave,  but  rather  a  record 
of  the  changes  in  pressure,  their  succession  and  time  relations,  which 
follow  each  beat  of  the  heart.  The 
artery  usually  selected  for  obtaining  a 
sphygmogram  is  the  radial.  This  artery 
lies  quite  superficially,  covered  only  by 
connective  tissue  and  skin  and  supported 
by  the  flat  surface  of  the  radial  bone, 
conditions  most  favorable  to  technical 
investigation.  FlG  i?i._The  pulse-curve  or 

The     sphygmogram    or    pulse-curve  Sphygmogram. 

may  be  divided  into  two  portions:  viz., 

a  line  of  ascent  from  a  to  b,  and  a  line  of  descent  from  b  to  d  (Fig. 
171).  In  normal  tracings  the  former  is  almost  vertical  and  caused  by 
the  sudden  expansion  of  the  artery  immediately  following  the  ventric- 
ular contraction;  the  latter  is  in  general  oblique,  due  to  the  recoil  of 
the  arterial  walls,  occupies  a  longer  period  of  time,  and  is  marked  by 
several  elevations  and  depressions,  both  of  which  indicate  that  the 
restoration  to  equilibrium  is  neither  immediate  nor  uncomplicated. 
One  of  these  elevations  is  quite  constant  and  known  as  the  dicrotic  wave, 
c;  the  depression  or  notch  just  preceding  it  is  known  as  the  dicrotic 
notch.  Pre-  and  post-dicrotic  waves  are  not  infrequently  present.  The 
summit  is  generally  sharp  and  pointed. 

The  vertical  direction  of  the  line  of  ascent  is  taken  as  an  indica- 
tion that  the  arterial  walls  expand  readily,  that  the  blood  is  discharged 
quickly,  and  that  the  ventricular  action  is  not  impeded.  An  oblique 
direction  of  the  line  of  ascent  is  an  indication  that  the  reverse  condi- 
tions obtain.  The  height  varies  inversely  as  the  arterial  pressure, 
other  things  being  equal;  being  high  with  a  low  pressure,  and  low  with 
a  high  pressure. 

The  dicrotic  elevation  shows  that  a  second  expansion  wave  is  de- 
veloped which  interrupts  temporarily  the  recoil  of  the  arterial  walls. 
The  origin  of  this  second  expansion  has  been  the  subject  of  much 
investigation,  and  at  present  it  may  be  said  that  the  question  is  not 
fully  decided.  It  is  asserted  by  some  investigators  that  it  is  central 
in  origin,  beginning  at  the  base  of  the  aorta  and  passing  to  the  per- 
iphery; by  others,  that  it  is  peripheral  in  origin,  beginning  near  the 


:66 


TEXT-BOOK  OF  PHYSIOLOGY. 


capillary  region  and  reflected  to  the  heart.  The  former  view  is  the 
one  more  generally  accepted.  According  to  it,  the  expansion  is  the 
result  of  the  sudden  closure  of  the  aortic  valves,  and  a  backward  surge 
of  the  blood  column  against  them.  The  sudden  arrest  of  the  blood  and 
its  accumulations  again  expands  the  aorta. 

The  dicrotic  notch  is  therefore  taken  as  the  moment  at  which  the 
ventricular  systole  ceases  -and  the  aortic  valves  close.  From  this  fact 
it  is  evident  that  immediately  after  the  first  expansion  the  pressure 
begins  to  fall,  even  though  the  ventricular  systole  continues,  owing 
to  the  discharge  of  blood  from  the  arterial  into  the  capillary  and  venous 
systems.  The  height  of  the  dicrotic  wave  or  the  depth  of  the  dicrotic 
notch  is  favored  by  low  arterial  tension  and  highly  elastic  arteries. 
Both  features  are  diminished  by  the  reverse  conditions.  The  apex  is 
sometimes  rounded  and  even  flat,  indicative  of  a  great  diminution  in 
arterial  elasticity.     The  sphygmogram  not  infrequently  varies  con- 


AJ 


Fig.  172. — Mosso's  Plethysmography  G.  Glass  vessel  for  holding  a  limb.  F. 
Flask  for  varying  the  water-pressure  in  G.  T.  Recording  apparatus. — (Landois  and 
Stirling.) 


siderably  from  the  normal  type  in  different  pathologic  conditions  of 
the  circulatory  apparatus.  A  consideration  of  these  variations  does 
not  fall  within  the  scope  of  this  work. 

The  Volume  Pulse. — If  an  individual  artery  expands  with  each 
systole  and  recoils  with  each  diastole  of  the  heart,  the  same  is  true  of 
all  arteries,  and  as  a  result  the  volume  of  any  organ  or  part  of  the  body 
must  undergo  similar  changes.  To  such  alternate  changes  in  volume 
the  term  volume  pulse  is  given.  The  extent  to  which  an  organ  will 
increase  in  volume  will  depend  to  some  extent  on  its  elasticity.  The 
reason  for  the  increase  in  volume  is  the  resistance  offered  to  the  flow 
of  blood  into  and  through  the  capillaries;  the  decrease  in  volume  to 
the  overcoming  of  the  resistance  through  the  arterial  recoil. 

The  variations  in  volume  may  be  recorded  by  enclosing  the  organ 
in  a  rigid  glass  or  metal  vessel,  which  at  one  point  is  in  communication 
with  a  recording  apparatus,  e.  g.,  a  tambour  with  a  lever  or  mer- 
curial manometer  with  float  and  pen.  The  space  between  the  organ 
and  vessel  is  filled  with  normal  saline,  air,  or  oil.  Such  an  apparatus 
is  known  as  a  plelhy sinograph.     A  well-known  form  of  plethysmograph 


THE  CIRCULATION  OF  THE  BLOOD. 


367 


is  that  of  Mosso  (Fig.  172).  Many  forms  of  this  apparatus  have  been 
devised  in  accordance  with  the  character  of  the  organ — spleen,  kidney, 
etc. — to  be  investigated,  though  the  principle  underlying  them  is  es- 
sentially the  same.  In  addition  to  changes  in  volume  due  to  the  heart's 
action,  most  organs  undergo  additional  changes  in  volume  from  vaso- 
motor and  respiratory  causes. 

Indeed  the  plethysmographic  is  the  most  generally  employed  method 
to  show  the  action  of  vaso-motor  nerves  in  changing  the  resistance 
of  the  arterioles  and  hence  the 
outflow  of  blood.  Thus  when 
an  organ  is  enclosed  in  a  plethys- 
mograph  and  the  arterial  pres- 
sure raised  by  either  a  direct 
stimulation  of  vaso-motor  nerves 
or  a  reflex  stimulation  of  the 
vaso-constrictor  center,  there  is 
always  a  decrease  in  the  volume 
of  the  organ  under  observation; 
and  on  the  contrary,  when  the 
vaso-dilatator  nerves  or  centers 
are  stimulated  and  the  vessels 
dilated,  there  will  be  a  fall  of 
pressure,  an  increased  outflow  of 
blood  and  an  increase  in  the 
volume  of  the  organ.  From  this 
it  is  learned  that  the  functional 
activity  of  an  organ  which  is 
attended  and  conditioned  by  an 
increased  blood-supply  is  always 
associated  with  an  increase  in 
volume.  On  plethysmographic 
records  large  undulations  are  frequently  observed  which  are  regarded 
as  of  respiratory  origin. 


Fig.  173. — The  Vessels  of  the  Frog's 
Web.  a.  Trunk  of  vein,  and  (b,  b)  its 
tributaries  passing  across  the  capillary  net- 
work. The  darkispots  are  pigment  cells. — 
(Yeo's  "Physiology.") 


THE  CAPILLARY  CIRCULATION. 

In  certain  regions  of  the  body  of  many  animals  it  is  possible,  on 
account  of  the  delicacy  and  transparency  of  the  tissues,  to  observe 
not  only  the  flow  of  blood  through  the  smaller  arteries,  capillaries, 
and  veins,  but  many  of  the  phenomena  connected  with  it,  to  which 
reference  has  already  been  made.  The  structures  usually  selected 
for  the  observation  of  these  phenomena  are  the  interdigital  membranes 
(Fig.  173),  the  tongue,  the  lung,  the  bladder,  and  the  mesentery  of  the 
frog.  Though  any  one  of  these  structures  will  afford  an  admirable 
view  of  the  blood-flow,  the  mesentery  for  many  reasons  is  the  most 
satisfactory.  For  a  comparison  of  the  phenomena  observed  in  the  cold- 
blooded animals  with  those  in  the  warm-blooded  animals  the  omentum 


368  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  guinea-pig  may  be  employed.  If  the  frog  is  the  subject  of  ex- 
periment, it  should  be  slightly  curarized  and  the  brain  destroyed  by 
pithing.  The  animal  is  then  placed  on  a  small  board  capable  of  ad- 
justment to  the  stage  of  the  microscope.  The  abdomen  is  then  opened 
along  the  side  and  a  loop  of  intestine  withdrawn  and  placed  around 
a  cork  ring  which  surrounds  an  opening  in  the  side  of  the  frog  board. 
The  loop  of  the  intestine  should  be  so  placed  that  it  will  lie  between 
the  observer  and  the  body  of  the  frog.  The  mesentery  thus  exposed 
must  be  kept  moist  with  normal  saline  solution. 

When  examined  with  low  powers  of  the  microscope,  arteries,  veins, 
and  capillaries  will  be  found  occupying  the  field  of  vision.  Their 
general  arrangement,  their  size  and  connections,  can  be  readily  deter- 
mined. After  a  few  preliminary  adjustments  a  region  will  be  found 
in  which  the  blood  is  flowing  in  opposite  directions.  The  vessel 
apparently  carrying  blood  away  from  the  observer  is  an  artery;  the 
vessel  apparently  carrying  blood  toward  the  observer  is  a  vein;  the 
smallest  vessels  are  capillaries.  The  blood  in  the  artery  is  of  a  brighter 
color  than  the  blood  in  the  vein;  the  blood  in  the  capillaries  is  almost 
colorless.  The  arterial  blood-stream  not  infrequently  shows  remit- 
tancy,  an  alternate  acceleration  and  retardation,  corresponding  to  each 
heart-beat;  the  capillary  and  venous  streams  are  uniform  and  con- 
tinuous. The  relative  velocities  in  the  three  sets  of  vessels  are  indicated 
by  the  movement  of  the  red  corpuscles.  In  the  arteries  they  pass 
before  the  eye  so  rapidly  that  they  can  not  be  distinguished;  in  the 
capillaries  they  pass  so  slowly  that  both  form  and  structure  may  be 
determined;  in  the  veins,  though  again  moving  rapidly,  they  can  often 
be  distinguished. 

The  relative  positions  of  the  red  and  white  corpuscles  in  the 
blood-stream  are  also  apparent;  the  former  occupy  the  central,  the 
latter  the  peripheral  portion,  at  the  same  time  adhering  to  the  sides 
of  the  vessel.  Between  the  axial  portion  of  the  stream  occupied  by 
the  red  curpuscles  and  the  wall  of  the  vessel  there  is  a  clear  still  layer 
of  plasma,  the  result  of  an  adhesion  of  the  plasma  to  the  wall.  It  is 
this  feature  which  gives  rise  to  the  friction  between  successive  layers 
of  the  blood-stream,  the  resistance  to  the  blood-flow,  and  the  devel- 
opment of  blood-pressure.  The  relative  breadth  of  the  still  layer 
and  amount  of  friction  are  greater  in  small  than  in  large  vessels. 

The  volume  of  blood  passing  into  any  given  capillary  area  is  de- 
termined by  the  degree  of  contraction  of  the  arterioles.  Thus  on  the 
application  of  warm  saline  solution,  which  relaxes  the  arterioles,  there 
is  a  large  increase  in  the  inflow  of  blood;  vessels  previously  invisible 
suddenly  come  into  view  as  the  blood  with  its  corpuscles  passes  into  them. 
On  the  application  of  cold  water,  which  contracts  the  arterioles  and 
diminishes  the  inflow,  many  of  the  smaller  vessels  entirely  disappear 
from  view.  The  alternate  contraction  and  relaxation  of  the  arterioles 
will  therefore  determine  the  quantity  of  blood  flowing  into  and  through 
the  capillary  system. 


THE  CIRCULATION  OF  THE  BLOOD. 


369 


Migration  of  the  White  Corpuscles. — A  phenomenon  fre- 
quently observed  in  the  capillary  vessels  of  the  mesentery  or  of  the 
bladder  of  the  frog  is  the  passage  of  the  white  corpuscles  through  the 
walls  into  the  surrounding  lymph-spaces.  To  this  process  the  term 
migration  or  diapedesis  is  given.  After  the  tissues  have  been  exposed 
to  the  air  for  some  time  or  subjected  to  an  irritant,  the  vessels  dilate 
and  become  distended  with  blood.  In  a  short  time  the  blood-stream 
slows,  and  finally  comes  to  rest.  The  condi- 
tion of  stasis  is  then  established.  During  the 
development  of  this  condition  the  white  cor- 
puscles accumulate  in  large  numbers  along  the 
inner  surface  of  the  vessels  and  soon  begin  to 
pass  through  the  vessel-walls.  This  they  do 
by  protruding  a  portion  of  their  substance  and 
inserting  it  into  and  through  the  vessel-wall. 
This  once  accomplished,  the  remainder  of  the 
cell  in  due  time  follows  until  it  has  entirely 
passed  out  into  the  tissue-space.  The  opening 
in  the  cell-wall  now  closes.  The  successive 
steps  in  this  process  are  shown  in  Fig.  174. 
As  this  migration  occurs  mainly  after  the  cir- 
culation has  ceased  or  when  the  tissues  present 
the  phenomena  of  approaching  inflammation, 
it  is  difficult  to  state  in  how  far  it  is  strictly  a 
physiologic  process. 

The  Venous  Circulation. — The  blood, 
having  passed  through  the  capillary  vessels,  is 
gathered  up  by  the  veins  and  conveyed  to  the 
right  side  of  the  heart.  As  the  veins  converge 
and  unite  to  form  larger  and  larger  trunks  the 
sectional  area  gradually  diminishes,  and  hence 
the  velocity  of  the  blood-flow  increases,  though 
it  never  attains  the  velocity,  even  in  the  venae 
cavae,  that  it  had  in  the  aorta,  for  the  reason 
that  the  sectional  area  of  the  venae  cavae  is 
considerably  larger  than  that  of  the  aorta. 
The  pressure  also  is  very  low  in  the  larger 
veins  because  the  friction  still  to  be  overcome  is  relatively  very  slight. 

The  capacity  of  the  venous  system  is  considerably  greater  than 
that  of  the  arterial  system,  as  there  are  usually  two  and  even  three 
veins  accompanying  each  artery.  This,  taken  in  connection  with  its 
greater  distensibility,  makes  of  the  venous  system  a  reservoir  in  which 
blood  can  be  stored.  On  this  reservoir  the  arterial  system  can  call 
for  that  amount  of  blood  necessary  for  the  maintenance  of  its  normal 
volume  and  pressure,  and  into  it  any  excess  can  be  discharged.  The 
relative  amounts  contained  in  the  two  systems  are  regulated  by  the 
nervous  svstem.     The  movement  of  the  blood  through  the  veins  is 


Fig.  174. — Diagram  to 
show  Various  Stages  in 
the  diapedesis  or  mi- 
GRATION oe  White  Cor- 
puscles. 


37o  TEXT-BOOK  OF  PHYSIOLOGY. 

accomplished  by  the  cooperation  of  several  forces,  reference  to  which 
will  be  made  in  a  following  paragraph. 

THE  PULMONIC  VASCULAR  APPARATUS. 

The  pulmonic  vascular  apparatus  consists  of  a  closed  system 
of  vessels  extending  from  the  right  ventricle  to  the  left  auricle,  and 
includes  the  pulmonary  artery,  capillaries,  and  pulmonary  veins.  In 
its  anatomic  structure  and  physiologic  properties  it  closely  resembles, 
if  it  is  not  identical  with,  the  systemic  apparatus. 

The  stream-bed  widens  from  the  beginning  of  the  pulmonary 
artery  to  the  middle  of  the  capillary  system;  it  again  narrows  from 
this  point  to  the  terminations  of  the  pulmonary  veins. 

The  movement  of  the  blood  from  the  beginning  to  the  end  of  the 
system  is  due  to  a  difference  of  pressure  between  these  two  points, 
the  result  of  the  friction  between  the  blood  and  the  vascular  walls. 
From  the  difference  in  the  extent  of  the  pulmonic  and  systemic  systems 
it  is  evident  that,  other  things  being  equal,  the  friction  .is  less,  and 
therefore  also  the  pressure  is  less  in  the  former  than  in  the  latter.  This 
view  is  supported  by  the  difference  in  the  thickness  of  the  walls  of 
the  right  and  left  sides  of  the  heart.  The  pressure  in  the  pulmonary 
artery  of  the  dog  was  shown  by  Beutner  to  be  about  one-third  that 
in  the  aorta;  by  Bradford  and  Dean  to  be  one-fifth.  The  velocity 
of  the  blood-stream  in  each  of  the  three  divisions  of  the  system  can 
not  well  be  determined.  The  time  occupied  by  a  particle  of  blood 
in  passing  from  the  right  to  the  left  ventricle  has  been  estimated  at 
one-fourth  the  time  required  to  pass  from  the  left  to  the  right  ven- 
tricle. Assuming  the  latter  to  be  thirty  seconds,  the  former  would  be 
seven  and  one-half  seconds. 

The  capillary  vessels  are  spread  out  in  a  very  elaborate  manner 
just  beneath  the  inner  surface  of  the  pulmonary  air-cells,  and  form, 
by  their  close  relation  to  it,  a  mechanism  for  the  excretion  of  carbon 
dioxid  and  the  absorption  of  oxygen.  The  extent  of  the  capillary 
surface  is  very  great.  It  has  been  estimated  at  200  square  meters. 
The  amount  of  blood  flowing  through  this  system  hourly  and  exposed 
to  the  respiratory  surface  is  about  800  liters.  The  reason  for  the  ex- 
istence of  the  pulmonary  circulation  is  the  renewal  of  the  oxygen 
volume  in  the  blood  and  the  elimination  of  the  carbon  dioxid;  for  the 
accomplishment  of  both  objects  ample  provision  is  here  made.  The 
flow  of  blood  through  the  cardio-pulmonary  vessels  is  subject  to  varia- 
tion during  both  inspiration  and  expiration  in  consequence  of  their 
relation  to  the  respiratory  apparatus.  The  mechanism  by  which  these 
variations  are  produced  will  be  considered  in  the  chapter  devoted  to 
Respiration. 

FORCES  CONCERNED  IN  THE  CIRCULATION  OF  THE  BLOOD. 

1.  The  Contraction  of  the  Heart. — The  primary  forces  which  keep 
the  blood  flowing  from  the  beginning  of  the  aorta  to  the  right 


THE  CIRCULATION  OF  THE  BLOOD.  371 

side  of  the  heart  and  from  the  beginning  of  the  pulmonary  artery 
to  the  left  side  are  the  contractions  of  the  left  and  right  ventricles 
respectively.  This  is  evident  from  the  fact  that  each  ventricle 
at  each  contraction  not  only  overcomes  the  pressure  in  the  aorta 
and  pulmonary  artery,  the  sum  of  all  resistances,  but  imparts  a 
given  velocity  to  the  blood.  Since  the  pressure  continuously 
falls  from  the  beginning  to  the  end  of  each  system,  it  follows  that 
the  blood  must  flow  from  the  point  of  high  to  the  point  of  low 
pressure.  During  the  interval  of  the  heart's  activity  the  walls  of 
the  arteries,  to  which  the  heart's  energy  was  largely  transferred, 
now  take  up  and  continue  the  work  of  the  heart,  and  by  recoiling 
drive  the  blood  forward  and  into  the  venous  system.  Though 
the  heart's  energy  is  probably  sufficient  to  drive  the  blood  into 
the  opposite  side  of  the  heart,  it  is  supplemented  by  other 
forces — e.  g.: 

2.  Muscle  Contraction. — As  a  result  of  the  relation  which  the  veins 

bear  to  the  muscles  in  all  parts  of  the  body  it  is  clear  that  with 
each  contraction  and  relaxation  of  the  muscles  there  will  be  exerted 
an  intermittent  pressure  on  the  veins.  With  each  contraction, 
the  blood  on  the  proximal  side  will  at  once  be  driven  forward 
with  increased  velocity,  while  that  on  the  distal  side  will  be  re- 
tarded, will  accumulate  and  distend  the  veins,  owing  to  the 
closure  of  the  valves;  with  the  relaxation  of  the  muscle  the  elastic 
and  contractile  tissues  in  the  walls  of  the  veins  will  come  into 
play  and  force  the  blood  forward. 

3.  Thoracic  Aspiration. — The  inspiratory  movement  aids  the  flow 

of  blood  through  the  venae  cavae  and  their  tributaries.  With 
each  inspiration  the  pressure  within  the  thorax  but  outside  the 
lungs  undergoes  a  diminution  more  or  less  pronounced  in  ac- 
cordance with  the  extent  of  the  movement.  As  a  result,  the 
blood  in  the  large  veins  outside  of  the  thorax,  being  now  subjected 
to  a  pressure  greater  than  that  in  the  thorax,  flows  more  rapidly 
toward  the  heart.     With  each  expiration  the  reverse  obtains. 

4.  Action  of  the  Valves. — It  is  quite  probable  that  gravity  opposes 

to  some  extent  the  flow  of  blood  through  the  veins  below  the  level 
of  the  heart.  This  opposition  to  the  upward  flow  is  largely  pre- 
vented by  the  valves,  for  each  retardation  is  immediately  checked 
by  their  closure  and  support  given  to  the  column  of  blood.  The 
influence  of  gravity  is  shown  when  the  relation  of  the  arm  to 
the  heart  is  changed.  Thus,  if  the  arm  be  allowed  to  hang  pas- 
sively by  the  side  of  the  body,  the  veins,  especially  on  the  back  of 
the  hand,  will  become  distended  with  blood.  If  now  the  arm  be 
raised,  the  blood  will  flow  rapidly  toward  the  heart,  as  shown  by 
the  rapid  emptying  of  the  veins. 
Work  Done  by  the  Heart. — The  work  which  the  left  ventricle 

performs  at  each  contraction  when  it  discharges  its  contained  volume 

of  blood  into  the  aorta  is: 


372  TEXT-BOOK  OF  PHYSIOLOGY. 

i.  To  overcome  the  total  resistance  of    the  systemic  vascular  appa- 
ratus expressed  in  terms  of  aortic  pressure;  and — 
2.  To  impart  velocity  to  the  blood. 

The  pressure  in  the  aorta  is  not  absolutely  determined,  though 
for  many  reasons  it  may  be  assumed  to  be  about  150  mm.  Hg.,orits 
equivalent,  a  column  of  blood  1.93  meters  in  height.  If  the  volume 
of  blood  which  the  heart  discharges  is  assumed  to  be  188  grams,  the 
work  done  may  be  calculated  by  multiplying  the  weight  by  the  height : 
viz.,  0.188  X  1.93  =  0.3628  kilogrammeter. 

The  velocity  of  the  blood  in  the  aorta  has  been  approximately 
estimated  at  0.5  meter  per  second.  The  work  done  in  imparting  this 
velocity  to  188  grams  is  estimated  by  squaring  the  velocity  and  dividing 
by  the  accelerating  force  of  gravity  (oX598l)  and  multiplying  the 
quotient  by  0.188.  The  quotient  of  the  first  two  values  represents 
the  distance  a  body  would  have  to  fall  to  acquire  this  velocity:  viz., 
0.0127  meter.  The  work  done  is  therefore  0.188  X  0.0127,  or  0.0023 
kilogrammeter. 

The  entire  work  of  the  left  ventricle  is  the  sum  of  these  two  amounts, 
or  0.3651  kilogrammeter.  Assuming  that  the  heart  beats  72  times  per 
minute,  the  work  done  daily  would  be  0.604  X  72  X  60  X  24,  or 
37,857.6  kilogrammeters.  The  right  ventricle  approximately  per- 
forms about  one-third  of  this  amount  of  work  in  overcoming  the 
resistance  offered  by  the  pulmonary  system  and  in  imparting  velocity 
to  its  contained  volume  of  blood.  The  work  of  the  entire  heart  would 
therefore  be  for  the  twenty-four  hours  about  50,476  kilogrammeters. 


THE  NERVE  MECHANISM  OF  THE  VASCULAR  APPARATUS. 

The  middle  coat  of  the  arteries,  and  especially  of  those  in  the 
peripheral  region  of  the  arterial  system,  consists  of  well-defined 
layers  of  non-striated  muscle-fibers  arranged  in  a  circular  direction  or 
at  right  angles  to  the  long  axis  of  the  vessel.  In  the  physiologic  con- 
dition these  fibers  are  in  a  state  of  continuous  contraction,  more  or 
less  pronounced,  and  give  to  the  arteries  a  certain  average  caliber 
which  permits  a  definite  volume  of  blood  to  flow  through  them  in  a 
given  unit  of  time. 

The  cause  of  this  tonic  contraction  is  not  definitely  known.  It 
has  been  attributed  to  the  action  of  local  nerve-ganglia,  to  the  pres- 
sure of  blood  from  within,  to  the  influence  of  organic  substances  in  the 
blood,  the  products  of  gland  activity:  e.  g.,  adrenalin  or  cpinephrin. 

This  tonic  contraction  of  the  vascular  muscle  is  subject  to  increase 
or  decrease  in  accordance  with  the  action  of  various  agents.  In- 
creased contraction  will  result  in  a  decrease  of  the  caliber  and  a 
reduction  in  the  outflow  of  blood.  Decrease  of  the  contraction  or 
relaxation  will  result  in  an  increase  both  of  the  caliber  and  outflow 
of  blood.     The  small  arteries  thus  determine  the  volume  of  blood 


THE  CIRCULATION  OF  THE  BLOOD.  373 

passing  to  any  given  area  or  organ  in  accordance  with  its  functional 
activities. 

The  Vaso-motor  Nerves. — The  activities  of  the  vascular  muscle 
are  regulated  by  the  central  nerve  system  through  the  intermedia- 
tion of  nerve-fibers,  termed  vaso-motor  nerves.  Of  these  there  are 
two  kinds,  one  which  increases  or  augments  the  contraction,  the 
vaso-constrictors  or  vaso-augmentors;  another  which  decreases  or 
inhibits  the  contraction,  the  vaso-dilatators  or  vaso-inhibitors. 

The  vaso-motor  nerves  of  both  classes,  unlike  the  ordinary  motor 
nerves,  do  not  pass  directly  to  the  muscle-fiber,  but  indirectly  by  way 
of  the  ganglia  of  the  sympathetic  nerve  system.  In  these-  ganglia 
the  vaso-motor  nerves,  which  come  from  the  central  nerve  system, 
terminate,  breaking  up  into  tufts,  which  arborize  around  the  nerve- 
cells.  From  the  cells  new  nerve-fibers  arise  which  then  pass  without 
interruption  to  their  final  destination. 

The  nerve-fibers  which  emerge  from  the  central  nerve  system 
are  extremely  fine  in  caliber  and  medullated;  those  which  emerge 
from  the  sympathetic  ganglia  are  equally  fine,  but  non-medullated. 
The  former  are  termed  pre-ganglionic,  the  latter  post- ganglionic 
fibers.  The  ganglion  in  which  the  pre-ganglionic  fibers  end  is  not 
necessarily  found  in  the  pre- vertebral  or  lateral  chain;  it  may  be 
found  in  the  collateral  or  even  in  the  peripheral  group  of  ganglia. 

The  Vaso-constrictor  Nerves.— The  vaso-constrictor  nerves  take 
their  origin  from  nerve-cells  located  in  the  anterior  horns  and  lateral 
gray  matter  of  the  spinal  cord.  They  emerge  from  the  cord  in  company 
with  the  fibers  which  compose  the  anterior  roots  of  the  spinal  nerves 
from  the  second  thoracic  to  the  second  or  third  lumbar  nerves  inclusive. 
A  short  distance  from  the  cord  they  leave  the  anterior  roots  as  the 
white  rami  communicantes  and  enter  the  pre-vertebral  or  lateral 
sympathetic  ganglia.  From  the  results  of  many  observations  and 
experiments  it  is  probable  that  the  great  majority  of  the  vaso-con- 
strictor nerves  terminate  in  these  ganglia;  that  is  to  say,  it  is  here  that 
the  pre-ganglionic  fibers  arborize  around  the  contained  nerve-cells. 
From  the  nerve-cells  new  fibers  arise,  the  post-ganglionic,  which  pass 
to  the  blood-vessels  of  the  head,  to  the  upper  and  lower  extremities, 
and  to  the  thoracic  and  abdominal  viscera. 

The  vaso-constrictors  for  the  head  emerge  from  the  spinal  cord 
in  the  first  four  thoracic  nerves,  thence  pass  successively  into  and 
through  the  ganglion  stellatum  (the  first  thoracic),  the  annulus  of 
Vieussens,  the  inferior  cervical  ganglion,  the  sympathetic  cord  to 
the  superior  cervical  ganglion,  around  the  cells  of  which  they  arborize. 
From  this  ganglion  the  new  fibers  follow  the  carotid  artery  and  its 
branches  to  their  terminations. 

The  vaso-constrictors  for  the  fore-limbs  emerge  from  the  cord  in 
the  roots  of  the  fourth  to  the  tenth  thoracic  nerves  inclusive.  Through 
the  white  rami  they  pass  into  the  sympathetic  chain,  after  which  they 
take  an  upward  direction  and  terminate  around  the  cells  of  the  gan- 


374  TEXT-BOOK  OF  PHYSIOLOGY. 

glion  stellatum.  From  this  ganglion  the  new  fibers  enter,  by  way  of 
the  gray  rami  communicantes,  the  trunks  of  the  cervical  nerves  which 
unite  to  form  the  brachial  plexus  and  by  this  route  pass  to  the  blood- 
vessels. 

The  vaso-constrictors  for  the  hind-limbs  emerge  from  the  cord 
in  the  roots  of  the  eleventh  dorsal  to  the  second  or  third  lumbar  nerves 
inclusive.  They  then  pass  through  the  white  rami  to  the  lower 
lumbar  and  upper  sacral  ganglia.  Thence  by  way  of  the  gray  rami 
they  pass  into  the  nerve-trunks  which  unite  to  form  the  sacral  nerves 
and  by  this  route  pass  to  the  blood-vessels. 

The  vaso-constrictors  for  the  viscera  of  the  abdominal  cavity  pass 
by  way  of  the  splanchnic  nerves  directly  into  the  collateral  ganglia, 
the  semilunar,  the  superior  mesenteric,  the  inferior  mesenteric,  and 
the  sacral.  From  these  ganglia  an  elaborate  network  of  non-medul- 
lated  fibers  passes  to  the  blood-vessels  of  the  stomach,  intestines,  and 
other  viscera.  The  great  splanchnic  nerve  is  one  of  the  most  im- 
portant vaso-constrictor  trunks  of  the  body,  on  account  of  the  large 
vascular  area  it  controls. 

The  existence,  course,  distribution,  and  functions  of  the  vaso- 
constrictor nerves  have  been  determined  by  a  variety  of  methods, 
physiologic  and  anatomic.  Stimulation  of  the  nerve-trunks  under 
appropriate  conditions  gives  rise  to  a  contraction,  division  to  a  dilata- 
tion of  the  blood-vessels.  The  physiologic  continuity  of  the  pre- 
ganglionic fibers  with  the  nerve-cells  of  the  sympathetic  ganglia 
has  been  shown  by  the  intra-vascular  injection  or  the  local  applica- 
tion of  nicotin.  This  agent,  as  shown  by  Langley,  has  a  selective 
action  on  the  arborizations  of  the  pre-ganglionic  fibers,  and  when 
given  in  sufficient  doses  suspends  their  conductivity;  hence  stimu- 
lation of  the  pre-ganglionic  fibers  is  without  effect,  though  stimulation 
of  the  post-ganglionic  fibers  is  followed  by  the  usual  contraction. 

The  following  will  serve  as  illustrations  of  the  functions  of  vaso- 
constrictor nerves.  Division  of  the  great  splanchnic  is  followed  by 
a  marked  dilatation  of  the  blood-vessels  of  the  intestinal  tract;  stimu- 
lation of  the  peripheral  end  by  their  contraction.  Division  of  the 
cervical  cord  of  the  sympathetic  is  followed  by  dilatation  of  the  blood- 
vessels of  the  side  of  the  head;  stimulation  of  the  peripheral  end  by 
their  contraction. 

The  Vaso-dilatator  Nerves. — The  vaso-dilalalor  nerves  have 
their  origin  for  the  most  part  in  nerve-cells  situated  in  the  region  of  the 
spinal  cord  included  between  the  origins  of  the  second  dorsal  to  the 
second  lumbar  nerves  inclusive,  though  they  are  not  confined  to  this 
region.  Some  vaso-dilatator  fibers  have  their  origin  in  the  medulla 
oblongata,  others  in  the  sacral  region  of  the  spinal  cord. 

The  general  course  of  the  dilatator  fibers  for  the  intestinal  tract 
is  the  same  as  that  of  the  vaso-constrictors,  though  instead  of  becom- 
ing related  to  the  nerve-cells  in  the  pre-vertebral  ganglia,  they  pass 
by  way  of  the  splanchnics  to  the  collateral  ganglia,  the  semilunar, 


THE  CIRCULATION  OF  THE  BLOOD.  .      375 

the  superior  and  inferior  mesenteric,  and  perhaps  to  peripheral  ganglia 
in  or  near  the  blood-vessels  themselves. 

The  vaso-dilatators  for  the  limbs  are  found  in  the  common  nerve- 
trunks  associated  with  the  usual  motor  and  sensor  fibers,  though  the 
exact  route  by  which  they  pass  from  the  spinal  cord  to  the  peripheral 
nerves  has  not  in  all  cases  been  determined.  Their  cell  stations  have 
not  been  definitely  located.  The  vaso-dilatator  nerves  for  the  blood- 
vessels of  the  submaxillary  gland  arise  in  the  medulla,  pass  outward 
in  the  trunk  of  the  facial  nerve,  and  reach  the  gland  by  way  of  the 
chorda  tympani  branch.  Their  cell  station  is  in  the  ganglion  near 
the  hilum  of  the  gland.  The  vaso-dilatator  nerves  for  the  blood- 
vessels of  the  corpora  cavernosa  of  the  penis,  the  nervi  erigentes, 
have  their  origin  in  the  sacral  region  of  the  spinal  cord;  and  emerge 
in  the  roots  of  the  second  and  third  sacral  nerves.  Their  cell  station 
is  in  the  ganglion  near  the  organ. 

The  existence,  course,  distribution,  and  functions  of  the  vaso- 
dilatator fibers  have  been  determined  by  the  same  methods  employed 
in  the  investigation  of  the  vaso-constrictors.  Thus  division  and 
stimulation  of  the  peripheral  end  of  the  chorda  tympani  nerve  are 
at  once  followed  by  an  active  dilatation  of  the  blood-vessels  of  the 
submaxillary  gland.  The  inflow  of  blood  is  so  great  that  the  gland 
becomes  bright  red  in  color.  Its  tissues  being  unable  to  appropriate 
all  the  oxygen,  the  blood  emerges  in  the  veins  almost  arterial  in  char- 
acter. Stimulation  of  the  peripheral  ends  of  the  divided  nervi  erigentes 
is  followed  by  similar  effects  in  the  blood-vessels  of  the  corpora  caver- 
nosa. Slow  stimulation,  once  per  second,  of  the  peripheral  end  of  a 
divided  sciatic  nerve  is  followed  by  dilatation  of  the  blood-vessels  of 
the  leg. 

From  these  and  many  other  facts  of  a  similar  character  it  is  highly 
probable  that  the  blood-vessels  of  each  organ  are  under  the  control 
of  two  antagonistic  classes  of  nerve-fibers,  one  augmenting  the  degree 
of  their  contraction,  the  vaso-constrictors,  the  other  diminishing  it 
through  inhibition,  the  vaso-inhibitors.  Through  the  cooperative 
antagonism  of  these  two  classes  of  nerves  the  caliber  of  the  blood- 
vessels and  thereby  the  volume  of  the  blood  is  accurately  adapted  to  the 
needs  of  each  organ  both  during  rest  and  during  activity.  It  is  also 
to  the  alternate  activity  of  these  nerves  that  the  variations  occurring 
from  time  to  time  in  the  volume  of  organs  are  to  be  attributed. 

Physiologic  Properties. — The  vaso-constrictors  and  the  vaso- 
dilatators differ  somewhat  in  their  physiologic  properties,  as  shown  by 
the  results  of  experiment.  Thus,  when  a  mixed  nerve,  i.  e.,  one  con- 
taining both  classes  of  fibers — e.  g.,  the  sciatic — is  stimulated  with 
frequently  repeated  induced  currents,  the  constrictor  effect  is  the  more 
pronounced,  the  dilatator  effect  being  wanting  or  prevented;  when 
stimulated  with  slowly  repeated  induced  currents,  the  dilatator  effect 
is  the  more  pronounced.  These  different  effects  are  strikingly  shown 
in  Fig.  175,  A  and  B. 


376 


TEXT-BOOK  OF  PHYSIOLOGY. 


In  the  experiment  of  which  these  tracings  are  the  result  the  leg 
of  a  cat  was  enclosed  in  a  plethysmograph  and  the  variations  in  volume 
due  to  dilatation  or  contraction  of  the  vessels,  following  stimulation 
of  the  sciatic  nerve,  were  recorded  by  means  of  a  tambour  and  lever 
on  a  slowly  revolving  cylinder.  In  A  the  fall  of  the  curve  indicates 
a  diminution  of  volume,  from  contraction  of  blood-vessels  following 
a  rate  of  stimulation  of  the  sciatic  nerve  of  16  per  second  for  fifteen 
seconds.  In  B  the  rise  of  the  curve  indicates  an  increase  in  volume 
from  dilatation  of  the  vessels  following  a  rate  of  stimulation  of  i  per 
second  for  fifteen  seconds  (Bowditch  and  Warren).  With  different 
rates  of  stimulation  somewhat  different  results  are  obtained. 

After  division  of  a  mixed  nerve  the  vaso-constrictors  degenerate 
and  lose  their  influence  over  the  blood-vessel  in  from  four  to  five 


^ 

1 

f^i^ 

Fig.  175. — Plethysmograms  of  the  Hind-leg  of  the  Cat  following  Stimulation 
of  the  Sciatic  Nerve.  In  A  the  rate  of  stimulation  was  sixteen  per  second,  in  B  one 
per  second  for  fifteen  seconds. 


days,  the  vaso-dilatators  in  from  seven  to  ten  days,  as  shown  by  the 
response  to  electrical  stimulation. 

When  a  nerve  is  cooled,  the  vaso-constrictors  lose  their  irritability 
before  the  vaso-dilatators. 

Vaso-motor  Centers. — The  nerve-cells  throughout  the  spinal 
cord  from  which  the  vaso-motor  nerves  take  their  origin  may  be 
regarded  as  nerve-centers  which  through  their  related  nerve-fibers 
exert  from  time  to  time  either  a  constrictor  or  a  dilatator  influence 
over  the  blood-vessels.  Though  both  the  vaso-constrictor  and  vaso- 
dilatator centers  are  in  a  state  of  continuous  activity,  the  former 
decidedly  preponderates,  as  shown  by  the  maintenance  of  a  tonic 
contraction  of  the  blood-vessels.  The  activity  of  both  centers  may 
be  increased  or  decreased,  augmented  or  inhibited,  by  nerve  impulses 
reflected  to  them  from  the  periphery  through  afferent  nerves  or  through 
nerve-fibers  descending  the  cord  from  higher  levels  of  the  nerve- 
system.  Experiment  has  shown  that  when  a  definite  region  of  the 
medulla  oblongata  is  punctured  or  in  anywise  destroyed  there  is  an 


THE  CIRCULATION  OF  THE  BLOOD.  377 

immediate  dilatation  of  the  blood-vessels  throughout  the  body  and  a 
fall  of  blood-pressure  below  one-half  or  one-third  of  the  normal  value. 
This  region  has  a  width  of  one  and  a  half  millimeters  and  extends 
longitudinallv  for  a  distance  of  four  or  five  millimeters,  terminating 
at  a  point  four  millimeters  above  the  tip  of  the  calamus  scriptorius. 

A  transection  of  the  medulla  above  the  upper  limit  of  this  area  is 
without  effect  on  the  blood-pressure.  A  similar  section  below  it. 
however,  is  at  once  followed  by  vascular  dilatation,  a  loss  of  vascular 
tone,  and  a  general  fall  of  blood-pressure.  Subsequent  stimulation 
of  the  peripheral  end  of  the  divided  medulla,  the  animal  being  curar- 
ized  and  artificial  respiration  maintained,  will  give  rise  to  a  marked 
contraction  of  the  blood-vessels  and  a  rise  of  blood-pressure  up  to 
and  Tar  beyond  the  normal  value. 

If  the  experimental  lesion  is  limited  to  the  area  mentioned  in  the 
foregoing  paragraph,  the  vascular  dilatation  passes  away  after  a 
time,  the  blood-vessels  regain  their  normal  tone,  and  the  pressure 
again  rises.  These  facts  indicate  that  there  is  in  the  gray  matter 
beneath  the  floor  of  the  fourth  ventricle,  a  restricted  area  composed  of 
nerve-cells,  which  maintains  through  efferent  nerve-fibers  the  tonus 
of  the  blood-vessels  by  virtue  of  its  dominating  influence  over  the  vaso- 
motor centers  in  the  cord,  and  which  is  therefore  to  be  regarded  as  the 
general  vaso-motor  {constrictor)  center.  The  vaso-motor  centers  through- 
out the  cord  are  to  be  regarded  as  subsidiary  centers.  The  nerve- 
fibers  which  transmit  the  regulative  nerve  impulses  from  the  general  to 
the  subsidiary  centers  are  to  be  found  in  the  lateral  columns  of  the 
spinal  cord. 

Since  the  blood-vessels  maintain  a  more  or  less  constant  tone,  it  is 
assumed  that  the  vaso-motor  center  is  in  a  state  of  continuous  activity. 
In  how  far,  however,  this  activity  is  the  result  of  chemic  changes 
between  the  cells  and  the  surrounding  lymph  and  blood,  or  the  result 
of  constantly  arriving  nerve  impulses  reflected  from  the  periphery 
or  from  higher  regions  of  the  nerve  system,  is  not  readily  deter- 
minable.    Both  factors  are  probably  involved. 

A  general  vaso-dilatator  center  has  never  been  located  and  there  are 
many  reasons  for  thinking  that  such  a  center  has  no  anatomic  existence. 
There  are,  however,  special  or  local  vaso-dilatator  centers  in  the 
medulla  oblongata  and  in  various  regions  of  the  spinal  cord  and 
especially  in  the  lumbar  region. 

Direct  Stimulation  of  the  Vaso-motor  Centers. — The  general 
vaso-motor  (constrictor)  center  at  least  is  markedly  influenced  by 
the  quantity  and  quality  of  blood  and  lymph  circulating  around  and 
through  it.  If  the  blood-supply  to  the  medulla  and  associated  struc- 
tures be  diminished  by  compression  of  the  carotid  arteries,  the  activity 
of  the  center  is  at  once  increased,  as  shown  by  increased  vascular 
contraction  and  a  rise  of  pressure.  Restoration  of  the  blood-supply 
is  followed  by  a  return  of  the  center  to  its  normal  degree  of  activity. 
Increased  blood-supply,  as  in  cerebral  hyperemia,  is  attended  by  a 


378  TEXT-BOOK  OF  PHYSIOLOGY. 

fall  in  blood-pressure  indicating  a  decrease  in  the  activity  of  the  center. 
A  diminution  in  the  percentage  of  oxygen  or  an  increase  in  the  per- 
centage of  C02  in  the  blood  will  increase  the  activity  of  the  center. 
In  asphyxia  especially,  the  center  is  extremely  excitable,  as  shown  by 
a  rise  of  the  arterial  tension.  The  subsidiary  centers  in  the  spinal 
cord  are  influenced  by  corresponding  conditions. 

Reflex  Stimulation  of  the  Vaso-motor  Centers. — The  results 
of  experiment  make  it  certain  that  the  degree  of  vascular  contraction 
maintained  by  the  vaso-motor  centers  can  be  increased  or  decreased 
by  nerve  impulses  reflected  to  the  cord  and  medulla  from  the  periphery 
or  from  the  brain.  The  effect  may  be  general,  or  local  and  confined 
to  the  area  from  which  the  impulses  arise.  The  following  experi- 
ments may  be  cited  as  illustrations: 

Stimulation  of  the  central  end  of  a  divided  posterior  root  of  a 
spinal  nerve  gives  rise  to  increased  vascular  contraction,  as  shown 
by  the  rise  of  blood-pressure.  Stimulation  of  the  central  end  of  the 
divided  sciatic  will  give  rise  to  opposite  results,  according  to  the  strength 
of  the  stimulus,  weak  stimuli  producing  dilatation,  strong  stimuli 
producing  contraction  of  the  vessels.  Stimulation  of  the  central  end 
of  the  divided  vagus  gives  rise  to  dilatation  of  the  vessels  of  the  lips, 
cheeks,  and  nasal  and  palatal  mucous  membranes.  Stimulation  of 
the  tongue  is  followed  by  dilatation  of  the  vessels  of  the  submaxillary 
gland.  Stimulation  of  certain  branches  of  the  vagus  nerve  is  followed 
by  a  passive  dilatation  of  blood-vessels  and  a  marked  fall  of  pressure. 
Stimulation  of  the  peripheral  terminations  of  the  afferent  nerves  of 
the  glans  penis  will  give  rise  to  an  active  dilatation  of  the  blood-vessels 
of  the  corpora  cavernosa,  etc. 

A  satisfactory  explanation  of  these  different  results  is,  however, 
wanting.  By  some  investigators  it  is  believed  that  the  usual  variations 
in  the  arteriole  contraction  are  the  outcome  of  corresponding  varia- 
tions in  the  activity  of  the  general  vaso-constrictor  center  the  result 
of  nerve  impulses  coming  through  afferent  nerves. 

The  preceding  statements  as  to  the  effects  on  the  degree  of  vascular 
contraction,  and  hence  on  the  blood-pressure  which  follow  stimulation 
of  different  afferent  nerves,  has  led  to  the  assumption  that  there  are  in 
most  afferent  nerves  two  classes  of  nerve-fibers,  though  perhaps  in 
varying  proportions,  one  of  which  when  in  activity  augments,  the 
other  of  which  when  in  activity  inhibits  the  activity  of  the  vaso-con- 
strictor center.  The  former  class  is  generally  termed  pressor,  the 
latter  depressor  fibers. 

It  is  possible,  therefore,  that  under  physiologic  conditions,  physio- 
logic stimuli  act  on  the  peripheral  terminations  of  either  the  one  or  the 
other;  according  as  they  do  will  the  center  be  augmented  or  inhibited 
in  its  activity,  and  attended  by  either  an  increase  or  a  decrease  in  the 
degree  of  the  previous  vascular  contraction. 

Again  it  may  be  assumed,  from  the  results  of  experimentation  on 
afferent  nerves,  that  the  physiologic  stimuli  may  act  simultaneously 


THE  CIRCULATION  OF  THE  BLOOD. 


379 


on  the  peripheral  terminations  of  both  classes  of  fibers  and  that  the 
vaso-constrictor  center  is  acted  on  by  the  two  antagonistic  influences. 
In  this  assumption  the  resultant  effect  on  the  blood-vessels,  viz., 
increased  or  decreased  contraction,  will  be  the  resultant  of  their  action 
on  the  vaso-constrictor  center.  If  the  stimuli  act  preponderantly 
on  the  depressor  fibers  the 
center  will  be  depressed 
and  the  vessels  will  dilate; 
if  they  act  preponderantly 
on  the  pressor  fibers  the 
center  will  be  stimulated 
and  the  vessels  will  con- 
tract. 

Inasmuch  as  the  vas- 
cular dilatation  is  often 
greater  than  the  dilatation 
which  follows  division  of 
the  vaso-motor  fibers  them- 
selves, it  has  been  assumed 
by  some  that  the  general 
vascular  tonus,  as  well  as 
its  variations  from  time  to 
time,  is  the  resultant  of  the 
simultaneous  activity  and 
variations  in  activity  of 
both  vaso-constrictor  and 
vaso-dilatator  centers;  that 
in  the  afferent  nerves  there 
are  two  sets  of  fibers,  one 
of  which  when  stimulated 
augments  the  activity  of  the 
vaso-constrictor  center  and 
inhibits  the  activity  of  the 
vaso-dilatator  center;  the 
other  of  which  auguments 
the  activity  of  the  vaso- 
dilatator center  and  in- 
hibits the  activity  of  the 
vaso-constrictor  center. 
The  result,  either  contrac- 
tion   or    dilatation,    which 

follows  stimulation  of  their  peripheral  terminations  will  depend  on  the 
character  of  the  physiologic  stimulus. 

In  those  particular  instances  in  which  stimulation  of  the  peripheral 
terminations  of  afferent  nerves,  e.  g.,  the  nervi  erigentes  and  chorda 
tympani,  is  followed  by  active  dilatation  of  the  blood-vessels,  it  has  been 
assumed  that  there  are  afferent  nerve-fibers  which  directly  stimulate 


sym.n 
depr.n 
vag.n 


car.  a 


Fig.  176. — Diagram  showing  the  Origin 
and  Relation  of  the  Depressor  Nerve  in  the 
Rabbit.  Depr.  n.,  depressor  nerve;  vag.  n.,  vagus 
nerve;  sup.  1.  n.,  superior  laryngeal  nerve;  inf.  e.g., 
inferior  cervical  ganglion;  sym.  n.,  sympathetic  nerve ; 
car.  a.,  carotid  artery;  dig.  m.,  digastric  muscle; 
hyp.  n.,  hypoglossal  nerve;  sup.  c.  g.,  superior  cer- 
vical ganglion;  inf.  1.  n.,  inferior  laryngeal  nerve. 


380  TEXT-BOOK  OF  PHYSIOLOGY. 

or  augment  the  activity  of  a  special  vaso-dilatator  center  and  for  this 
reason  should  be  termed  "reflex  vaso-dilatator  nerves"  (Hunt). 

The  Influence  of  Emotional  States. — The  vaso-motor  centers 
are  capable  of  being  influenced  in  their  activities  by  emotional  states, 
doubtless  as  a  result  of  the  arrival  of  nerve  impulses  from  the  cortex 
of  the  cerebrum.  Thus  it  is  well  known  that  fear  causes  a  contraction 
of  the  blood-vessels  of  the  head  and  face  and  that  shame  causes  a 
dilatation  of  the  same  vessels.  With  the  cessation  or  the  disappear- 
ance of  the  emotional  state,  the  blood-vessels  return  to  their  former 
degree  of  contraction. 

The  Depressor  Nerve. — A  striking  illustration  of  the  depressor  or 
inhibitor  action  of  afferent  nerves  upon  the  vaso-constrictor  center  is 
furnished  by  the  result  of  stimulation  of  a  branch  of  the  vagus,  the  so- 
called  "depressor  nerve."  In  the  rabbit,  Fig.  176,  there  is  a  small 
nerve  formed  bv  the  union  of  a  branch  from  the  trunk  of  the  vagus 


FIG  j77, — Fall  of  Blood-pressure  from  Excitation  of  the  Depressor  Nerve. 
The  cylinder  was  stopped  in  the  middle  of  the  curve  and  the  excitation  maintained  for 
seventeen  minutes.  The  line  of  zero  pressure  (0,0)  should  be  30  mm.  lower  than  here 
shown. — (Bayliss.) 

with  a  branch  from  the  superior  laryngeal.  The  peripheral  distribu- 
tion of  this  nerve  is  over  the  wall  of  the  ventricle  and  perhaps  to  some 
extent  to  the  structures  of  the  arota  near  its  origin.  A  similar  ana- 
tomic arrangement  is  met  with  in  the  horse,  pig,  and  hedge  hog.  In 
some  other  animals,  as  the  dog,  it  is  bound  up  in  the  vago-sympathetic. 
In  man  it  is  also  present,  though  shortly  after  its  origin  it  enters  the 
trunk  of  the  vagus.  Division  of  this  nerve  is  without  effect  either  on 
the  heart  or  the  vessels.  Stimulation  of  the  peripheral  end  has  neither 
an  accelerator  nor  an  inhibitor  action  on  the  heart.  Stimulation  of 
the  central  end  is  followed  by  a  fall  in  blood-pressure,  frequently 
to  a  level  below  one-half  the  normal  value;  at  the  same  time  there  is  a 
diminution,  brought  about  reflexly,  in  the  rate  of  the  heart-beat  (Fig. 
[77).  The  fall  in  pressure,  however,  is  not  due  to  this  cause,  for  it 
occurs  equally  well  after  division  of  all  the  cardiac  nerves.     For  this 


THE  CIRCULATION  OF  THE  BLOOD.  381 

reason  the  nerve  was  termed  the  depressor  nerve  of  the  vaso-motor 
center. 

On  exposure  of  the  abdominal  cavity,  it  is  observed  during  stimula- 
tion of  the  depressor  that  there  is  a  notable  dilatation  of  the  intestinal 
vessels.  From  this  fact  it  was  assumed  that  the  action  of  the  depres- 
sor nerve  was  to  lower  the  general  pressure  through  reflex  dilatation 
of  these  vessels.  It  has  been  shown  by  Porter  and  Beyer  that  if  the 
splanchnics  are  divided  and  the  peripheral  end  stimulated  so  as  to 
maintain  the  tonus  of  the  intestinal  vessels,  and  hence  the  general  pres- 
sure, stimulation  of  the  depressor  nerve  will  nevertheless  be  followed 
by  a  fall  of  the  blood-pressure  almost  as  great  as  when  the  splanchnics 
are  intact.  From  this  it  is  evident  that  the  depressor  nerve  is  related 
to  centers  which  influence  the  vascular  apparatus  in  its  entirety. 
It  has  been  supposed  that  through  it  the  heart  can  protect  itself  from 
injurious  results  of  an  excessive  rise  of  arterial  pressure. 

Thus,  when  the  intra-cardiac  pressure  or  the  intra-aortic  pressure 
rises  beyond  a  normal  amount  from  increased  resistance,  the  peripheral 
terminations  of  this  nerve  are  stimulated  with  the  result  that  the  vaso- 
motor center  is  inhibited  and  the  arterioles  relaxed.  Through  this 
means  the  pressure  falls  and  the  work  of  the  heart  is  lessened. 


CHAPTER  XV. 
RESPIRATION. 

Respiration  is  a  process  by  which  oxygen  is  introduced  into,  and 
carbon  dioxid  removed  from,  the  body.  The  assimilation  of  the 
former  and  the  evolution  of  the  latter  take  place  in  the  tissues  as  a 
part  of  the  general  process  of  nutrition.  Without  a  constant  supply 
of  oxygen  and  an  equally  constant  removal  of  the  carbon  dioxid, 
those  chemic  changes  which  underlie  and  condition  all  life  phenom- 
ena could  not  be  maintained. 

The  general  process  of  respiration  may  be  considered  under  the 
following  headings,  viz. : 

i.  The  anatomy  and  general  arrangement  of  the  respiratory  appa- 
ratus. 

2.  The  mechanic  movements  of  the  thorax  by  which  an  interchange 

of  atmospheric  and  intra-pulmonary  air  is  accomplished. 

3.  The  chemistry  of  respiration,  the  changes  in  composition  under- 

gone by  the  air,  blood,  and  tissues. 

4.  The   nerve    mechanism  by  which  the  respiratory  movements  are 

maintained. 

THE  RESPIRATORY  APPARATUS. 

The  respiratory  apparatus  consists  essentially  of: 

1.  The  lungs  and  the  air-passages  leading  into  them:  viz.,  the  nasal 

chambers,  mouth,  pharynx,  larynx,  and  trachea. 

2.  The  thorax  and  its  associated  structures. 

The  nasal  chambers  are  the  natural  entrances  for  the  inspired 
air.  Their  complicated  structure  slightly  retards  the  movement  of 
the  air,  in  consequence  of  which  its  temperature  and  moisture  are 
adjusted  to  the  physiologic  conditions  of  the  lower  respiratory  pas- 
sages. The  mouth,  though  frequently  serving  as  an  entrance  for  air, 
is  not  primarily  a  respiratory  passage.  Both  the  nasal  chambers 
and  the  mouth  communicate  posteriorly  with  the  pharynx,  in  which 
the  respiratory  and  the  deglutitory  passages  cross  each  other,  the 
former  leading  directly  into  the  larynx. 

The  larynx  is  a  complicated  mechanism  serving  the  widely  dif- 
ferent though  related  functions  of  respiration  and  phonation.  It 
consists  of  a  framework  of  cartilages,  articulating  one  with  another, 
united  by  ligaments  and  moved  by  muscles;  it  is  covered  externally 
with  fibrous  tissue  and  lined  with  mucous  membrane.  The  superior 
opening  of  the  larynx,  the  glottis,  is  triangular  in  shape,  the  base  ' 

382 


RESPIRATION. 


383 


being  directed  upward  and  forward,  the  apex  downward  and  back- 
ward.    The  inclination  of  the  glottic  opening  is  almost  vertical. 

The  cavity  of  the  larynx  is  partially  subdivided  by  the  interposition 
of  the  vocal  bands  into  a  superior  and  an  inferior  portion.  The 
opening,  bounded  by  the  vocal  bands,  is  also  triangular  in  shape, 


Fig.  178.  —  Trachea  axd  Bronchial  Tubes,  i,  2.  Larynx.  3,  3.  Trachea.  4. 
Bifurcation  of  trachea.  5.  Right  bronchus.  6.  Left  bronchus.  7.  Bronchial  division 
to  upper  lobe  of  right  lung.  8.  Division  to  middle  lobe.  9.  Division  to  lower  lobe. 
10.  Division  to  upper  lobe  of  left  lung.  n.  Division  to  lower  lobe.  12,  12,  12,  12. 
Ultimate  ramifications  of  bronchi.  13,  13,  13,  13.  Lungs,  represented  in  contour 
14,  14.  Summit  of  lungs.     15,  15.  Base  of  lungs. — (Sappey.) 


though  in  this  case  the  base  is  directed  backward,  the  apex  forward. 
(See  chapter  on  Voice  and  Speech.) 

The  introduction  of  the  vocal  bands  narrows  at  this  level  the  air- 
passage  and  to  some  extent  interferes  with  the  free  entrance  of  air. 
According  to  the  investigations  of  Semon,  the  area  of  the  air-passage 
above  and  below  the  phonatory  apparatus  is  about  200  sq.  mm.; 
while  the  area  bounded  by  the  vocal  apparatus  is  but  155  sq.  mm. 
during  quiet  respiration. 


584 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  trachea  is  a  tube,  some  12  centimeters  in  length,  from  one- 
half  to  three-fourths  of  a  centimeter  in  breadth,  extending  from  the 
lower  border  of  the  larynx  to  a  point  opposite  the  fifth  dorsal  verte- 
bra. It  consists  of  an  external  fibrous  and  an  internal  mucous  mem- 
brane, between  which  is  a  series  of  superposed  C-shaped  arches  or 
rings  of  elastic  cartilage,  some  18  or  20  in  number.  Between  the 
fibrous  and  mucous  coats  posteriorly,  and  occupying  the  space  be- 
tween and  attached  to  the  free  ends  of  the  cartilages,  there  is  a  layer 
of  transversely  arranged  non-striated  muscle-fibers,  known  as  the 
tracheal  muscle.  The  alternate  contraction  and  relaxation  of  this 
muscle  would  by  varying  the  distance  between  the  ends  of  the  cartil- 
ages, either   diminish  or  increase  the  caliber  of   the  trachea.     The 

surface  of   the  mucous  membrane 


is  covered  by  a  layer  of  stratified 
columnar  ciliated  epithelium  (Fig. 
179).  In  the  submucous  tissue 
there  are  a  number  of  glands  the 
ducts  of  which  open  on  the  free 
surface. 

Opposite  the  fifth  dorsal  verte- 
bra the  trachea  divides  into  a 
right  and  a  left  bronchus.  Each 
bronchus  again  subdivides  into  two 
or  three  branches,  which  penetrate 
the  corresponding  lung. 

The  lungs,  in  the  physiologic 
condition,  occupy  the  greater  part 
of  the  cavity  of  the  thorax.  They  are  separated  from  each  other  by 
the  contents  of  the  mediastinal  space:  viz.,  the  heart,  the  large 
blood-vessels,  the  esophagus,  etc.  Each  lung  is  somewhat  pyramidal 
in  shape  with  the  apex  directed  upward.  The  outer  surface  is  con- 
vex and  corresponds  to  the  general  conformation  of  the  thorax.  The 
inner  surface  is  concave  and  accommodates  the  contents  of  the  me- 
diastinal space.  At  about  the  middle  of  the  inner  surface  there 
enter  the  lung,  the  bronchi,  and  blood-vessels.  The  under  surface 
of  the  lung  is  concave  and  rests  on  the  diaphragm.  The  posterior 
border  is  convex;  the  anterior  border  is  thin. 

A  histologic  analysis  of  the  lung  shows  it  to  consist  of  the  branches 
of  the  bronchi,  their  subdivisions  and  ultimate  terminations,  blood- 
vessels, lymphatics  and  nerves,  imbedded  in  a  stroma  of  fibrous  and 
elastic  tissue.  The  anatomic  relations  which  these  structures  bear 
one  to  another  is  as  follows: — 

Within  the  substance  of  the  lung  the  bronchi  divide  and  subdivide, 
giving  origin  to  a  large  number  of  smaller  branches,  the  bronchial 
tubes,  which  penetrate  the  lung  in  all  directions.  With  this  repeated 
subdivision  the  tubes  become  narrower,  their  walls  thinner,  their 
structure  simpler.      In   passing  from   the  larger  to  the  smaller  tubes 


Fig.   179. — Transverse  Section  of 
the  Trachea  of  a  Kitten. — (Stirling.) 


RESPIRATION. 


385 


the  cartilaginous  arches  become  shorter  and  thinner,  and  finally  are 
represented  by  small  angular  and  irregularly  disposed  plates.  In  the 
smallest  tubes  the  cartilage  entirely  disappears.  With  the  diminution 
of  the  caliber  of  the  tube  and  a  decrease  in  the  thickness  of  its  walls, 
there  appears  a  layer  of  non-striated  muscle-fibers,  the  so-called  bron- 
chial muscle,  between  the  mucous  and  submucous  tissues,  which  com- 
pletely surrounds  the  tube  and  becomes  especially  well  developed  in 
those  tubes  devoid  of  cartilage.  The  fibrous  and  mucous  coats  at  the 
same  time  diminish  in  thickness. 

The  bronchial  muscles  are  presumably  in  a  state  of  tonic  contraction 
and  impart  to  the  bronchial  tubes  a  certain  average  caliber  best 
adapted  for  respiratory  purposes.     Experimental  investigations  indicate 


Bronchiole 


Infundibulum 


Air-cell. 


Fig.  180. — Scheme  of  a  Bronchiole  Ter- 
minating in  Alveolar  Passages,  those  Leading 
into  Infundibula  beset  with  Air-cells.— 
(Landois  and  Stirling.) 


Fig.  181. — Single  Lob- 
ule of  Human  Lung.  a. 
Alveolar  passage,  b.  Cav- 
ity of  lobule  or  infundib- 
ulum. c.  Pulmonarv  sacs. 
— (Dalton.) 


that  they  are  innervated  by  efferent  fibers  of  the  vagus  nerve,  inas- 
much as  stimulation  of  this  nerve  is  usually  followed  by  a  contraction 
of  the  muscles  and  a  narrowing  of  the  lumen  of  the  bronchial  system. 
These  muscles  may  also  be  thrown  into  increased  activity  by  the 
inhalation  of  irritating  gases  and  into  a  tetanus  by  pathologic  causes 
as  seen  in  the  various  forms  of  asthma. 

When  the  bronchial  tube  has  been  reduced  to  the  diameter  of  about 
one  millimeter,  it  is  known  as  a  bronchiole  or  a  terminal  bronchus. 
From  the  sides  of  the  terminal  bronchus  and  from  its  final  termination 
there  is  given  off  a  series  of  short  branches  which  soon  expand  to  form 
lobules  or  alveoli  (Fig.  180).  The  cavity  of  the  alveolus  is  termed 
the  infundibulum.  From  the  inner  surface  of  the  alveolus  and  of  the 
passageway  leading  into  it,  there  project  thin  partitions  which  sub- 
25 


386 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  182. — Section  of  Silvered  Lung  of  Kitten, 
including  Portions  of  Infundibulum  and  Air- 
sac,  a.  Small  polyhedral  epithelial  cells  covering 
the  wall  of  the  infundibulum.  b.  Fibro-elastic 
framework,  c.  Large  flattened  epithelial  plates  lining 
air-sac,  among  which  lie  small  groups  of  small  cells 
(d).— (Pier  soil) 


divide  the  outer  portion  of  the  general  cavity  or  infundibulum  into 
small  spaces,  the  so-called  air-sacs  or  air-cells  (Fig.  181).  The  wall 
of  the  alveolus  is  extremely  thin  and  consists  of  fibro-elastic  tissue, 

supporting  a  very  elab- 
orate capillary  network 
of  blood-vessels.  The 
bronchial  system  as  far 
as  the  aveolar  passages 
is  lined  by  ciliated  epi- 
thelium. The  air-sacs 
are  lined  by  flat  epithe- 
lial plates  of  irregular 
shape,  termed  the  respi- 
ratory epithelium  (Fig. 
182).  The  alveoli  are 
united  one  to  another  by 
fibro-elastic  tissue. 

The  bronchial  arter- 
ies which  supply  nutri- 
tive material  to  the  pul- 
monary structures  arise 
from  the  aorta  as  a  rule, 
though  sometimes  from 
an  intercostal  artery.  Each  lung  receives  two  arteries  which  ac- 
company the  bronchi  as  far  as  the  distal  ends  of  the  alveolar  passages. 
From  the  capillary  network  formed  out  of  the  terminals  of  these  ar- 
teries, two  systems  of  veins  arise, 
one  of  which  returns  the  blood  from 
the  larger  tubes  and  empties  it  into 
the  azygos  vein;  the  other  of  which 
returns  the  blood  from  the  smaller 
tubes  and  the  alveolar  passages, 
and  empties  it  into  the  pulmonary 
veins.'  The  blood  in  the  pulmonary 
veins,  though  largely  arterialized, 
nevertheless  contains  some  venous 
blood  derived  from  the  veins  aris- 
ing from  the  capillary  network  of 
the  bronchial  arterioles. 

The  nerves  distributed  to  the 
muscle-fibers  of  the  bronchial  arter- 
ies, and  of  the  bronchial  tubes  and 
to  the  mucous  membrane,  are  de- 
rived from  the  vagus  and  the  sym- 
pathetic and  enter  the  substance  of  the  lung  at  and  around  its  root. 

In  consequence  of  the  presence  of  the  elastic  tissue,  the  lungs 
are  distensible  and  elastic.     After  removal  from  the  body  the  elastic 


Fig.  183. — The  Relation  of  the 
Pulmonary  Artery,  PA,  and  the 
Pulmonary  Vein,  PV,  to  the  Lobules, 
AA.     B.  The  Bronchiole. 


RESPIRATION. 


387 


tissue  at  once  recoils,  forcing  out  a  portion  of  the  contained  air.  The 
condition  of  the  lung  is  now  one  of  collapse.  Under  pressure,  how- 
ever, the  lung  can  be  readily  distended  or  inflated.  These  proper- 
ties endure  for  a  long  period  after  death,  if  not  indefinitely,  if  the  lungs 
are  properly  preserved.  The  capacity  of  the  lungs  can  be  made  to 
vary  within  rather  wide  limits  in  virtue  of  the  presence  of  the  elastic 
tissue. 

The  Pulmonary  Blood-vessels. — The  pulmonary  artery  which 
conducts  the  venous  blood  from  the  heart  to  the  lungs  divides  beneath 


Fig  184. — Bronchi  and  Lungs,  Posterior  View,  i,  i.  Summit  of  lungs.  2,  2. 
Base  of  lungs.  3.  Trachea.  4.  Right  bronchus.  5.  Division  to  upper  lobe  of  lung. 
6.  Division  to  lower  lobe.  7.  Left  bronchus.  8.  Division  to  upper  lobe.  9.  Division 
to  lower  lobe.  10.  Left  branch  of  pulmonary  artery.  11.  Right  branch.  12.  Left 
auricle  of  heart.  13.  Left  superior  pulmonary  vein.  14.  Left  inferior  pulmonary  vein. 
15.  Right  superior  pulmonary  vein.  16.  Right  inferior  pulmonary  vein.  17.  Inferior 
vena  cava.     18.  Left  ventricle  of  heart.     19.  Right  ventricle. — (Sappey.) 


the  arch  of  the  aorta  into  a  right  and  a  left  branch.  Each  branch  with 
its  subdivisions  enters  the  lung  at  the  hilum  in  company  with  the  larger 
divisions  of  the  bronchi.  Within  the  lung  the  arteries  divide  and  sub- 
divide in  a  manner  corresponding  to  that  of  the  bronchial  tubes, 
which  they  follow  to  their  ultimate  terminations.  As  the  pulmonary 
lobules  are  approached,  a  small  arterial  branch  plunges  into  the  wall 
of  the  lubule  (Fig.  183),  in  which  it  forms  an  elaborate  capillary  net- 
work which  surrounds  and  embraces  the  air-sacs  on  all  sides.  As 
this  network  is  to  subserve  the  respiratory  exchange  of  gases  it  lies 
nearer  the  inner  than  the  outer  surface  of  the  lobule  and  in  close  rela- 


288 


TEXT-BOOK  OF  PHYSIOLOGY. 


tion  to  the  respiratory  epithelium.  The  air  and  blood  are  thus  brought 
into  intimate  relationship,  being  separated  only  by  the  respiratory 
epithelium  and  the  wall  of  the  capillary  vessel.  The  blood  emerging 
from  the  capillary  vessels  is  conducted  by  a  corresponding  converging 
system  of  vessels,  the  pulmonary  veins,  out  of  the  lungs  and  into  the 
left  auricle  of  the  heart.  The  main  function  of  the  pulmonary  appara- 
tus and  the  pulmonary  division  of  the  circulatory  apparatus  is  to  afford 
a  ready  means  for  the  exhalation  of  the  carbon  dioxid  and  the  absorption 

of  oxygen.  In  consequence  of  this 
exchange  of  gases  the  blood  changes 
in  color  from  dark  bluish-red  to 
scarlet  red.  The  relations  of  the 
heart  and  its  vessels  to  the  lungs 
and  bronchial  tubes  are  shown  in 
Fig.  184. 

The  Thorax.— The  thorax,  in 
which  the  respiratory  organs  and 
their  associated  structures  are 
lodged,  is  conic  in  shape,  though 
somewhat  compressed  from  before 
backward.  Its  apex  is  directed  up- 
ward, its  base  downward.  The 
walls  of  the  thorax  are  composed, 
first,  of  a  bony  framework  or  skel- 
eton and,  second,  of  muscles  and 
fascia.  The  bony  framework  is 
formed  posteriorly  by  the  thoracic 
vertebrae  and  the  posterior  extremi- 
ties of  the  ribs,  laterally  by  the 
ribs,  and  anteriorly  by  the  costal 
cartilages  and  the  sternum.  The 
superior  opening,  through  which 
pass  the  trachea,  esophagus,  and 
blood-vessels,  is  oval  in  outline  and 
measures  from  side  to  side  about  12.5  cm.,  and  from  before  backward 
about  6.25  cm.  The  inferior  opening  is  of  large  size,  but  irregular  in 
its  boundaries  from  the  upward  inclination  of  the  ribs  and  the  down- 
ward projection  of  the  sternum. 

The  ribs  which  form  a  large  part  of  the  thoracic  walls  constitute  a 
series  of  bony  arches  attached  posteriorly  to  the  vertebrae  and  anteriorly 
to  the  sternum  through  the  intermediation  of  their  cartilages.  The 
last  two  form  an  exception.  The  ribs  are  somewhat  twisted  upon  them- 
selves and  pursue  an  oblique  direction  from  above  downward  and 
forward.  As  a  result  the  anterior  extremity  lies  at  a  lower  level  than  the 
posterior.  The  costal  cartilages  are  directed  upward  and  forward,  with 
the  exception  of  the  upper  three,  which  are  almost  horizontal.  The 
general  arrangement  and  appearance  of  the  thorax  are  shown  inFig.  185. 


Fig.  185. — Thorax,  Anterior  View 
1.  Manubrium  sterni.  2.  Gladiolus.  3 
Ensiform  cartilage  of  xiphoid  appendix 
4.  Circumference  of  apex  of  thorax.  5 
Circumference  of  base.  6.  First  rib 
7.  Second  rib.  8,  8.  Third,  fourth, 
fifth,  sixth,  and  seventh  ribs.  9.  Eighth, 
ninth,  and  tenth  ribs.  10.  Eleventh  and 
twelfth  ribs.     11,   n.  Costal  cartilages. 


RESPIRATION. 


389 


The  costo-vertebral  and  costo-chondral  and  the  chondro-sternal 
articulations  are  diarthrodial  in  character  and  endow  the  thoracic 
walls  with  a  considerable  degree  of  mobility.  The  costo-vertebral 
joints  are  two  in  number,  the  first  being  formed  by  the  beveled  head 
of  the  rib  and  the  bodies  of  two  adjoining  vertebrae;  the  second,  by 
the  tubercle  of  the  rib  and  the  transverse  process.  The  costo-chon- 
dral and  the  chondro-sternal  articulations,  as  their  names  imply, 
are  formed  by  the  ribs,  cartilages,  and  sternum. 

The  muscles  which  complete  the  formation  of  the  thoracic  walls 
are  as  follows:  the 
diaphragm,  the  in- 
tercostales  externi 
and  interni,  the 
levatores  costarum, 
t  h  e  triangularis 
sterni,  and  the  in- 
fra-costales. 

The  diaphragm 
is  the  musculo- 
membranous  sheet 
which  closes  the 
inferior  opening  of 
the  thorax  and 
completely  sepa- 
rates its  cavity 
from  that  of  the  ab- 
domen. It  con- 
sists of  two  mus- 
cles which  arise 
from  the  bodies  of 
the  first  three  or 
four  lumbar  verte- 
brae and  neighbor- 
ing fascia,  from  the 
border   of   the    six 

lower  ribs,  and  from  the  ensiform  cartilage  (Fig.  186).  From  this  ex- 
tensive origin  the  muscle-fibers  pass  centrally  to  be  inserted  into  a 
common  tendon.  As  the  direction  of  the  fibers  is  from  below  up- 
ward and  inward,  the  diaphragm  is  somewhat  dome-shaped.  Its 
inferior  border  is  for  a  short  distance  in  contact  with  the  sides  of  the 
thorax. 

The  intercostales  externi,  eleven  in  number  on  each  side,  occupy 
the  spaces  between  the  ribs  to  which  they  are  attached  from  the 
tubercle  to  the  anterior  extremity  (Figs.  187  and  188).  Their  fibers, 
which  are  arranged  in  parallel  bundles,  are  directed  from  above 
downward  and  from  behind  forward.  The  point  of  attachment, 
therefore,  of  anv  given  bundle  of  fibers  to  the  rib  above,  lies  nearer 


Fig.  186. — Diaphragm,  Inferior  Aspect,  i.  Anterior 
and  middle  leaflet  of  central  tendon.  2.  Right  leaflet.  3. 
Left  leaflet.  4.  Right  cms.  5.  Left  cms.  6,  6.  Intervals 
for  phrenic  nerves.  7.  Muscular  fibers,  from  which  the 
ligamenta  arcuata  originate.  8.  Muscular  fibers  that  arise 
from  the  inner  surface  of  the  six  lower  ribs.  9.  Fibers 
that  arise  from  ensiform  cartilage.  10.  Opening  for  inferior 
vena  cava.  11.  Opening  for  esophagus.  12.  Aortic  open- 
ing. 13,  13.  Upper  portion  of  transversalis  abdominis, 
turned  upward  and  outward.  14.  Anterior  leaflet  of  trans- 
versalis aponeurosis.  15,  15.  Quadratus  lumborum.  16, 
16.  Psoas  magnus.     17.  Third  lumbar  vertebra. 


39° 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  vertebral  column,  nearer  the  fulcrum,  than  the  point  of  attach- 
ment below. 

The  intercostales   inter ni,   eleven   in   number,   occupy  the   spaces 
between,  and  are  attached  to,  the  ribs  from  the  tubercle  to  the  anterior 

extremity  of   the  cartil- 


ages, 


The 


lr 


fibers, 


which  are  also  arranged 
in  parallel  bundles,  are 
directed  from  above 
downward  and  back- 
ward (Figs.  188  and  189). 
The  levatores  costa- 
rum  are  twelve  in  num- 
ber on  either  side.  They 
arise  from  the  tips  of  the 
transverse  processes  of 
the  last  cervical  and  the 
thoracic  vertebrae  with 
the  exception  of  the  last. 
From  the  point  of  origin 
the  fibers  pass  down- 
ward and  outward  in  a 
111  %  s  diverging  manner  to  be 

inserted  into  the  ribs 
between  the  tubercle  and 
the  angle.  Their  action, 
as  their  name  implies,  is 
to  elevate  the  posterior 
portion  of  the  ribs. 

The  triangularis 
sterni  arises  from  the  side 
of  the  posterior  surface 
of  the  lower  third  of  the 
sternum  and  is  inserted 
by  fleshy  slips  into  the 
cartilages  of  the  ribs 
from  the  second  to  the 
sixth. 

From  the  fact  that 
the  inferior  opening  of 
the  thorax  as  well  as  the  intercostal  spaces  are  completely  closed  by 
the  foregoing  muscles,  and  from  the  further  fact  that  the  superior  is 
closed  by  fascia  except  at  those  points  through  which  pass  the 
trachea,  blood-vessels  and  esophagus,  the  cavity  of  the  thorax  is  abso- 
lutely air-tight. 

The  Pleurae. — Each  lung  is  surrounded  by  a  closed  invaginatcd 
serous  sac,  the  pleura,  of  which  the  inner  portion  is  reflected  over 


T 
Fig.  187. — Showing  the  Situation,  the  Points 
of  Attachment,  and  Direction  of  the  Intercostal 
Muscles,     i.  The  intercostales  exlerni.     2.  The  inter- 
costales interni.     3.  The  intercartilaginei. — (Deaver.) 


RESPIRATION. 


39i 


and  is  closely  adherent  to  the  surface  of  the  entire  lung  as  far  as  its 
root;  the  outer  portion  is  reflected  over  the  inner  wall  of  the  thorax, 
the  superior  surface  of  the  diaphragm,  and  the  viscera  of  the  mediasti- 
num. Under  normal  conditions  these  two  layers  of  the  pleura,  the 
visceral  and  parietal,  are  in  contact,  or  at  most  separated  only  by  a 
thin  capillary  layer  of  lymph.  The  presence  of  this  fluid  prevents 
appreciable  friction  as  the  two  surfaces  play  against  each  other  in 
consequence  of  the  movements  of  the  lungs. 

THE  MECHANIC  MOVEMENTS  OF  THE  THORAX. 

The  blood  receives  oxygen  from,  and  yields  carbon  dioxid  to,  the 
alveoli  of  the  lungs,  as  it  flows  through  the  pulmonary  capillaries. 
That  this  exchange  of  gases  may  continue,  it  is  of  primary  import- 
ance that  the  air  within  the  alveoli  be  removed  as  rapidly  as  it  is 
vitiated.  This  is  accomplished  by  an 
alternate  increase  and  decrease  in  the 
capacity  of  the  thorax,  accompanied 
by  corresponding  changes  in  the  ca- 
pacity of  the  lungs.  During  the  former 
there  is  an  inflow  of  atmospheric  air 
(inspiration),  during  the  latter  an  out- 
flow of  intra-pulmonary  air  (expira- 
tion). The  continuous  recurrence  of 
these  two  movements  brings  about  that 
degree  of  pulmonary  ventilation  neces- 
sary to  the  normal  exchange  of  gases 
between  the  blood  and  the  air.  The 
two  movements  together  constitute  a 
respiratory  act  or  cycle. 

In  the  course  of  the  respiratory 
cycles  the  thorax  presents  alternately 
a  short  period  of  rest — viz.,  between 
the  end  of  an  expiration  and  the  be- 
ginning of  an  inspiration — and  a 
relatively  long  period  of  activity,  in- 
cluding both  inspiration  and  expiration.  The  former  may  be  regarded 
as  the  static,  the  latter  as  the  dynamic  condition  of  the  thorax.  In  the 
static  condition,  the  thorax  and  its  contained  and  associated  organs 
sustain  a  definite  relation  one  to  another;  in  the  dynamic  conditions 
these  relations  undergo  a  change  the  extent  of  which  is  proportional  to 
the  extent  of  the  movements.* 

*  It  is  a  matter  of  dispute  as  to  whether  or  not  there  is  an  absolute  cessation  of  move- 
ment of  the  thoracic  walls  at  the  end  of  expiration.  A  graphic  record  of  the  movement 
shows  that  if  there  is  no  absolute  cessation,  the  movement  is  so  slight  that,  for  the  pur- 
poses here  intended,  a  pause  may  be  admitted.  With  this  admission  it  is,  however,  recog- 
nized that  the  forces,  both  elastic  and  muscular,  which  are  always  acting  on  the  thoracic 
walls,  though  in  opposite  directions,  have  not  ceased  to  act,  but  have  become  so  nearly 
equal  that  for  a  brief  period  they  are  practically  in  a  condition  of  equilibrium,  during 
which  the  thoracic  walls  are  stationary. 


Fig.  188. — View  from  behind  of 
Four  Dorsal  V  e  r  t  e  b  r  .e  and 
Three  Attached  Ribs,  showing 
the  Attachment  of  the  Elevator 
Muscles  of  the  Ribs  and  the 
Intercostals.  1.  Long  and  short 
elevators.  2.  External  intercostal.  3 
Internal  intercostal.-—  (Allen  Thomson  ) 


392  TEXT-BOOK  OF  PHYSIOLOGY. 

THE  STATIC  CONDITION. 

Relation  of  the  Thoracic  Organs. — Intra-pulmonary  Pres- 
sure :  Intra-thoracic  Pressure. — In  the  static  condition  of  the 
thorax  the  lungs,  by  virtue  of  their  distensibility,  completely  fill  all 
parts  of  the  thorax  not  occupied  by  the  heart  and  great  blood-vessels 
(Fig.  189).  This  condition  is  maintained  by  the  pressure  of  the  air 
within  the  lungs,  the  intra-pulmonary  pressure,  which  with  the  respira- 
tory passages  open,  is  that  of  the  atmosphere,  760  mm.  Ffg.  This 
relation  persists  so  long  as  the  thorax  remains  air-tight.  If  the  skin 
and  muscles  covering  an  intercostal  space  be  removed  the  lung  can 
be  seen  in  close  contact  with  the  parietal  layer  of  the  pleura  gliding  by 
with  each  inspiration  and  expiration.  If,  however,  an  opening  be 
now  made  in  the  pleura  sufficient  to  admit  air,  the  lung  immediately 
collapses  and  a  pleural  cavity  is  established.  The  pressure  of  air  within 
and  without  the  lung  counterbalancing,  at  the  moment  the  air  is 
admitted,  the  elastic  tissue  at  once  recoils  and  forces  a  large  part  of 
the  air  out  of  the  lung.  This  is  a  proof  that  in  the  normal  condition, 
the  lungs,  distended  by  atmospheric  pressure  from  within,  are  in  a 
state  of  elastic  tension  and  ever  endeavoring  to  pull  the  visceral  layer 
of  the  pleura  away  from  the  parietal  layer.  That  they  do  not  succeed 
in  doing  so  is  due  to  the  fact  that  the  atmospheric  pressure  from  with- 
out is  prevented  from  acting  on  the  lung  by  the  firm  unyielding  walls 
of  the  thorax. 

Intra-thoracic  Pressure.- — As  a  result  of  the  elastic  tension  of 
the  lungs  a  fractional  part  of  the  intra-pulmonary  pressure,  760  mm. 
Hg.,  is  counterbalanced  or  opposed,  so  that  the  heart  and  great  vessels 
and  other  intra-thoracic  viscera  are  subjected  to  a  pressure  somewhat 
less  than  that  of  the  atmosphere;  the  amount  of  this  pressure  will  be 
that  of  the  atmosphere  less  that  exerted  by  the  elastic  tissue  of  the 
lung  in  the  opposite  direction,  expressed  in  terms  of  millimeters  of 
mercury.  In  the  thorax  but  outside  the  lungs,  there  then  prevails 
a  pressure,  negative  to  the  pressure  inside  the  lungs  and  which  is  known 
as  the  intra-thoracic  pressure. 

The  amount  of  this  intra-thoracic  pressure  can  be  approximately 
determined  in  several  ways.  Thus,  if  shortly  after  death  a  mer- 
curial manometer  be  inserted  air-tight  into  the  trachea  of  a  human 
being  and  the  thorax  opened,  the  lungs  will  recoil  and  compress  their 
contained  air.  The  mercurial  manometer  will  at  once  show  an 
excess  of  pressure  in  the  trachea  of  about  6  mm.  This  was  taken 
by  Donders  as  a  measure  of  the  force  with  which  the  lungs  endeavor 
to  recoil.  The  intra-thoracic  pressure  would  be,  therefore,  atmos- 
pheric pressure,  760  mm.,  less  6  mm.,  or  754  mm.  Hg.  Another 
method  is  to  insert  a  rubber  catheter  through  a  small  opening  in  an 
intercostal  space  into  the  thoracic  cavity.  The  air  which  enters 
through  the  open  extremities  of  the  catheter  and  leads  to  a  collapse 
of  the  lungs  may  be  subsequently  aspirated,  when  the  lung  returns  to 


RESPIRATION. 


393 


its  normal  position.  The  catheter  is  then  placed  in  connection  with 
a  water  manometer.  On  establishing  a  communication  between 
them,  by  the  turning  of  a  stopcock,  the  water  will  rise  in  the  proximal 
and  fall  in  the  distal  limb  of  the  manometer,  indicating  a  pressure 
in  the  thorax  negative  to  that  in  the  lung.  The  difference  in  the 
level  of  the  water  in  the  two  limbs  of  the  manometer,  expressed  in 
millimeters  of  mercury,  would  also  represent  the  force  with  which 
the  elastic  tissue  strives  to  recoil,  and  the  extent  to  which  it  opposes 
the   atmospheric   pressure.     This   subtracted   from   the    atmospheric 


Fig.  189. — Section  of  Thorax  with  the  Lungs,  Heart,  and  Principal  Vessels. 
5.  Catheter  introduced  into  the  pleural  space  and  connected  with  a  manometer. — (After 
Moral  and  Doyen.) 


pressure  would  give  the  intra-thoracic  pressure.  In  the  living  dog 
this  latter  is  less  than  the  former,  to  the  extent  of  from  3.5  to  5.5  mm. 
For  the  same  reason  the  superior  surface  of  the  diaphragm  also  ex- 
periences a  pressure  less  than  that  of  the  atmosphere.  Owing  to  the 
soft  and  yielding  character  of  the  abdominal  walls  the  atmospheric 
pressure  is  transmitted  through  the  abdominal  organs  to  the  inferior 
surface  of  the  diaphragm.  The  pressure  being  greater  from  below 
than  above,  the  diaphragm  is  forced  upward  until  it  assumes  the  dome- 
like appearance  it  usually  presents.  (These  relations  are  shown  in 
Fig.  189.) 

The  cause  of  the  negativity  of  the  intra-thoracic  pressure  is  con- 
nected with  the  change  in  the  relation  of  the  lungs  to  the  thorax  at- 
tending the  first  inspiration.     Previous  to  birth  the  walls  of  the  alveoli 


394  TEXT-BOOK  OF  PHYSIOLOGY. 

and  bronchioles  are  collapsed  and  in  apposition.  The  larger  bron- 
chial tubes  in  all  probability  contain  fluid.  The  lungs  therefore  are 
devoid  of  air  (atelectatic),  and,  having  a  specific  gravity  greater  than 
water,  readily  sink  when  placed  in  this  fluid.  The  capacity  of  the 
thorax  does  not  exceed  the  volume  of  the  lungs.  With  the  first  inspira- 
tion, however,  the  thoracic  walls  take  a  new  position.  The  air  at  once 
rushes  into  the  lungs  and  distends  them.  But  as  the  capacity  of  the 
thorax  even  at  the  end  of  the  expiration  is  now  greater  than  the  volume 
which  the  lungs  could  assume  without  considerable  distention,  there 
at  once  arises  the  elastic  recoil  in  the  opposite  direction,  the  condition 
which  gives  rise  to  the  negativity  of  the  pressure  in  the  thoracic  cavity. 
It  is  also  probable  that  as  the  child  develops,  the  thorax  grows  more 
rapidly  than  the  lungs,  giving  rise  to  a  condition  which  would  increase 
and  accentuate  the  elastic  tension  and  thus  increase  the  negativity 
of  the  intra-thoracic  pressure. 

THE  DYNAMIC  CONDITION. 

In  the  dynamic  condition  as  previously  stated  these  relations 
and  pressures  change.  Thus  the  diaphragm  descends,  the  ribs 
ascend,  the  sternum  advances  and  the  lungs  expand.  The  intra- 
pulmonary  pressure  varies  during  both  inspiration  and  expiration. 
With  the  enlargement  of  the  thorax  through  muscle  activity,  there 
goes  a  corresponding  increase  in  the  size  and  capacity  of  the  lungs, 
in  consequence  of  the  expansion  and  pressure  of  the  air  in  the  pulmonary 
alveoli.  With  the  expansion  of  the  air  its  pressure  falls;  but  though  it 
is  now  less  than  atmospheric,  it  is  yet  much  greater  than  the  opposing 
force  of  the  lung  tissue.  As  a  result  of  the  fall  of  intra-pulmonary  pres- 
sure there  is  a  rapid  inflow  of  air,  which  continues  until  atmospheric 
pressure  is  restored;  that  is,  at  the  end  of  the  inspiration.  With  the 
diminution  of  the  thorax,  through  the  recoil  of  the  elastic  tissue  of  the 
thoracic  and  abdominal  walls,  there  goes  a  corresponding  decrease  of 
lung  capacity,  in  consequence  of  the  recoil  of  the  elastic  tissue  of  the 
lungs.  As  a  result,  the  air  in  the  lungs  becomes  compressed,  its  pres- 
sure rises  above  that  of  the  atmosphere,  and  a  rapid  outflow  of  air 
takes  place,  which  continues  until  atmospheric  pressure  is  again  re- 
stored; that  is,  at  the  end  of  the  expiration. 

The  cause  for  the  fall  of  intra-pulmonary  pressure  during  in- 
spiration and  the  rise  during  expiration  is  to  be  found  in  the  resist- 
■  ance  offered  by  the  air-passages  to  the  movement  of  the  air,  through- 
out their  entire  extent,  and  especially  at  the  level  of  the  vocal  bands. 
The  greater  the  resistance,  from  whatever  cause,  physiologic  or  patho- 
logic, the  greater  the  variations  of  the  pressure. 

In  quiet  inspiration  the  fall  of  pressure,  as  indicated  by  a  man- 
ometer inserted  into  one  nostril,  seldom  amounts  to  more  than  1.5 
mm.  of  Hg.,  the  rise  in  expiration,  2.5  to  3  mm.  of  Hg.  In  forcible 
inspiratory  and  expiratory  efforts  these  limits  may  be  largely  exceeded. 


RESPIRATION. 


395 


Insp. 


Intra-pulmonary  pressure. 
B  ExP- 


Thus  it  was  found  by  Donders  that  with  one  nostril  closed  and  a  mer- 
curial manometer  inserted  into  the  other  the  pressure  by  voluntary 
efforts  could  be  made  to  fall  57  mm.  during  inspiration  and  to  rise 
87  mm.  during  expiration.  The  changes  in  intra-pulmonary  pressure 
are  graphically  represented  in  the  upper  half  of  Fig.   190. 

The  infra-thoracic  pressure  also  varies  during  both  inspiration  and 
expiration.  As  the  intra-pulmonary  pressure  falls,  the  recoil  of  the 
elastic  tissue  increases,  with  the  result  of  diminishing  the  intra-thoracic 
pressure,  though  not  in  a  steadily  progressive  manner.  The  fall  of 
intra-thoracic  pressure  at  the  end  of  a  quiet  inspiration  amounts  to 
about  9  mm.  Hg.  In  forcible  inspiratory  efforts  this  fall  in  intra- 
thoracic pressure  may  amount  to  30  or  40  mm.  of  Hg.  As  the  intra- 
pulmonary  pressure  rises  above  the  atmospheric  pressure  during 
expiration,  the  recoil  of  the  elastic  tissue  is  again  opposed,  with  the 
result  of  increasing  the 
intra-thoracic  pressure, 
though  not  in  a  steadily 
progressive  manner. 
The  changes  in  intra- 
thoracic pressure  are 
graphically  represented 
in  the  lower  half  of  Fig. 
190. 

Respiratory  Move- 
ments.— As  previously 
stated,  the  ventilation 
of  the  lungs  is  accom- 
plished by  an  alternate 
increase  and  decrease  in 

the  capacity  of  the  thorax,  accompanied  by  corresponding  changes 
in  the  lungs,  the  two  movements  being  known  as  inspiration-  and 
expiration  respectively.  During  the  increase  in  the  thoracic  capacity, 
the  air  passively  flows  into  the  lungs;  during  the  decrease  in  the  thora- 
cic capacity,  the  air  passively  flows  out  of  the  lungs.  In  both  move- 
ments the  lungs  play  an  entirely  passive  part,  their  movements  being 
determined  by  the  pressure  of  air  within  them  and  by  the  thoracic 
walls,  with  which  they  are  in  close  contact. 

1.  Inspiration  is  an  active  process,  the  result  of    muscle  activity. 

2.  Expiration  is  a  passive  process,  the  result  mainly  of  the  recoil  of 

the  elastic  tissue  of  the  walls  of  the  thorax  and  abdomen  and  of 
the  elastic  tissue  of  the  lungs. 
Ih  inspiration  the  thorax  is  enlarged  in  all  its  diameters:     viz., 
vertical,  transverse,  and  antero-posterior.     In  expiration  these  diam- 
eters are  in  turn  decreased  as  the  thorax  returns  to  its  previous  con- 
dition. 

Inspiratory   Muscles. — The    muscles   which   from   their   origin. 
direction,  and  insertion  contribute  to  the  enlargement  or  expansion  of 


760  mm 


C  760  mm 


Intra-thoracic  pressure. 

Fig.  190. — Representing  the  Changes,  i,  in 
the  Intra-pulmonary,  and  2,  in  the  Intra-tho- 
racic Pressures  during  Inspiration  and  Ex- 
piration. 


396 


TEXT-BOOK  OF  PHYSIOLOGY 


the  thorax  are  quite  numerous,  and  include  those  muscles  which 
enter  into  the  formation  of  the  thoracic  walls  (intrinsic  muscles), 
as  well  as  certain  muscles  which,  having  their  origin  elsewhere,  are 
attached  to  the  thoracic  walls  at  different  points  (extrinsic  muscles), 
though  the  extent  to  which  they  are  called  into  activity  depends 
on  the  necessity  for  either  tranquil  or  energetic  inspirations.  The 
gradations  between  a  minimum  and  a  maximum  inspiration  are 
very  slight,  and  it  is  difficult  to  state  at  what  particular  instant  any 
given  muscle  begins  to  act.  It  is  customary,  however,  to  divide 
the  muscles  into  two  groups:  (i)  Those  active  in  the  average  or 
ordinary  inspirations,  and  (2)  those  active  in  maximum  or  extra- 
ordinary inspirations.  Among  the  muscles  active  in  ordinary  inspira- 
tions  may   be    mentioned   the   diaphragm,   the   inter co stales   externi, 

the  intercartilagenei,  the  levator es  costarum, 
the  scaleni,  and  the  serratus  posticus  superior. 
Among  the  muscles  active  in  extraordinary 
inspirations  may  be  mentioned,  in  addition  to 
the  foregoing,  the  sterno-cleido-mastoideus ,  the 
trapezius,  and  the  pectorales  minor  and  major. 
The  vertical  diameter  is  increased  by  the 
contraction  and  descent  of  the  diaphragm, 
and  more  especially  of  its  lateral  muscular 
portions.  At  the  end  of  an  expiration  the 
diaphragm  is  relaxed,  and  the  lower  portion 
closely  applied  to  the  walls  of  the  thorax.  At 
the  beginning  of  an  inspiration  the  muscle- 
fibers  contract,  shorten,  and  approximate  a 
straight  line,  whereby  not  only  is  the  con- 
vexity of  the  diaphragm  diminished,  but  that 
portion  in  contact  with  the  thorax  is  drawn 
away,  thus  making  a  large  free  space  into 
which  the  lateral  and  posterior  portions  of  the 
The  attachment  of  the  central  tendon  of  the 
diaphragm  to  the  pericardium  prevents  any  marked  descent  of  this 
portion  except  in  forcible  inspiratory  efforts  (Fig.  191).  The  vertical 
diameters  are  thus  enlarged,  though  unequally  in  different  regions  of 
the  thorax. 

As  the  diaphragm  descends  it  displaces  the  abdominal  viscera, 
forcing  them  downward  against  the  abdominal  walls,  which  advance 
and  become  more  convex.  In  forcible  inspiration  the  diaphragm, 
acting  from  the  central  tendon  as  the  more  fixed  point,  would  draw 
the  lower  portion  of  the  thorax  inward  were  this  not  prevented  by  the 
outward  pressure  of  the  displaced  viscera. 

The  anlero- posterior  and  transverse  diameters  arc  increased  by 
the  elevation  and  outward  rotation  of  the  ribs  and  an  advance  of  the 
sternum,  both  movements  made  possible  by  the  construction  and 
arrangement  of  the  costo-vertebral  and  costo-chrondral  and  chrondro- 


Fig.  19T. —  Diagram 

SHOWING  INTERVAL  BE- 
TWEEN the  Position  of 
the  Diaphragm  in  Ex- 
piration (e,  e)  and  In- 
spiration (i,  i).  The  In- 
crease in  Capacity  is 
Shown  by  the  White 
Areas. — (Yeo.) 

lungs  at  once  descend. 


RESPIRATION. 


397 


sternal  articulations.  The  construction  of  these  articulations  is  such 
as  to  permit  at  the  first  a  slight  elevation  and  depression  of  the  head 
of  the  rib,  and  at  the  second  a  gliding  of  the  tubercle  on  the  transverse 
process.  The  axis  around  which  the  rib  rotates  practically  coincides 
with  the  axis  of  the  rib  neck,  which  in  the  upper  part  of  the  thorax  is 
almost  horizontal,  in  the  lower  part  somewhat  sagittal  in  direction. 
Hence  when  the  ribs  are  elevated  the  upper  part  of  the  thorax  increases 
in  its  antero-posterior,  the  lower  part  in  its  transverse  diameters. 
At  the  same  time,  the  lower  portion  of  the  sternum  is  pushed  forward 
and  upward  by  the  elevation  of  the  anterior  extremity  of  the  ribs  and 
the  widening  of  the  angle  of  the  costo-chondral  articulation.  With 
the  elevation  of  the  ribs  there  goes  an  eversion  or  outward  rotation 


Fig.  192. — Diagram  illustrating  the  Action  of  A,  the  External  Intercostal 
and  B,  the  Internal  Intercostal  Muscles.  V,  V.  Vertical  support.  R,  R'. 
Parallel  bars.  S.  Vertical  strip,  representing  respectively  the  vertebral  column,  two  ribs, 
and  sternum. 


which  gives  an  additional  increase  to  the  transverse  diameters.  This 
elevation  and  outward  rotation  of  the  ribs  is  the  resultant  of  the  coop- 
eration of  the  following  muscles,  viz.:  the  intercostales  externi,  the 
intercartilagenei,  the  levatores  costarum,  the  scaleni  and  the  serratus 
posticus  superior. 

The  action  of  the  external  intercostal  muscles  has  been  a  subject 
of  much  discussion.  Some  investigators  have  maintained  that  they 
are  elevators  of  the  ribs,  and  therefore  inspiratory;  others  that  they  are 
depressors  of  the  ribs,  and  therefore  expiratory  in  function.  At  the 
present  time  the  general  consensus  of  opinion  is  that  the  former  view 
is  the  one  most  in  accordance  with  the  facts.  The  situation  of  the 
muscles  and  the  shortness  of  their  fibers  render  it  extremely  diffi- 
cult to  obtain  myographic  tracings  of  their  action.  This  is  supposed, 
however,  to  be  disclosed  by  the  play  of  the  apparatus  suggested  orig- 
inally by  Bernouilli,  which  consists,  as  shown  in  Fig.  192,  of  a  vertical 
support  carrying  two  freely  movable  parallel  bars  united  at  their  outer 
extremities  by  a  short  vertical  strip,  representing  respectively  the 
vertebral  column,  two  adjoining  ribs,  and  a  piece  of  the  sternum. 


398  TEXT-BOOK  OF  PHYSIOLOGY. 

The  parallel  bars  are  joined  to  each  other  by  a  short  elastic  band 
having  the  direction  of  and  representing  the  external  intercostal  muscles. 
If  the  bars  are  depressed,  the  elastic  band  is  elongated  and  made 
tense.  On  releasing  the  bars  the  band  at  once  recoils  and  elevates 
them.  Although  the  elastic  force  is  the  same  in  both  directions,  the 
bars  are  yet  elevated  for  the  reason  that  in  accordance  with  the  parallel- 
ogram of  forces  the  component  acting  upward  on  the  long  arm  of 
the  lever  preponderates  over  the  component  acting  downward  on  the 
short  arm  of  the  lever.  The  action  of  the  band  is  supposed  to  disclose 
and  illustrate  the  action  of  the  muscle. 

The  inter  car  tilaginei,  those  portions  of  the  intercostales  interni 
which  occupy  the  space  between  the  costal  cartilages  from  the  sternum 
to  their  outer  extremity,  bear  the  same  relation  to  the  cartilages  in 
reference  to  the  sternum  that  the  external  intercostals  bear  to  the  ribs 
in  reference  to  the  vertebral  column;  that  is,  the  point  of  attachment 
to  the  cartilage  above,  lies  nearer  the  sternum,  nearer  the  fulcrum, 
than  the  point  of  attachment  below.  Hence  the  same  action  is  at- 
tributed to  them  as  to  the  external  intercostals:  viz.,  elevation  of  the 
cartilages  and  the  anterior  extremities  of  the  ribs. 

The  levatores  costarum,  as  is  evident  from  their  points  of  origin 
and  insertion,  elevate  the  ribs  posteriorly. 

The  scaleni  muscles,  anticus,  medius,  and  posticus,  arise  from  the 
transverse  processes  of  the  cervical  vertebrae,  and  after  pursuing  a 
downward  and  forward  direction  are  inserted  into  the  sternal  end  of 
the  first  and  second  ribs.  The  action  of  the  first  two,  at  least,  is  to 
elevate  the  first  rib  and  thus  establish  a  fixed  point  from  which  the 
intercostal  muscles  can  act.  The  posticus  has  doubtless  a  similar 
action  on  the  second  rib. 

The  serratus  posticus  superior,  a  quadrilateral  sheet  of  muscle- 
fibers,  arises  mainly  from  the  spines  of  the  last  cervical  and  first  and 
second  thoracic  vertebras.  The  anterior  extremity  is  serrated  and 
attached  to  the  outer  surfaces  of  the  second,  third,  fourth,  and  fifth 
ribs  beyond  the  angle.  The  action  of  the  muscle  is  the  elevation  of 
the  ribs  to  which  it  is  attached. 

In  forcible  or  extraordinary  inspirations,  whereby  the  capacity  of 
the  thorax  is  still  further  increased,  the  foregoing  muscles  are  rein- 
forced by  the  sterno-cleido-mastoideus ,  the  trapezius,  and  the  pectorales 
minor  and  major.  Their  functions  will  become  apparent  from  a 
consideration  of  their  origins  and  insertions. 

Expiratory  Forces  and  Muscles. — Expiration,  as  previously 
stated,  is  a  passive  process  brought  about  by  the  recoil  of  the  elastic 
tissues  of  the  thoracic  and  abdominal  walls,  and  of  the  lungs,  all  of 
which  have  been  stretched  and  made  tense  during  inspiration.  With 
the  cessation  of  the  inspiratory  effort  the  elastic  forces,  assisted  by  the 
weight  of  the  ribs,  sternum,  and  soft  tissues,  return  the  thorax  to  its 
former  condition.  The  result  is  a  diminution  of  all  the  diameters  of 
the  thorax.     The  vertical  diameter  is  diminished   by  the  recoil  of 


RESPIRATION.  399 

the  tense  abdominal  walls,  the  replacement  of  the  abdominal  organs 
and  the  consequent  ascent  of  the  diaphragm  to  its  former  position. 
The  transverse  and  antero- posterior  diameters  are  diminished  by  the 
descent  of  the  ribs,  sternum,  and  lungs.  It  is  somewhat  uncertain 
if  a  normal  expiratory  movement  necessitates  active  muscle  contrac- 
tion. If,  however,  there  is  any  impairment  of  the  elasticity  of  the 
lungs  or  ribs,  or  any  interference  with  the  free  exit  of  the  intra-pulmo- 
nary  air,  it  is  highly  probable  that  the  elastic  forces  are  assisted  by  the 
internal  intercostal  and  triangularis  sterni  muscles. 

The  action  of  the  internal  intercostals  is  less  clearly  understood 
than  that  of  the  externals,  and  for  the  same  reasons.  If,  however, 
Bernouilli's  model  discloses  the  action  of  the  latter,  it  equally  well 
discloses  the  action  of  the  former.  Thus,  if  the  parallel  bars  be  joined 
by  an  elastic  band  having  the  direction  of  and  representing  the  inter- 
nal intercostals,  and  then  forcibly  elevated,  the  band  is  elongated  and 
made  tense.  On  releasing  the  bars,  the  elastic  band  at  once  recoils 
and  depresses  them.  Here,  again,  though  the  elastic  force  is  the  same 
in  both  directions,  the  bars  are  depressed,  for  the  reason  that  the 
component  acting  downward  on  the  long  arm  of  the  lever  prepon- 
derates over  that  acting  upward  on  the  short  arm  of  the  lever.  The 
action  of  the  band  is  supposed  to  disclose  and  illustrate  the  action  of 
the  muscle. 

The  triangularis  sterni  muscle,  judging  from  its  anatomic  re- 
lations, in  all  probability  assists  in  expiration  by  depressing  the  car- 
tilages to  which  it  is  attached  and  as  a  further  result  the  anterior 
extremities  of  the  ribs. 

After  the  elastic  forces  have  ceased  to  act  and  the  normal  expira- 
tory movement  has  been  brought  to  a  close,  the  thorax  can  be,  to  a 
considerable  extent,  still  further  diminished  in  all  its  diameters  by 
the  contraction,  through  volitional  effort,  of  abdominal  and  thoracic 
muscles.  To  this  decrease  in  the  capacity  of  the  thorax,  as  a  result 
of  which  a  much  larger  volume  of  air  is  expelled  from  the  lungs  than 
during  passive  expiration,  the  term  forced  expiration  has  been  given. 
With  the  cessation  of  muscle  activity  the  elastic  forces  of  the  now- 
compressed  thoracic  walls,  aided  by  the  return  of  upward  displaced 
abdominal  organs,  at  once  restore  the  thoracic  walls  to  the  position 
they  had  attained  at  the  end  of  passive  expiration.  Of  the  muscles 
active  in  forced  expiration  in  addition  to  the  intercostales  interni  and 
the  triangularis  sterni,  the  following  may  be  mentioned,  viz.:  the 
abdominales,  the  serratus  posticus  inferior,  and  the  quadratus  lum- 
borum. 

The  externus  abdominis  arises  by  a  series  of  muscle  slips  from  the 
outer  surface  of  the  lower  eight  ribs.  After  pursuing  an  oblique 
direction  downward  and  forward,  the  slips  blend  to  form  a  single 
muscle,  which  is  inserted  mainly  into  the  outer  lip  of  the  anterior 
half  of  the  crest  of  the  ilium  and  into  the  central  abdominal  tendon 
or  aponeurosis. 


4oo  TEXT-BOOK  OF  PHYSIOLOGY. 

The  internus  abdominis  arises  mainly  from  the  anterior  two-thirds 
of  the  inner  crest  of  the  ilium  and  the  lumbar  fascia.  Its  fibers  pass 
upward  and  forward  to  be  inserted  into  the  cartilages  of  the  last  three 
ribs  and  into  the  central  abdominal  tendon. 

The  rectus  abdominis  arises  from  the  crest  of  the  pubes  and  is 
inserted  above  into  the  cartilages  of  the  fifth,  sixth,  and  seventh  ribs, 
and  occasionally  into  the  ensiform  cartilage. 

The  transversalis  arises  from  the  cartilages  of  the  last  six  ribs, 
the  lumbar  fascia,  and  the  anterior  half  of  the  crest  of  the  ilium. 
After  passing  transversely  across  the  abdomen,  the  fibers  are  inserted 
mainly  into  the  linea  alba. 

The  conjoint  action  of  these  muscles  is  to  diminish  the  convexity 
of  the  abdominal  walls  and  to  exert  a  pressure  on  the  abdominal 
organs.  These,  taking  the  line  of  least  resistance,  are  forced  upward 
against  the  inferior  surface  of  the  diaphragm,  which  in  consequence 
becomes  more  strongly  curved  and  ascends  higher  into  the  thorax. 
The  vertical  diameter  of  the  thorax  is  thus  diminished.  Acting  from 
the  pelvis  as  a  fixed  point,  these  muscles  will  also  draw  downward 
and  inward  the  lower  end  of  the  sternum  and  the  lower  ribs  and 
diminish  the  antero-posterior  and  transverse  diameters. 

The  serratus  posticus  inferior  arises  from  the  spines  of  the  last 
two  thoracic  and  first  two  lumbar  vertebras.  The  fibers  pass  upward 
to  be  inserted  into  the  lower  border  of  the  last  four  or  five  ribs  beyond 
the  angle.  Their  action  is  to  depress  the  ribs  and  assist  in  expira- 
tion. 

The  quadratus  lumborum  has  a  similar  action  on  the  last  rib. 

Movements  of  the  Lungs. — As  the  thorax  is  enlarging  in  all  its 
diameters  during  inspiration,  through  muscle  activity,  the  lungs  are 
correspondingly  enlarging  in  all.  their  diameters,  by  virtue  of  their 
distensibility,  through  the  pressure  of  the  air  within  them.  The  lungs 
must  therefore  move  downward,  outward  and  forward.  That  this 
is  the  case  is  made  evident  both  by  an  examination  of  the  lungs  through 
an  intercostal  space  after  removal  of  the  skin  and  intercostal  muscles 
and  by  the  methods  of  percussion.  The  inferior  border  of  each  lung 
descends  from  the  lower  border  of  the  sixth  to  the  eleventh  rib,  insert- 
ing itself  into  the  space  developed  between  the  thorax  and  diaphragm 
as  the  latter  contracts  and  is  drawn  away  from  the  former.  In  con- 
sequence of  the  lateral  expansion  the  anterior  border  of  each  lung 
advances  toward  the  middle  line  until  the  heart  is  almost  covered. 
With  the  beginning  and  continuance  of  expiration  the  lungs  exhibit 
a  reverse  movement  which  continues  until  they  reach  their  original 
position.  At  all  times,  however,  the  movements  of  the  lungs  are  en- 
tirely passive  and  determined  by  the  movements  of  the  thorax. 

The  succession  of  events  in  the  thorax  at  the  time  of  a  respiratory 
act  may  be  summarized  as  follows: 

During  Inspiration. 
i.  Enlargement  of  the  thoracic  diameters  by  muscle  action. 


RESPIRATION.  401 

2.  Expansion  of  intra-pulmonary  (alveolar)  air. 

3.  Expansion  of  the  lungs. 

4.  Lowering  of  the  intra-pulmonary  air  pressure  below  the  atmos- 

pheric air  pressure. 

5.  Increase  in  the  negativity  of  the  intra-thoracic  pressure. 

6.  Inflow  of  atmospheric  air,  in  consequence  of  its  higher  press- 

ure, until  the  intra-pulmonary  air  pressure  rises  to  that  of 
the  atmosphere. 
During  Expiration. 

1.  Diminution  of  the  thoracic  diameters  by  the  action  of  elastic 

forces. 

2.  Recoil  of  the  lungs. 

3.  Compression  of  the  intra-pulmonary  (alveolar)  air. 

4.  Rise  of  intra-pulmonary  air  pressure  above  the  atmospheric 

air  pressure. 

5.  Decrease  in  the  negativity  of  the  intra-thoracic  pressure. 

6.  Outflow  of  intra-pulmonary  air,  in  consequence  of  its  higher 

pressure,  until  the  intra-pulmonary  air  pressure  falls  to  that 
of  the  atmosphere. 

Respiratory  Movements  of  the  Upper  Air-passages. — The 
resistance  to  the  entrance  of  air  into  and  through  the  respiratory  tract 
is  much  diminished  by  respiratory  movements  of  the  nares  and  larynx 
which  are  associated  and  occur  synchronously  with  the  movement 
of  the  thorax. 

The  nares  at  each  inspiration  are  dilated  by  the  outward  move- 
ment of  their  alae  or  wings,  the  result  of  muscle  activity.  At  each 
expiration  they  are  diminished  by  the  return  of  their  cartilages  through 
the  play  of  elastic  forces.  The  larynx,  as  shown  by  observation  with 
the  laryngoscope,  exhibits  corresponding  movements  of  the  vocal 
membranes.  Their  introduction  at  this  level  naturally  narrows  the 
tract,  and  would  interfere  with  both  the  entrance  and  the  exit  of  air 
were  they  not  kept  widely  asunder  during  the  time  they  are  not  re- 
quired for  purposes  of  phonation.  This  is  accomplished  by  the  tonic 
contraction  of  the  posterior  crico-arytenoid  muscles,  which  are  entirely 
respiratory  in  function. 

It  is  not  infrequently  stated  that  these  membranes  exhibit  consider- 
able oscillations,  outward  and  inward,  corresponding  to  the  periods 
of  inspiration  and  expiration.  The  statements  of  the  majority  of 
laryngologists  do  not  favor  this  view.  During  tranquil  breathing 
the  membranes  are  widely  separated  and  almost  stationary,  seldom 
moving  in  either  direction  more  than  a  few  millimeters.  In  labored 
respirations  these  movements  are  naturally  increased  in  extent.  The 
irregular  movements  of  the  membranes  occasioned  by  the  unskilful  use 
of  the  laryngoscope,  especially  with  nervous  patients,  are  not  to  be  re- 
garded as  strictly  physiologic.  The  respiratory  space  in  quiet  breath- 
ing is  an  isoceles  triangle,  with  a  length  of  20  mm.  and  a  width  at  the 
base  of  15.5  mm.  with  an  area  of  155  mm. 
26 


4o2  TEXT-BOOK  OF  PHYSIOLOGY. 

Respiratory  Types. — Observation  of  the  respiratory  movements 
in  the  two  sexes  shows  that  while  the  enlargement  of  the  thoracic 
cavity  is  accomplished  both  by  the  descent  of  the  diaphragm  (as  shown 
by  the  protrusion  of  the  abdomen)  and  the  elevation  of  the  thoracic 
walls,  the  former  movement  preponderates  in  the  male,  the  latter  in 
the  female,  giving  rise  to  what  has  been  termed  in  the  one  case  the  dia- 
phragmatic or  abdominal  and  in  the  other  the  thoracic  or  costal  type  of 
respiration.  The  cause  of  this  greater  mobility  and  actvity  of  the  tho- 
rax in  the  female  has  been  a  subject  of  much  discussion.  It  has  been 
attributed,  on  the  one  hand,  to  the  necessity  for  a  physiologic  adjust- 
ment between  respiration  and  child-bearing,  and  therefore  a  specific 
sex  peculiarity;  on  the  other  hand,  it  has  been  attributed  to  persistent 
constriction  of  the  waist,  in  consequence  of  which  the  full  play  of  the 
diaphragm  is  prevented  and  the  burden  of  inspiration  is  thrown  on  the 
thoracic  muscles.  It  has  been  assumed  that  if  inspiration  were  con- 
fined in  women  to  the  diaphragm,  there  would  arise  in  the  latter  stages 
of  gestation  such  an  increase  in  intra-abdominal  pressure  that  not  only 
would  respiratory  exchanges  be  interfered  with,  but  fetal  life  might  be 
unfavorably  influenced,  if  not  endangered.  Modern  investigations 
have  not  confirmed  this  assumption,  but,  on  the  contrary,  have  corrob- 
orated the  view  that  the  preponderance  of  thoracic  movement  is  due 
to  the  influences  of  dress  restrictions,  for  with  their  removal  the  so-called 
costal  type  of  breathing  entirely  disappears.  While  gestation  may 
lead  to  a  greater  activity  of  the  thorax,  this  is  but  temporary,  for  with 
its  termination  there  is  a  return  to  the  diaphragmatic  type  of  breathing. 

Number  of  Respirations  per  Minute. — The  number  of  respira- 
tions which  occur  in  a  unit  of  time  varies  with  a  variety  of  conditions, 
the  most  important  of  which  is  age.  The  results  of  the  observations  of 
Quetelet  on  this  point,  which  are  generally  accepted,  are  as  follows: 

Age.  Respirations  per  Minute.  Age.  Respirations  per  Minute 

o-  i  year, 44  20-25  years, 18.7 

5  years, 26  25-30      "       16.0 

15-20      "      20  30-5°      "      I^-° 

From  these  observations  it  may  be  assumed  that  the  average  number 
of  respirations  in  the  adult  is  eighteen  per  minute,  though  varying 
from  moment  to  moment  from  sixteen  to  twenty.  During  sleep,  how- 
ever, the  respiratory  movements  often  diminish  in  number  as  much 
as  30  per  cent.,  at  the  same  time  diminishing  in  depth. 

Rhythm. — Each  respiratory  act  takes  place  normally  in  a  regular 
methodic  manner,  each  event  occurring  in  a  definite  sequence  and 
occupying  the  same  relative  period  of  time.  This  rhythm,  however, 
is  not  infrequently  temporarily  disturbed  by  emotions,  volitional  acts, 
muscle  activity,  phonation,  changes  in  the  composition  of  the  blood, 
etc.;  with  the  removal  of  these  disturbing  factors,  the  respiratory 
mechanism  soon  returns  to  its  normal  condition. 

A  graphic  representation  of  the  excursions  of  the  thoracic  walls, 
rhythmic  or  otherwise,  is  obtained  by  fastening  to  the  thorax  an  appara- 


RESPIRATION. 


40; 


Fig.  193. — Pneumograph. — (Fits.) 


tus,  a  stethometer  or  a  pneumograph,  which  by  means  of  a  tambour 
takes  up  and  transmits  the  movement  to  a  second  tambour  provided 
with  a  recording  lever.  A  simple  form  of  pneumograph,  suggested 
by  Fitz  (Fig.  193),  consists  of  a  coil  of  wire  two  and  a  half  centimeters 
in  diameter  and  about  40  centimeters  in  length,  enclosed  by  thin  rubber 
tubing,  one  end  of  which  is  closed,  the  other  placed  in  communication 
with  either  a  tambour  and  lever  or  with  a  piston  recorder.  By  means 
of  an  inelastic  cord  or  chain  the  apparatus  is  securely  fastened  to  the 
chest.  With  each  inspiration  the  spring  is  elongated,  the  air  within 
the  system  is  rarefied, 
and  as  a  result  the  lever 
falls;  with  each  expira- 
tion the  reverse  condi- 
tions obtain  and  the 
lever  rises.  If  the  lever 
be  applied  to  the  record- 
ing surface  of  a  moving 
cylinder,  a  curve  of  the 
thoracic  movement,  a  pneumatogram,  is  obtained  (Fig.  194),  from 
which  it  is  apparent  that  inspiration  takes  place  more  abruptly  and 
occupies  a  shorter  period  of  time  than  expiration;  that  expiration  im- 
mediately follows  inspiration,  but  that  there  is  a  slight  pause  between 
the  end  of  the  expiration  and  the  beginning  of  the  inspiration.  The 
time  relations  of  the  two  movements  can  be  obtained  by  a  magnet- 
signal  actuated  by  an  electric  current  interrupted  once  a  second.  The 
ratio  of  inspiration  to  expiration  has  been  represented  as  5  to  6,  or  6  to  8. 
Volumes  of  Air  Breathed. — The  volumes  of  air  which  enter 
and  leave  the  lungs  with  each  inspiration  and  expiration  naturally 

vary  with  the  extent  of  the  move- 
ment, though  four  at  least  may  be 
determined:  (1)  that  of  an  ordinary 
inspiration;  (2)  that  of  an  ordinary 
expiration;  (3)  that  of  a  forced  in- 
spiration; (4)  that  of  a  forced  ex- 
piration. 

The  apparatus  employed  for 
the  determination  of  these  different 
volumes  is  the  spirometer,  a  modification  of  the  gasometer.  The 
form  introduced  by  Jonathan  Hutchinson  (Fig.  195)  consists  of  two 
metallic  cylinders,  one  (a)  containing  water,  the  other  (b)  containing 
air,  the  latter  being  inserted  into  the  former.  The  air  cylinder  is 
balanced  by  weights  so  accurately  that  it  remains  stationary  in  any 
position.  A  tube,  penetrating  the  base  of  the  water  cylinder,  is  con- 
tinued upward  through  and  above  the  level  of  the  water.  The  air- 
space above  is  thus  placed  in  free  communication  with  the  external  air. 
A  stopcock  at  the  outer  end  of  this  tube  prevents  the  escape  of  the  air 
when  this  is  not  desirable.     To  the  free  end  of  the  tube  a  rubber  tube 


Fig.  194. — A  Pneumatogram. 
Marey.) 


{After 


404 


TEXT-BOOK  OF  PHYSIOLOGY. 


provided  with  a  suitable  mouthpiece  is  attached,  through  which  air 
can  be  breathed  into  or  out  of  the  air  cylinder.  With  each  inspiration 
the  air  cylinder  descends;  with  each  expiration  it  ascends.  A  scale, 
on  one  of  the  side  supports,  graduated  in  cubic  inches  or  centimeters, 
indicates  the  volume  of  air  inspired  or  expired. 

With  this  apparatus  Hutchinson,  from  a  long  series  of  observa- 
tions, defined  and  determined  the  above-mentioned  four  volumes 
as  follows: 

i.  The  tidal  volume,  that  which  flows  into  and  out  of  the  lungs  with 

each  inspiration  and  expiration,  which 
varies  from  20  to  30  cubic  inches  (312 
to  468  c.c). 

2.  The  complemental  volume,  that  which 
flows  into  the  lungs,  in  addition  to  the 
tidal  volume,  as  a  result  of  a  forcible 
inspiration,  and  which  amounts  to  about 
no  cubic  inches  (1748  c.c). 

3.  The  reserve  volume,  that  which  flows  out 
of  the  lungs,  in  addition  to  the  tidal 
volume,  as  a  result  of  a  forcible  expira- 
tion, and  which  amounts  to  about  100 
cubic  inches  (1562  c.c). 

After    the    expulsion  of    the   reserve 
volume  there  yet  remains  in  the  lungs  an 
unknown  volume  of  air  which  serves  the 
mechanic  function  of  distending  the  air- 
cells  and  alveolar  passages,  thus  main- 
taining the  conditions  essential  to   the 
free   movement  of   blood    through  the 
capillaries  and  to  the  exchanges  of  gases 
between  the  blood  and  alveolar  air.     As 
this  air   can  not  be  displaced  by  voli- 
tional effort,  but  resides  permanently  in   the    alveoli  and   bron- 
chial tubes  though  constantly  undergoing  renewal,  it  was  termed — 
The  residual  volume,  the  amount  of  which  is  difficult  of  determina- 
tion, but  has  been  estimated  by  different  observers  at  914  c.c, 
1562  c.c,  1980  c.c. 
The  Vital  Capacity  of  the  Lungs. — From  foregoing  statements 
it  is  clear  that  the  thorax  and  lungs  are  capable  of  a  maximum  degree 
of  expansion,  at  which  moment  the  lungs  contain  their  maximum 
volume  of  air.     This  volume,  whatever  it  may  be,  represents  the  entire 
capacity  of  the  lungs  in  the  physiologic  condition,  and  includes  the 
tidal,  the  complemental,  the  reserve,  and  the  residual  volumes.     Mr. 
Hutchinson,  however,  defined  the  vital  or  respiratory  capacity  of  the 
lungs  as  the  amount  of  air  which  can  be  expelled  by  the  most  forcible 
expiration  after  the  most  forcible  inspiration,  and  which  therefore  ex- 
cludes the  residual  volume.     The  vital  capacity  was  supposed  to  be 


Fig.    195. — Spirometer. 
(Hutchinson.) 


4- 


RESPIRATION. 


405 


Fig.  196. — Pneumatograph. — (Gad.) 


an  indication  of  an  individual's  respiratory  power,  not  only  in  phys- 
iologic but  also  in  pathologic  conditions.  Though  averaging  about 
230  cubic  inches  (3593  c.c.)  for  an  individual  5  feet  7  inches  in  height, 
the  vital  capacity  varies  with  a  number  of  conditions,  the  most  im- 
portant of  which  is  stature.  It  is  found  that  between  5  and  6  feet  the 
capacity  increases  8  inches  (125  c.c)  for  each  inch  increase  in  height. 

The  volume  changes  of  the 
thorax  indicated  by  the  volumes 
of  air  entering  and  leaving  the 
lungs  can  be  not  only  determined 
but  graphically  represented  by 
means  of  an  apparatus  similar  in 
principle  to  the  spirometer,  de- 
vised by  Gad  and  known  as  the 
pneumatograph  or  aero  pie  thy sino- 
graph (Fig.  196).  This  consists 
of  a  quadrangular  box  with  double  walls,  the  space  between  which 
is  filled  with  water.  The  center  of  the  box  is  an  air  chamber.  A  thin- 
walled  mica  box  sinks  into  the  water.  Posteriorly  it  is  attached  to 
and  rotates  around  an  axis,  which  permits  of  an  elevation  or  depres- 
sion of  the  anterior  portion.  It  is  also  carefully  counterpoised.  A 
light  lever  attached  to  the  mica  box  records  its  movements.  The 
interior  of  the  box  communicates  by  a  tube  with  a  large  reservoir  into 

which  the  individual  breathes,  the 
object  being  to  prevent  a  too  rapid 
vitiation  of  the  air.  Inspiration 
causes  the  lever  to  descend,  expira- 
tion to  ascend.  Previous  gradua- 
tion of  the  apparatus  is  necessary 
to  determine  the  volumes  breathed. 
A  graphic  record  of  the  volume 
changes  is  shown  in  Fig.  197. 

Respiratory  Sounds. — On  ap- 
plying the  ear  over  the  trachea  and 
bronchi  there  is  heard  during  both 
inspiration  and  expiration  a  well- 
defined  sound,  loud,  harsh,  and 
blowing  in  character,  which  from 
its  situation  is  known  as  the  bron- 
chial sound.  It  is  especially  well 
heard  between  the  scapula?  above 
the  fourth  dorsal  vertebra.  This  sound  is  produced  in  the  larynx, 
for  with  its  separation  from  the  trachea  the  sound  disappears.  The 
cause  of  the  sound  is  to  be  found  in  the  narrowing  of  the  air-passage 
at  the  level  of  the  vocal  membranes,  though  the  mechanism  of  its  pro- 
duction is  uncertain.  On  applying  the  ear  to  almost  anv  portion  of 
the  chest-wall,  but  especially  to  the  infrascapular  area,  there  is  heard 


•-W 


ao  c.in 

TIDALVOL 


Fig.  197. — Representing  the  Vol- 
ume Changes  of  the  Thorax  and 
Lungs  (Diagrammatic). 


4o6  TEXT-BOOK  OF  PHYSIOLOGY. 

during  both  inspiration  and  expiration  a  delicate,  sighing,  rustling 
sound,  which  from  its  supposed  seat  of  origin,  the  air-vesicles  or  -cells, 
is  known  as  the  vesicular  sound.  This  sound  is  supposed  to  be  due 
to  the  sudden  expansion  of  the  air-cells  during  inspiration  and  to  the 
friction  of  the  air  in  the  alveolar  passages. 

THE  CHEMISTRY  OF  RESPIRATION. 

The  general  nutritive  process  as  it  takes  place  in  the  tissues  involves 
the  assimilation  of  oxvgen  and  the  evolution  of  carbon  dioxid.  The 
former  is  the  first,  the  latter  the  last,  of  a  series  of  chemic  changes 
the  continuance  of  which  is  essential  to  the  maintenance  of  all  life 
phenomena.  A  constant  supply  of  oxygen  and  an  equally  constant 
removal  of  carbon  dioxid  are  necessary  conditions  for  tissue  activity. 
The  respiratory  movements  constitute  the  means  by  which  the  oxygen 
of  the  air  is  brought  into,  and  the  carbon  dioxid  expelled  from,  the 
lungs  into  the  surrounding  air.  The  blood  is  the  medium  by  which 
the  oxygen  is  transported  from  the  lungs  to  the  tissues  and  the  carbon 
dioxid  from  the  tissues  to  the  lungs. 

The  exchanges  between  blood  and  tissues  constitute  internal  res- 
piration, in  contradistinction  to  the  thoracic  movements  by  which 
the  air  is  brought  into  relation  with  the  blood,  and  which  constitute 
external  respiration.  The  transfer  of  the  oxygen  by  the  blood  from 
the  interior  of  the  lungs  to  the  tissues,  and  of  the  carbon  dioxid  from 
the  tissues  to  the  interior  of  the  lungs,  is  the  outcome  of  a  series  of 
chemic  changes  which  are  related  to  the  exchange  of  gases  between 
the  air  in  the  lungs  and  the  blood,  on  the  one  hand,  and  between  the 
blood  and  tissues,  on  the  other. 

In  consequence  of  the  many  and  complex  chemic  changes  which 
attend  these  gaseous  exchanges,  there  arise  changes  in  composition  of: 
i.  The  air  breathed. 

2.  The  blood,  both  arterial  and  venous. 

3.  The  tissue  elements  and  the  lymph  by  which  they  are  surrounded. 
The  investigation  of  the  nature  of  these  changes,  the  mechanism 

of  their  production,  and  their  quantitative  relations  constitutes  the 
subject-matter  of  the  chemistry  of  respiration. 

CHANGES  IN  THE  COMPOSITION  OF  THE  AIR. 

Experience  teaches  that  the  air  during  its  sojourn  in  the  lungs 
undergoes  such  a  change  in  composition  that  it  is  rendered  unfit  for 
further  breathing.  Chemic  analysis  has  shown  that  this  change  in- 
volves a  loss  of  oxygen,  a  gain  in  carbon  dioxid,  watery  vapor  and 
organic  matter.  For  the  correct  understanding  of  the  phenomena 
of  respiration  it  is  essential,  that  not  only  the  character  but  the  extent 
of  these  changes  be  known.  This  necessitates  an  analysis  of  both  the 
inspired  and  expired  airs,  from  a  comparison  of  which  certain  deduc- 
tions can  be  made 


RESPIRATION.  407 

The  results  which  have  been   obtained   are   represented   in   the 
following  table: 

Inspired  Air.                                                                              Expired  Air. 
f  Oxygen, 20.80.  f  Oxygen, 16.02. 


100  J  Carbon  dioxid, traces.  |  Carbon  dioxid,  .  .  .   4.38. 

vols.  I  Nitrogen, 79.20.  ,     <  Nitrogen, 79.60. 

I  Watery  vapor, variable.  '  |  Watery  vapor,  .  .  .  .saturated. 

[  Organic  matter. 

These  analyses  indicate  that  under  ordinary  conditions  the  air 
loses  oxygen  to  the  extent  of  4.78  per  cent,  and  gains  carbon  dioxid 
to  the  extent  of  4.38  per  cent.;  that  it  gains  in  nitrogen  to  the  extent 
of  0.4  per  cent,  and  in  watery  vapor  from  its  initial  amount  to  the 
point  of  saturation,  as  well  as  in  organic  matter.  It  is  to  these  changes 
in  their  totality  that  those  disturbances  of  physiologic  activity  are 
to  be  attributed  which  arise  when  expired  air  is  re-breathed  for  any 
length  of  time  without  having  undergone  renovation. 

Special  forms  of  apparatus  have  been  devised  for  the  collection 
and  analysis  of  gases.  Their  construction  as  well  as  the  methods 
of  analysis  involved  are  complicated  and  need  not  be  described  in 
this  connection.  The  presence  of  the  carbon  dioxid,  however,  may 
be  readily  shown  by  breathing  through  a  glass  tube  into  a  vessel  con- 
taining barium  or  calcium  hydrate.  The  turbidity  which  immediately 
follows  is  due  to  the  formation  of  barium  or  calcium  carbonate,  which 
can  be  due  only  to  the  presence  of  carbon  dioxid.  That  this  turbidity 
is  not  due  to  the  carbon  dioxid  normally  present  in  the  air  is  shown  by 
the  fact  that  the  solution  remains  clear  until  the  passage  of  the  atmos- 
pheric air  has  been  maintained  for  some  time.  From  the  percentage 
loss  of  oxygen  and  gain  in  carbon  dioxid,  the  total  oxygen  absorbed  and 
carbon  dioxid  exhaled  may  be  approximately  calculated.  Thus,  if 
the  volume  of  air  breathed  daily  be  accepted  at  either  10,800  or  12,- 
240  liters,  and  the  percentage  loss  of  oxygen  be  4.78,  the  total  oxygen 
absorbed  may  be  obtained  by  the  rule  of  simple  proportion,  e.  g.: 

100  :  4. 78  ::  10,800  :  #  =  516  liters 
Or 

100  :  4.78  ::  12,240  :  #  =  585  liters. 

By  the  same  method  the  total  carbon  dioxid  exhaled  is  found  to  be 
either  473  or  526  liters;  volumes  in  both  instances  which  agree  very 
well  with  volumes  obtained  by  other  methods. 

From  the  fact  that  when  one  volume  of  oxygen  combines  with 
carbon  it  gives  rise  to  but  one  volume  of  carbon  dioxid,  it  is  evident 
that  of  the  oxygen  absorbed  the  greater  portion  by  far  is  utilized  in 
the  oxidation  of  the  carbon,  while  the  smaller  portion  is  utilized  in 
the  oxidation  of  other  substances,  but  especially  hydrogen,  as  shown 
by  the  increase  in  water  eliminated  beyond  that  consumed.  These 
amounts,  however,  are  not  fixed  but  variable,  and  depend  on  the 
quality  and  quantity  of  the  foods,  exercise,  etc.  The  ratio  of  the  volume 
of  the  carbon  dioxid  exhaled  to  the  volume  of  oxygen  absorbed  is 


4o8  TEXT-BOOK  OF  PHYSIOLOGY. 

known  as  the  respiratory  quotient,  and  is  usually  represented  by  the 
svmbol  -§~  Thus  in  the  foregoing  analysis  the  respiratory  quotient  is 
0.916. 

The  gain  in  nitrogen  is  a  variable  factor,  ranging  from  zero  to  0.9 
per  cent.  This  gain  is  probably  of  accidental  occurrence,  due  to  ab- 
sorption from  the  large  intestine,  in  which  decomposition  of  nitrogen- 
holding  compounds  is  taking  place.  It  is  generally  believed  that  free 
nitrogen  plays  no  part  in  any  phenomenon  of  combination  or  decom- 
position within  the  body. 

The  gain  in  watery  vapor  will  depend  on  the  amount  previously 
present  in  the  air.  This  is  conditioned  by  the  temperature.  With 
a  rise  in  temperature  the  percentage  of  water  increases;  with  a  fall, 
it  decreases.  By  breathing  into  a  vessel  containing  pumice  stone  satu- 
rated with  sulphuric  acid,  the  vapor  may  be  collected.  The  difference 
observed  between  the  weight  before  and  after  breathing  is  an  indica- 
tion of  the  amount  by  weight  of  water  exhaled  during  the  time  of 
breathing.  It  has  been  calculated  that  the  amount  of  water  exhaled 
daily  approximates  500  grams.  Though  invisible  at  ordinary  temper- 
atures, it  becomes  visible  at  low  temperature  as  soon  as  it  emerges  from 
the  respiratory  tract.  The  loss  of  heat  is  followed  by  a  condensation 
of  the  vapor,  which  appears  at  once  as  a  cloudy  precipitate. 

The  gain  in  organic  matter  is  also  variable.  The  amount  present 
is  not  sufficient  to  permit  of  a  thorough  chemic  analysis,  but  there  are 
reasons  for  believing  that  it  belongs  to  the  proteid  group  of  bodies. 
If  it  accumulates  in  the  air,  especially  at  high  temperatures,  it  readily 
undergoes  decomposition,  with  the  production  of  offensive  odors. 
Traces  of  free  ammonia  have  also  been  found  in  the  expired  air. 
In  addition  to  these  chemic  changes,  the  air  experiences  physical 
changes;  e.  g.,  a  rise  in  temperature  and  an  increase  in  volume.  The 
rise  in  temperature  can  be  shown  by  breathing  through  a  suitable 
mouthpiece  into  a  glass  tube  containing  a  thermometer.  By  this 
means  it  has  been  shown  that  inspired  air  at  20 °  C.  rises  in  temperature 
to  370  C;  at  6.3 °  to  29.8 °  C.  The  increase  in  the  temperature  will 
depend  upon  that  of  the  air  inspired  and  the  time  it  remains  in  the  lungs. 
If  retained  a  sufficient  length  of  time  it  will  always  become  that  of  the 
body.  As  a  result  of  the  heat  absorption  the  expired  air  increases  in 
volume  about  one-ninth  over  that  of  the  inspired  air.  When  corrected 
for  temperature  and  pressure  and  freed  from  aqueous  vapor,  the  volume 
of  the  expired  air  is  less  than  that  of  the  inspired  air  by  about  one-two 
hundred  and  fiftieth. 

The  Composition  of  the  Alveolar  Air. — The  foregoing  state- 
ment of  the  composition  of  the  expired  air,  derived  in  part  from  the 
upper  air-passages,  trachea,  and  bronchi,  does  not  necessarily  repre- 
sent Ihe  composition  of  the  alveolar  air.  It  is  very  probable  that  the 
percentage  of  carbon  dioxid  is  greater,  the  percentage  of  oxygen  less, 
in  the  latter  than  in  the  former.  This  is  made  evident  by  collecting 
in  several  portions  the  expired  air  as  it  escapes  from  the  respiratory 


RESPIRATION.  409 

tract  and  subjecting  it  to  analysis.  The  last  portion  always  contains 
a  larger  amount  of  carbon  dioxid  and  a  smaller  amount  of  oxygen 
than  the  first  portion.  The  determination  of  the  composition  of  the 
alveolar  air  is  extremely  difficult.  It  has  been  estimated  to  contain 
from  5  to  6  per  cent,  of  carbon  dioxid  and  from  14  to  18  per  cent,  of 
oxygen. 

Pulmonary  Ventilation. — It  is  owing  largely  to  this  inequality 
of  volumes  and  consequently  of  the  "partial  pressures"  of  these  two 
gases  in  the  trachea  and  alveoli  that  the  degree  of  ventilation  necessarv 
for  the  exchange  of  gases  between  lungs  and  air  is  maintained.  Though 
the  respiratory  movements  doubtless  create  currents  in  the  air-passages 
which  carry,  on  the  one  hand,  a  portion  of  the  inspired  air  directly 
into  the  alveoli,  and,  on  the  other  hand,  carry  a  portion  of  the  alveolar 
air  directly  out  of  the  body,  other  portions  find  their  way  into  and  out 
of  the  alveoli  in  accordance  with  the  laws  of  diffusion.  If  the  pressure 
of  the  oxygen  in  the  trachea  is  158  mm.  Hg.  and  in  the  alveoli  approxi- 
mately 122  mm.  Hg.,  diffusion  downward  will  take  place.  Equilibrium, 
however,  is  never  established,  as  the  oxygen  is  continually  disappear- 
ing by  passing  into  the  blood.  On  the  contrary,  if  the  carbon  dioxid 
pressure  in  the  alveoli  is  approximately  28  to  40  mm.  Hg.,  and  in  the 
trachea  0.3  mm.  Hg.,  diffusion  will  take  place  upward.  Equilibrium 
will  never  be  established,  however,  as  the  carbon  dioxid  is  constantly 
coming  out  of  the  blood.  Pulmonary  ventilation  may  also  be  aided 
by  those  alternate  changes  in  volume  of  the  heart,  great  vessels,  and 
lungs  occurring  as  the  result  of  the  heart-beat  and  producing  the  so- 
called  cardio-pneumatic  movements. 

CHANGES  IN  THE  COMPOSITION   OF  THE  BLOOD. 

The  blood  which  flows  into  the  lungs  through  the  pulmonary 
artery  is  dark  bluish-red,  that  which  flows  from  the  lungs  into  the 
pulmonary  veins  is  scarlet  red,  in  color.  The  blood  is  changed,  while 
flowing  through  the  lung  capillaries,  from  the  venous  to  the  arterial 
condition.  As  the  air  in  the  lungs  gains  carbon  dioxid  and  loses  oxygen, 
it  is  fair  to  assume  that  what  the  air  gains  the  blood  loses,  and  what  the 
air  loses  the  blood  gains.  In  other  words,  the  blood,  while  passing 
through  the  lungs,  is  changed  from  venous  to  arterial  by  the  loss  of 
carbon  dioxid  and  the  gain  of  oxygen.  The  change  in  color  of  venous 
blood  from  dark  bluish  to  scarlet  red  is  strikingly  shown  by  shaking 
it  in  a  test-tube  with  oxygen  or  atmospheric  air. 

The  blood  which  flows  into  the  tissues  through  the  arteries  is  red, 
that  which  flows  from  the  tissues  into  the  veins  is  bluish,  in  color. 
The  blood  while  flowing  through  the  tissue  capillaries  is  changed  from 
the  arterial  to  the  venous  condition.  Since  arterial  blood  when  de- 
prived of  oxygen  becomes  bluish-red,  the  indication  is  that  the  change 
in  color  is  associated  with,  if  not  entirely  due  to,  the  escape  of  oxygen 
into  the  tissues.     The  constant  elimination  of  carbon  dioxid  froni  the 


4io  TEXT-BOOK  OF  PHYSIOLOGY. 

blood  into  the  lungs  indicates  that  the  carbon  dioxid  is  as  constantly 
passing  from  the  tissues  through  the  capillary  walls  into  the  blood. 

These  considerations  are  confirmed  by  the  results  of  analyses  which 
have  been  made  of  both  venous  and  arterial  blood.  The  presence 
of  gas  in  the  blood  is  demonstrated  by  subjecting  it  under  appropriate 
conditions  to  the  vacuum  of  the  mercurial  air-pump,  into  which  it  at 
once  escapes.  From  ioo  volumes,  an  average  of  60  volumes  of  gas  at 
standard  pressure,  760  mm.  Hg.  and  temperature  o°  C,  can  thus  be 
obtained. 

Gases  of  the  Blood. — An  analysis  of  the  volumes  of  gas  removed 
from  both  venous  and  arterial  blood  shows  that  each  consists  of  oxygen, 
carbon  dioxid,  and  nitrogen,  though  in  different  amounts.  An  aver- 
age composition  of  the  gases  extracted  from  dog's  blood  obtained  from 
the  right  ventricle  and  carotid  artery  is  given  in  the  following  table: 

Venous  blood  J  ^jf  V  '  :\  9'12  ™ls-       Arterial  blood  \  °Xy,ge "' W  '  '.V      2°  v°ls- 
t  <  Carbon  dioxid,      4S  ,  {  Carbon  dioxid,     40 

IOO  VOlS.  -vr-,  ^J      i,  IOO  VOls.  ,T..  '       ^      ,, 

[  JNitrogen,    ....  1-  2  [  Nitrogen,  .  .  .  .  1-  2 

The  changes  produced  in  the  blood  by  respiration,  both  external 
and  internal,  become  apparent  from  a  comparison  of  these  analyses. 
The  venous  blood  while  passing  through  the  lungs  gains  from  eight 
to  eleven  volumes  per  cent,  of  oxygen  and  loses  five  volumes  per  cent, 
of  carbon  dioxid.  The  arterial  blood  while  passing  through  the  tissues 
loses  oxygen  and  gains  carbon  dioxid  in  corresponding  amounts.  The 
volume  of  nitrogen  is  not  appreciably  changed. 

The  Relation  of  the  Gases  in  the  Blood. — The  mechanism 
by  which  the  gases  become  associated  with  the  blood  at  the  moment 
of  their  entrance  into  it,  and  again  become  dissociated  just  prior  to 
their  exit  from  it,  as  well  as  their  relation  to  the  blood  while  in  transit, 
will  be  more  readily  understood  after  reference  to  a  few  elementary 
facts  relative  to  the  absorption  of  gases  by  liquids  in  general  and  the 
conditions  of  temperature  and  pressure  by  which  it  is  influenced. 

It  is  well  known  that  liquids  will  absorb  or  dissolve  at  any  constant 
pressure  unequal  volumes  of  different  gases  in  accordance  with  their 
solubilities  and  with  variations  in  temperature.  Water,  for  example, 
will  absorb,  in  accordance  with  the  foregoing  conditions,  oxygen,  carbon 
dioxid  and  nitrogen,  as  well  as  many  other  gases.  The  volume  of 
any  gas  thus  absorbed  is  known  as  the  coefficient-  of  absorption,  and  may 
be  defined  as  the  number  of  cubic  centimeters  of  the  gas  which  one 
cubic  centimeter  of  water  will  absorb  when  the  gas,  in  contact  with 
the  water,  stands  under  a  pressure  of  one  atmosphere  or  760  mm.  of 
mercury  and  at  a  temperature  of  o°  C.  The  volume  absorbed, ' 
however,  varies  inversely  as  the  temperature.  Thus  at  o°  C.  the 
volume  of  oxygen  absorbed  by  one  volume  of  water  is  0.0489  c.c. ; 
of  carbon  dioxid  1.713  c.c;  of  nitrogen  0.0234  c.c.  With  a  rise  of  tem- 
perature, the  pressure  remaining  constant,  the  absorptive  power  of 
water  for  each  of  these  gases  diminishes.     Thus  at  150  C,  the  volumes 


RESPIRATION.  411 

of  oxygen,  carbon  dioxid  and  nitrogen  absorbed  are  0.0310  c.c.,  1.0025 
c.c.  and  0.0168  c.c.  respectively.  Though  the  volume  of  the  gas 
absorbed  diminishes  as  the  temperature  rises,  it  is  independent  of 
pressure,  for  no  matter  to  what  extent  the  pressure  may  vary  the 
volume  absorbed  is  always  the  same.     (Law  of  Henry.) 

If  the  weight  of  the  gas  absorbed  be  considered  rather  than  the 
volume  (that  is  the  product  of  the  volume  and  the  density  or  the 
number  of  molecules  in  the  volume),  then  the  temperature  remaining 
constant,  the  weight  of  the  volume  absorbed  increases  and  decreases 
proportionately  as  the  pressure  rises  and  falls.  Thus  at  a  pressure  of 
760  mm.  of  mercury  and  at  a  temperature  of  o°  C,  the  volume  of 
oxygen  absorbed  by  one  volume  of  water  is  0.0489  c.c;  at  1520  mm. 
of  mercury,  the  same  volume  is  absorbed  but  its  weight  is  doubled. 
If  the  pressure  falls  below  760  mm.  of  mercury  the  same  volume  is  ab- 
sorbed but  its  weight  is  diminished.  (Law  of  Dalton.)  Because  of 
the  foregoing  facts,  it  is  necessary  in  all  gaseous  determinations  to 
reduce  for  purposes  of  comparison  the  obtained  volumes  to  standard 
temperature  (p°  C.)  and.  pressure  (760  mm.  of  mercury). 

When  the  liquid  is  once  saturated  with  a  gas  at  a  constant  pressure 
and  temperature,  there  is  coincidently  with  the  entrance  of  the  gas 
into  the  liquid,  an  equivalent  exit  of  the  gas  from  it,  tnough  the  volume 
retained  in  the  liquid  remains  constant.  The  reason  for  this  fact  is, 
that  under  the  conditions,  the  volume  of  the  gas  dissolved  by  the 
liquid  though  small  in  amount  exerts  a  pressure  in  the  opposite  direction 
equivalent  to  the  pressure  acting  upon  the  liquid.  If  one  cubic 
centimeter  of  water  absorbs  0.0489  c.c.  of  oxygen  at  760  mm.  and 
o°  C,  this  volume  will  exert  a  pressure  opposite  in  direction  of  760  mm. 
of  mercury.  For  this  reason  the  entrance  and  exit  of  the  gas  are 
equal  and  opposite. 

If  water  be  exposed  to  atmospheric  air  consisting  of  oxygen,  carbon 
dioxid,  and  nitrogen  in  the  ordinary  proportions,  at  any  given  tem- 
perature and  pressure,  the  water  will  absorb  unequal  volumes  of  each  of 
the  three  gases.  The  pressure  under  which  each  gas  is  absorbed  is  a 
part  only,  however,  of  the  total  atmospheric  pressure  at  the  time.  The 
pressure  exerted  by  any  one  of  these  gases  is  known  as  its  "  partial  pres- 
sure," and  depends  on  the  percentage  volume  of  the  gas  present.  If 
atmospheric  air  contains  at  standard  pressure  and  temperature  79.15 
volumes  per  cent  of  nitrogen,  its  partial  pressure  will  be  2f^  of  760, 
or  601.54  mm.  Hg.;  if  the  air  contains  0.04  volume  per  cent,  of  carbon 
dioxid  and  20.85  volumes  per  cent,  of  oxygen,  the  partial  pressure  of 
each  will  be  0.30  mm.  Hg.  and  158.46  mm.  Hg.  respectively.  The 
absorption  of  each  gas  is  independent  of  all  the  rest,  and  is  the  same 
for  nitrogen,  for  example,  as  if  it  alone  were  present  at  a  pressure  of 
601.54  mm.  Hg.  - 

Again,  if  water  holding  in  solution  a  certain  volume  of  a  gas — ■ 
carbon  dioxid,  for  example — be  exposed  to  an  atmosphere  containing 
but  0.04  volume  per  cent,  of  carbon  dioxid,  and  having  therefore  a 


4i2  TEXT-BOOK  OF  PHYSIOLOGY. 

pressure  of  but  0.3  mm.  Hg.,  the  gas  will  at  once  begin  to  leave  the 
water,  and  continue  to  do  so  until  the  pressure  of  the  carbon  dioxid 
in  the  atmosphere  balances  the  pressure  of  the  gas  in  the  water,  at  which 
moment  the  escape  of  the  gas  ceases.  The  pressure  of  a  gas  in  a  liquid 
is  equal  to  that  pressure  in  millimeters  of  mercury  of  the  same  gas  in 
the  atmosphere  which  is  required  to  keep  it  in  solution.  What  is 
true  for  the  carbon  dioxid  is  true  for  any  other  gas  that  may  be  in  solu- 
tion. If  a  liquid  has  a  greater  density  than  water  as  from  the  pres- 
ence of  inorganic  salts,  the  absorptive  power  under  standard  condi- 
tions of  temperature  and  pressure  becomes  less.  It  is  for  this  reason 
that  blood-plasma  contains  less  oxygen,  nitrogen  and  carbon  dioxid 
than  water. 

It  will  be  recalled  that  the  blood  yields  up  its  gases  when  subjected 
to  the  vacuum  of  the  mercurial  pump;  that  is,  to  a  diminution  or  com- 
plete removal  of  the  atmospheric  pressure.  From  this  it  might  be  in- 
ferred that  the  gases  are  merely  held  in  solution  by  pressure,  and  at 
once  escape  the  moment  they  are  exposed  to  a  space  in  which  there 
is  a  very  slight  or  a  total  absence  of  pressure.  In  other  words  that  the 
absorption  of  gases  by  the  blood  and  their  escape  from  it  follows  the  law 
of  pressure  as  stated  in  foregoing  paragraphs.  It  is  therefore  neces- 
sary to  test  this  supposed  condition  of  the  gases  in  the  blood  by  sub- 
jecting the  latter  to  gradually  diminishing  pressures,  with  a  view  of 
determining  in  how  far  the  discharge  of  the  gases  follows  the  law  of 
falling  pressures.  For  convenience  the  conditions  of  each  gas  will  be 
considered  separately. 

Oxygen. — If  blood  is  subjected  to  a  succession  of  pressures  pro- 
gressively less  than  the  standard,  it  is  found  that  though  oxygen  is 
evolved,  its  evolution  is  not  in  accordance  with  the  law  of  partial 
pressures;  that  is,  in  proportion  to  the  diminution  of  pressure.  Within 
wide  limits — e.  g.,  from  760  to  332  mm.  atmospheric  pressure,  to  which 
correspond  oxygen  pressures  of  160  and  70  mm.  respectively — there 
is  but  a  slight  increase  in  the  amount  of  oxygen  evolved;  and  it  is  not 
until  the  pressure  of  the  oxygen  falls  below  this  that  it  begins  to  be 
liberated  in  large  amounts.  From  this  on,  the  oxygen  continues  to  be 
liberated  with  decreasing  pressures,  until  the  zero  point  is  reached,  when 
all  gaseous  discharge  ceases.  Coincidently  the  blood  changes  in  color 
from  a  bright  red  to  a  deep  bluish-red.  It  is  evident  from  the  results 
of  this  procedure  that  the  condition  of  the  oxygen  in  the  blood  is  but 
to  a  slight  extent  one  of  physical  absorption.  The  indications  are  that 
the  union  is  of  the  nature  of  a  chemic  combination. 

If  the  red  corpuscles  are  removed  from  the  blood  and  the  plasma 
alone  treated  in  the  manner  above  described,  it  will  be  found  that  the 
oxygen  liberated  now  follows  the  law  of  partial  pressure.  The  amount 
so  liberated,  however,  is  small — about  one  per  cent,  of  the  total  oxygen 
of  the  blood.  The  agent  therefore  which  holds  the  oxygen  in  com- 
bination is  the  red  corpuscle,  and  more  especially  the  hemoglobin, 
which  constitutes  about  30  per  cent,  of  its  volume.     This  is  proved  by 


RESPIRATION.  413 

the  fact  that  a  solution  of  gas-free  hemoglobin  of  a  strength  equivalent 
to  that  of  the  blood  (14  per  cent.),  exposed  to  oxygen  under  a  gradually 
increasing  pressure  from  zero  up  to  50  to  70  mm.  pressure,  will  absorb 
large  quantities  of  oxygen;  beyond  this  point  the  amount  absorbed 
is  again  small  in  comparison.  At  70  mm.  pressure  the  hemoglobin 
is  almost  saturated.  Coincidently  with  this  absorption  the  hemo- 
globin changes  in  color  from  dark  blue  to  bright  red;  changes  from 
hemoglobin  to  oxyhemoglobin.  The  reverse  method,  that  of  subject- 
ing oxyhemoglobin  to  gradually  diminishing  pressures,  yields  opposite 
results,  that  is  the  oxygen  becomes  dissociated  and  the  force  by  which 
this  is  accomplished  is  known  as  the  force  of  dissociation.  As  one 
gram  of  hemoglobin  combines  with  1.59  c.c.  of  oxygen,  and  as  the  per- 
centage of  hemoglobin  is  13.50  to  14,  it  is  evident  that  there  is  sufficient 
hemoglobin  to  combine  with  practically  all  the  oxygen  usually  present 
in  the  blood. 

The  union  of  the  oxygen  with  the  hemoglobin  is  therefore  largely 
chemic  in  character,  dependent  however  on  pressure.  About  one-half 
of  one  per  cent,  is  physically  absorbed  by  or  dissolved  in  the  plasma; 
the  remainder  is  chemically  combined  with  the  hemoglobin. 

The  association  or  combination  of  oxygen  is  favored  by  a  pressure 
of  at  least  from  30  to  50  mm.  Hg.  and  upward;  the  dissociation,  by 
diminution  of  pressure.  In  the  conversion  of  hemoglobin  into  oxy- 
hemoglobin two  antagonistic  forces  are  at  work,  heat  and  chemic 
affinity.  The  former  endeavors  to  prevent,  the  latter  to  favor,  the 
union.  Chemic  affinity  increases  with  the  influence  of  mass,  that  is, 
in  proportion  to  the  number  of  atoms  in  a  unit  of  volume,  with  the 
density  and  with  the  partial  pressure  of  the  oxygen.  Diminution  of 
pressure  reduces  the  mass  influence  and  permits  the  heat  to  bring 
about  dissociation  (Bunge).  The  following  table  by  Hiifner  shows 
the  relative  proportion  of  hemoglobin  and  oxyhemoglobin  in  blood 
containing  14  per  cent,  hemoglobin  and  exposed  to  air  at  gradually 
diminishing  pressures: 


pheric  Pressure 

Partial  Pressure  of 

Hemoglobin- 

Oxyhemoglobin 

IN  MM.  Hg. 

Oxygen  in  mm. 

Hg. 

Percentage. 

Percentage. 

760 

159-3 

1.49 

98.51 

524.8 

no 

2.14 

97.86 

357-3 

75 

3-11 

96.89 

238-5 

5° 

4.60 

95-40 

"9-3 

25 

S.79 

91. 2L 

47-7 

10 

19.36 

S0.64 

23.8 

5 

32-5x 

67.49 

0.0 

0.0 

100.00 

O.OO 

Carbon  Dioxid. — The  blood  yields  up  its  contained  carbon  dioxid 
to  the  vacuum  of  the  gas-pump  as  completely  as  it  does  its  oxygen. 
The  same  is  not  the  case,  however,  if  the  red  corpuscles  are  first  re- 
moved and  the  experiment  made  with  either  plasma  or  serum.  Even 
at  zero  pressure  the  fluid  contains  carbon  dioxid,  as  shown  by  its  libera- 
tion on  the  addition  of  some  weak  acid,  as  tartaric  or  phosphoric,  an 


4i4  TEXT-BOOK  OF  PHYSIOLOGY. 

indication  that  it  exists  in  a  state  of  firm  combination.  The  same 
result  follows  the  addition  of  the  red  blood-corpuscles,  which  act  in  a 
manner  similar  to  the  acids  just  mentioned.  This  property  of  the  cor- 
puscles has  been  attributed  to  hemoglobin,  and  especially  when  in 
the  state  of  oxyhemoglobin.  It  is  for  this  reason  that  blood  yields  all 
its  carbon  dioxid  to  the  vacuum  of  the  gas-pump. 

The  limit  of  pressure  at  which  the  plasma  ceases  to  physically 
absorb  carbon  dioxid  and 'begins  to  chemically  combine  it  is  not  very 
clearly  defined.  It  has  been  estimated  that  of  the  entire  amount, 
38  to  45  volumes,  only  about  2.5  volumes  are  so  absorbed,  the  remain- 
der being  in  a  condition  of  both  loose  and  stable  combination. 

An  analysis  of  the  serum,  and  presumably  of  the  plasma,  shows 
the  presence  of  sodium  salts,  with  which  the  carbon  dioxid  could 
enter  into  combination,  viz.:  sodium  carbonate  and  dibasic  sodium 
phosphate.  The  sodium  is  thus  partly  divided  between  carbonic  acid 
and  phosphoric  acid.  The  amount  of  the  sodium  which  falls  to  carbon 
dioxid  will  depend  on  the  mass  influence  of  the  latter;  that  is,  its  partial 
pressure. 

At  its  origin  in  the  tissues  the  carbon  dioxid  acquires  a  consider- 
able tension,  and  its  mass  influence  is  correspondingly  large.  On 
entering  the  blood  it  combines  with  sodium  carbonate,  with  the  forma- 
tion of  sodium  bicarbonate,  as  shown  in  the  following  equation: 

Na2C03  +  CCV+  H20  =  2NaHC03. 

At  the  same  time,  having  a  greater  mass  influence  than  the  phos- 
phoric acid,  it  will  withdraw  from  the  dibasic  sodium  phosphate  one- 
half  of  its  sodium,  with  the  formation  of  sodium  bicarbonate  and  mono- 
basic sodium  phosphate,  as  shown  in  the  following  equation: 

Na2HP04  +  C02  +  H20  =  NaHC03  +  NaH2P04. 

With  the  diffusion  of  the  carbon  dioxid  from  the  blood  into  the  alveoli 
its  tension  in  the  venous  blood  falls,  its  mass  influence  diminishes, 
while  that  of  the  phosphoric  acid  relatively  increases.  As  a  result, 
the  sodium  is  withdrawn  from  the  sodium  bicarbonate,  an  additional 
liberation  of  carbon  dioxid  takes  place  and  dibasic  sodium  phosphate 
is  re-formed.  The  association  or  combination  of  the  carbon  dioxid 
with  the  basic  salts  depends  on  its  partial  pressure;  its  dissociation  in 
the  lungs,  on  a  diminution  of  pressure. 

Nitrogen. — This  gas  exists  in  both  arterial  and  venous  blood  in  a 
state  of  solution.  There  is  no  evidence  that  it  enters  into  combination 
with  any  other  element. 

Tension  of  the  Gases  in  the  Blood. — It  will  be  recalled  that  a 
Liquid  holding  in  solution  one  or  more  gases  will  on  exposure  to  an 
atmosphere  composed  of  the  same  gases  either  give  up  or  absorb  vol- 
umes varying  in  amount  and  in  accordance  with  their  partial  pressures 
until  equilibrium  is  established.  Jf  the  pressure  of  any  one  gas  in  the 
atmosphere  is  greater  than  the  pressure  of  the  same  gas  in  the  liquid, 


RESPIRATION.  415 

it  is  absorbed;  if  the  pressure  is  less  the  gas  is  discharged.  Knowing 
the  pressure  of  the  gases  in  percentages  of  an  atmosphere,  at  the  be- 
ginning and  the  end  of  an  experiment,  the  original  tension  or  pressure 
of  the  gases  in  the  liquid  can  be  easily  calculated.  On  this  principle 
various  forms  of  apparatus  known  as  aerotonometers  have  been  devised 
by  which  the  tension  of  the  gases  in  the  blood  can  be  determined. 

These  appliances  consist  essentially  of  a  glass  tube  containing 
oxygen,  carbon  dioxid  and  nitrogen  in  known  amounts  and  tensions. 
The  blood  from  an  animal  is  then  allowed  to  flow  directly  from  an  artery 
or  vein  into  the  tube.  As  it  flows  down  its  sides  in  a  thin  layer  it  pre- 
sents a  large  surface  to  the  action  of  the  contained  gases.  In  the  aero- 
tonometer  of  Fredericq  the  blood  made  non-coagulable  by  the  injection 
of  peptone  is  returned  from  the  opposite  extremity  of  the  tube  to  the 
animal.  This  enables  the  experiment  to  be  continued  for  an  hour  or 
more.  A  knowledge  of  the  tensions  of  the  blood  gases  is  of  interest, 
as  it  affords  a  clue  to  the  mechanism  by  which  the  interchange  takes 
place  between  the  lungs  and  the  blood,  on  the  one  hand,  and  the  blood 
and  tissues,  on  the  other.  The  results,  however,  of  different  observers 
are  not  sufficiently  in  accord  to  permit  of  positive  deductions. 

In  the  well-known  experiments  of  Strassburger,  the  tension  of  the 
oxygen  in  the  arterial  blood  of  the  dog  was  found  to  be  29.64  mm.  Hg., 
or  3.9  per  cent,  of  an  atmosphere,  and  in  the  venous  blood  22.04  mm. 
Hg.,  or  2.9  per  cent.  The  tension  of  the  carbon  dioxid  in  the  venous 
blood  was  found  to  be  41.14  mm.  Hg.,  or  5.4  per  cent. of  an  atmosphere, 
and  in  the  arterial  blood  21.8  mm.  Hg.,  or  2.8  per  cent.  Very  different 
results  have  been  obtained  by  Fredericq  with  the  aerotonometer 
devised  by  him  and  by  the  employment  of  a  method  different  from  that  of 
Strassburger.  Thus  he  states  that  the  oxygen  tension  in  the  pulmonary 
alveoli  is  136  mm.  Hg.,  or  18  per  cent,  of  an  atmosphere  while  in  the 
arterial  blood  it  is  106  mm.  Hg.,  or  14  per  cent.;  while  the  carbon-di- 
oxid  tension  in  the  tissues  varies  from  38  to  68  mm.  Hg.,  or  from  5  to  9 
per  cent. of  an  atmosphere;  while  in  the  venous  blood  it  varies  from  30  to 
41  mm.  Hg.,  or  from  ^.S  to  5.4  per  cent,  and  in  the  pulmonary  alveoli 
it  is  about  21  mm.  or  2.8  per  cent. 

CHANGES  IN  THE  COMPOSITION  OF  THE  TISSUES  AND  LYMPH. 

From  previous  statements  the  inferences  can  be  drawn  that  the 
oxygen  leaves  the  blood  as  the  latter  flows  through  the  capillaries; 
that  it  passes  through  the  capillary  wall  into  the  surrounding  lymph 
and  so  to  the  tissue-cells;  that  it  oxidizes  food  materials  in  the  tissue- 
cells  whereby  the  potential  energy  of  the  former  is  liberated  as  kinetic 
energy;  that  the  carbon  dioxid  so  evolved  passes  into  the  lymph  and 
through  the  wall  of  the  capillary  into  the  blood. 

While  this  is  doubtless  the  case,  the  presence  of  free  oxygen  in  the 
tissues  can  not  be  demonstrated  by  the  usual  methods  of  gas  analysis. 
Only  in  the  saliva  and  in  the  blood  of  the  placental  umbilical  vein  can 


4i6  TEXT-BOOK  OF  PHYSIOLOGY. 

it  be  shown  that  oxygen  has  directly  passed  through  the  capillary  wall. 
For  this  reason  it  has  been  claimed  by  a  few  investigators  that  oxygen 
does  not  leave  the  blood,  but  that  the  field  of  its  activity  as  an  oxidizing 
agent  is  limited  to  the  blood-current,  where  it  meets  with  and  oxidizes 
easily  reducible  substances  entering  from  the  tissues.  On  this  view 
the  potential  energy  of  the  food  would  be  liberated  by  mere  decom- 
position or  cleavage  in  consequence  of  cell  activity. 

Nevertheless  many  facts  from  the  fields  of  comparative  physi- 
ology and  physiologic  chemistry  combine  to  support  the  view  that 
oxygen  is  absolutely  necessary  to  the  maintenance  of  the  life  of  all 
tissue-cells.  Though  they  will  continue  to  manifest  their  character- 
istic activities — e.  g.,  contraction  on  the  part  of  a  muscle,  secretion  by  a 
gland,  the  conduction  of  a  nerve  impulse  by  the  nerve,  etc. — for  a 
variable  length  of  time  after  oxygen  is  prevented  from  gaining  access 
to  them,  nevertheless  they  will  in  due  time  die. 

The  necessity  for  oxygen  on  the  part  of  the  tissues  and  the  avidity 
with  which  they  absorb  it,  is  shown  by  their  power  of  reducing  pig- 
ments such  as  alizarine  blue.  If  this  pigment  be  injected  into  the 
blood-vessels  of  an  animal  and  the  animal  killed  in  about  ten  minutes, 
it  will  be  found  that  while  the  blood  exhibits  a  deep  blue  color  the 
tissues  present  their  usual  colors.  But  after  exposure  to  the  air  or  to 
free  oxygen  the  latter  also  acquire  the  characteristic  blue  color.  The 
explanation  offered  for  this  fact  is  that  the  tissues  in  their  need  for 
oxygen  absolutely  extract  it  from  the  pigment,  reducing  it  to  a  color- 
less compound,  which,  however,  on  exposure  recombines  with  oxygen 
and  regains  the  original  color. 

Though  free  oxygen  can  not  be  shown  to  be  present  in  the  tissues, 
there  are  many  reasons  for  believing  that  it  is  continually  passing  into 
them  by  way  of  the  lymph-stream.  Its  rapid  disappearance  would 
indicate  that  it  is  immediately  utilized  for  the  production  of  carbon 
dioxid  (which  is  improbable  on  other  grounds),  or  that  the  tissues 
possess  a  capacity  for  oxygen  storage,  of  placing  it  in  reserve  under 
some  combination  or  other,  by  which  it  can  be  securely  retained 
until  required  for  oxidation  purposes.  This  is  rendered  probable 
from  the  fact  that  the  carbon  dioxid  evolved  at  any  given  moment  is 
not  necessarily  dependent  on  the  oxygen  just  absorbed,  for  if  oxygen 
be  withheld  from  a  nutritive  fluid  which  is  being  artificially  circulated 
through  a  recently  isolated  organ,  carbon  dioxid  will  continue  to  be 
discharged  for  some  time.  A  muscle,  or  even  a  living  animal — e.  g., 
a  frog — placed  in  an  atmosphere  of  pure  nitrogen  will  remain  active 
and  evolve  C02  for  even  several  hours. 

Naturally  the  absorption  of  oxygen  and  the  discharge  of  carbon 
dioxid  and  the  changes  of  composition  which  arc  incident  to  nutri- 
tion will  be  most  marked  in  those  tissues  characterized  by  the  greatest 
degree  of  physiologic  activity.  Muscle-tissue  exhibits  these  changes 
to  a  greater  degree  than  bone.  Tissues  with  intermediate  degrees 
of  activity  should  exhibit  corresponding  degrees  of  respiratory  change. 


RESPIRATION. 


4i7 


Experiment  confirms  this  view.  Thus,  100  grams  each  of  muscle, 
spleen,  and  broken  bone  from  a  recently  living  animal  exposed  to  the 
air  for  twenty-four  hours  absorbed  respectively  50.8  c.c,  27.3  c.c, 
and  17.2  c.c.  of  oxygen,  while  each  discharged  during  the  same  period 
56.8  c.c,  15.4  c.c,  and  8.1  c.c.  of  carbon  dioxid  respectively.  In 
another  series  of  experiments  by  a  different  observer  100  grams  of 
muscle  absorbed  in  three  hours  23  c.c.  of  oxygen,  and  100  grams  of 
bone  5  c.c.  of  oxygen.  Both  tissues  discharged  carbon  dioxid  in 
amounts  proportional  to  the  oxygen  absorbed.  The  same  respiratory 
changes  may  be  more 
satisfactorily  demon- 
strated by  passing 
blood  through  the  tis- 
sues of  isolated  organs 
and  the  tissues  of  re- 
cently living  animals. 
The  analysis  of  the 
blood  before  and  after 
perfusion  shows  a  loss 
of  oxygen  and  a  gain 
in  carbon  dioxid. 

Tension    of    the 
Gases    in    the    Tis- 


ATMOSPHERIC  AIR. 
O—  I  SB    MM    MS,OR  20.85   PC 
CO     0.3    MM  HG    OR   0.04-  PC 


O-TEMSION   

22.04-  MM  HO  OR 


CO    TENSION 
•♦I.O*   MM  HG  OR 
5.4   P.C. 


ALVEOLUS 


VtNOUS 

BLOOD 


ARTERIAL 
BLOOD 


O -TENSIOI 
O.OO    M  M     H 

CO-TENSIO> 


O  —  TENSION 


CO —TENSION 


Fig.  198. — Diagram  showing  the  Relative  Tension 
of  Oxygen  and  Carbon  Dioxid  ln  the  Lungs,  ln  the 
Blood,  and  est  the  Tissues. 


sues. — As  the  pres- 
ence of  free  oxygen 
can  not  be  demon- 
strated, its  tension 
there  must  be  re- 
garded as  nil.  The 
tension  of  the  carbon 
dioxid  is  quite  high, 
though  difficult  of 
exact  determination. 
It  has  been  estimated 
at  from  45  to  68  mm. 
Hg.,  or  from  6  to  9  per  cent,  of  an  atmosphere. 

The  variations  of  tension  or  pressure  of  these  two  gases  in  the 
lungs,  in  different  parts  of  the  vascular  apparatus,  and  in  the  tissues, 
and  their  relations  to  each  other,  are  shown  in  Figure  198,  expressed 
in  mm.  Hg.  and  percentages  of  an  atmosphere.  The  figures  inserted 
are  those  of  Strassburger  and  are  of  less  value  as  far  as  the  oxygen 
tension  is  concerned  than  those  of  Fredericq. 

The  Mechanism  of  the  Gaseous  Exchange.— In  these  pressure 
differences  sufficient  cause  is  found  for  the  exchange  of  the  gases. 
The  oxygen  pressure  in  the  alveoli  being  in  excess  of  that  in  the  blood, 
the  gas  passes  through  the  thin  alveolo-capillary  wall  into  the  plasma. 
As  the  oxygen  pressure  in  the  plasma  rises  and  approximates  that  in 


4i8  TEXT-BOOK  OF  PHYSIOLOGY. 

the  alveoli,  a  portion  of  the  oxygen  combines  with  the  hemoglobin 
until  the  latter  is  almost  saturated.  The  corpuscle  is  then  carried 
through  the  arterial  system  surrounded  by  oxygen  under  a  definite 
pressure  which  is  sufficient  to  keep  the  absorbed  oxygen  in  union 
with  the  hemoglobin.  On  passing  into  the  systemic  capillaries,  the 
blood  enters  a  region  in  which  the  oxygen  tension  in  the  surrounding 
tissues  is  nil.  At  once  the  oxygen  dissolved  in  the  plasma  passes 
through  the  capillary  wall  into  the  surrounding  tissue-spaces.  The 
pressure  removed  from  the  corpuscle,  a  dissociation  of  the  oxygen 
and  of  the  hemoglobin  takes  place,  after  which  the  oxygen  also  passes 
through  the  capillary  wall  into  the  surrounding  lymph  and  so  io  the 
tissue-cells  where  it  is  stored  and  utilized.  On  passing  into  tnc 
venous  system  the  dissociation  of  the  oxygen  and  the  hemoglobin  is 
checked  by  the  rise  of  oxygen  pressure  in  the  plasma.  On  reaching 
the  lungs  the  oxygen  again  passes  into  the  blood  until  the  former  con- 
dition is  regained. 

The  sojourn  of  the  blood  in  the  capillaries  being  short,  the 
oxyhemoglobin  can  part  with  but  a  portion  of  its  oxygen,  sufficient, 
however,  to  satisfy  the  needs  of  the  tissues. 

The  carbon  dioxid  pressure  in  the  tissues  being  in  excess  of  that 
in  the  blood,  it  passes  through  the  capillary  wall  into  the  blood,  where 
it  exists  in  the  free  and  combined  states.  On  passing  into  the  pul- 
monic capillaries  the  blood  enters  a  region  in  which  the  carbon  dioxid 
in  the  alveoli  is  less  than  in  the  blood.  At  once  a  diffusion  and  dis- 
sociation of  the  carbon  dioxid  takes  place  through  the  alveolo-capillary 
wall  until  equilibrium  is  established.  This,  however,  is  of  very 
short  duration,  for  the  carbon  dioxid  so  eliminated  is  rapidly  removed 
from  the  lungs  by  the  respiratory  movements. 

While  diffusion,  in  response  to  physical  and  chemic  conditions, 
thus  plays  a  large  part  in,  and  is  sufficient  to  account  for,  the  ex- 
changes of  gases,  it  is  possible  that  the  alveolar  or  respiratory  epithe- 
lium may  also  play  an  essential  role.  It  is  believed  by  some  in- 
vestigators that  it  is  active  in  both  the  absorption  of  oxygen  and  the 
excretion  of  carbon  dioxid.  This  view  has  been  suggested  as  a  means 
of  interpreting  the  results  of  the  experiments  of  more  recent  investi- 
gators, made  with  a  view  of  determining  the  tension  of  the  blood 
gases.  It  was  found  by  Bohr  that  the  tension  of  the  oxygen  in  arterial 
blood  was  often  as  high  as  101  to  144  mm.  Hg.,  and  in  many  instances 
higher  than  the  tension  of  the  oxygen  in  the  trachea,  while  the  carbon 
dioxid  tension  in  the  trachea  was  higher  than  in  the  blood.  Haldane 
and  Smith  by  a  different  method  found  an  oxygen  tension  in  the 
arterial  blood  of  200  mm.  Hg.  If  these  results  should  prove  to  be 
correct,  though  they  are  at  present  subject  to  considerable  criticism 
and  not  generally  accepted,  some  other  force  than  diffusion  would 
have  to  be  found  to  explain  the  facts.  It  would  then  remain  to  deter- 
mine in  how  far  the  alveolar  epithelium  could  be  regarded  as  an  active 
agent  in  both  absorption  and  excretion  in  opposition  to  pressure. 


RESPIRATION.  419 

THE  TOTAL,  RESPIRATORY  EXCHANGE. 

The  total  quantities  of  oxygen  absorbed  and  carbon  dioxid  dis- 
charged by  a  human  being  in  twenty-four  hours  are  measures  of  the 
intensity  of  the  respiratory  process,  and  an  indication  of  the  extent  and 
character  of  the  chemic  changes  attending  all  life  phenomena.  Their 
determination  and  their  relation  to  each  other  are  matters  of  interest 
and  importance.  The  quantities  which  have  been  obtained  by  differ- 
ent observers  are  the  outcome  of  calculations  based  on  certain  groups 
of  data  and  of  experiments  made  with  special  forms  of  apparatus. 

Thus  from  the  total  air  breathed  daily,  estimated  from  the  amounts 
obtained  during  a  longer  or  shorter  period,  of  experiments  with  spiro- 
metric  apparatus,  and  from  the  percentage  loss  of  oxygen  and  gain  of 
carbon  dioxid  shown  by  an  analysis  of  the  respired  air,  it  can  be  cal- 
culated at  least  approximately  what  the  total  amounts  of  oxygen  ab- 
sorbed and  carbon  dioxid  exhaled  must  be.  If  it  be  assumed  that 
the  minimum  daily  volume  of  air  breathed  is  10,800  liters  and  the 
maximum  volume  12,240  liters,  and  the  percentage  loss  of  oxygen  is 
4.78,  then  the  total  volume  of  oxygen  absorbed  is  516  liters  (735.17 
grams)  or  585  liters  (836.42  grams).  By  the  same  method  the  total 
carbon  dioxid  exhaled  daily  is  found  to  be  either  473  liters  (931.8  grams) 
or  526  liters  (1036  grams).  The  direct  experiments  which  have  been 
made  with  specially  devised  forms  of  apparatus,  both  on  human  beings 
and  animals,  have  yielded  similar  results.  With  those  forms  which 
are  adapted  for  both  human  beings  and  animals — Scharling's,  Petten- 
kofer  and  Voit's — it  is  only  possible,  however,  to  determine  the  amount 
of  oxygen  absorbed.  This  is  done  by  deducting  the  loss  in  weight 
by  the  man  or  animal  during  the  experiment  from  the  combined 
weights  of  the  carbon  dioxid  and  water  discharged.  The  difference 
represents  the  oxygen  absorbed. 

The  Pettenkofer-Voit  apparatus  (Fig.  199)  consists  essentially 
of  a  chamber  large  enough  to  admit  a  man  and  capable  of  being 
made  air-tight  with  the  exception  of  an  inlet  for  air  for  breathing 
purposes.  The  respired  air  is  drawn  through  a  tube  and  measured 
by  a  large  meter  turned  by  a  water  or  gas  motor.  By  means  of  a  side 
tube  a  fractional  quantity  of  the  main  column  of  air  is  diverted  to  an 
absorption  apparatus  by  a  small  pump.  This  air  first  passes  into 
a  vessel  containing  H,S04,  by  which  the  water  is  collected;  then 
into  long  tubes  containing  barium  hydroxid,  by  which  the  carbon 
dioxid  is  absorbed;  thence  into  a  small  meter,  by  which  its  amount  is 
registered.  From  the  amount  of  water  and  carbon  dioxid  thus  ob- 
tained the  amounts  of  both  in  the  total  air  breathed  are  calculated. 
The  water  and  carbon  dioxid  previously  present  in  the  air  are  simulta- 
neously determined  by  a  corresponding  absorption  apparatus  and  de- 
ducted from  the  amounts  obtained  from  the  respired  air.  As  the 
apparatus  is  traversed  constantly  by  a  column  of  air  of  normal  com- 
position and  the  waste  products  removed  as  rapidly  as  discharged, 


420 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  experiment   can  be  continued  for  periods  varying  from  six  to 

twenty-four  hours  without  detriment  to  the  subject  of  the  experiment. 

AYith    those    forms    adapted    only    for    animals — Regnault's    and 


1      C  -C    ^ 


r/l     _     *J      O 


Reiset's,  or  Jolyet  and  Regnard's — it  is  possible  to  determine  simul- 
taneously the  absorption  of  oxygen  and  the  discharge  of  carbon 
dioxid.  As  the  apparatus  employed  is  completely  closed,  the  carbon 
dioxid   must  be  removed  as  soon  as  discharged  and  the  oxygen  re- 


RESPIRATION. 


421 


newed  as  soon  as  absorbed.  The  former  is  accomplished  by  the  as- 
piratory  action  of  moving  bulbs  containing  an  alkali,  the  latter  by  a 
steadily  acting  pressure  on  a  reservoir  of  oxygen.  This  apparatus 
(Fig.  200)  consists  essentially  of  a  bell-jar  in  which  the  animal  is 
placed.  This  is  brought  into  connection  by  tubes,  on  the  one  hand, 
with  the  oxygen  reservoir,  and,  on  the  other  hand,  with  the  aspiratory 


Fig.  200. — Regnault's  and  Reiset's  Respiration  Apparatus.  A.  Bell-jar  for  the 
reception  of  the  animal,  surrounded  by  a  compartment,  B,  containing  water.  N,  N,  N. 
Reservoirs  of  oxygen  communicating,  on  the  one  hand,  with  the  animal  chamber,  and, 
on  the  other  hand,  with  pressure  bottles,  P,  by  which  the  oxygen  is  driven  into  the  animal 
chamber.  G,  G.  Aspiratory  bulbs  containing  sodium  hydroxid  in  solution  for  the  absorp- 
tion of  the  carbon  dioxid.  The  bulbs  are  given  an  alternate  up-and-down  movement 
by  a  falling  weight  or  electric  motor. 


bulbs,  kept  in  motion  by  some  form  of  motor.  The  construction  of 
each  of  these  forms  of  apparatus  is  so  complex,  the  conduct  of  an 
experiment  and  the  final  determination  of  the  results  so  complicated, 
that  a  detailed  description  would  be  out  of  place  in  a  work  of  this 
character.* 

Among  the  results  obtained  by  these  and  other  methods  a  few  are 
given  in  the  following  table: 

*  Both  forms  of  apparatus  are  in  use  in  the  Physiological  Laboratory  of  the  Jefferson 
Medical  College  and  are  fully  described  by  Prof.  H.  C.  Chapman  in  his  text-book  on 
Physiology,  to  which  the  reader  is  directed  for  further  information. 


422  TEXT-BOOK  OF  PHYSIOLOGY. 


Oxygen  Absorbed. 

Observer. 

Carbon  Dioxid  Discharged, 

746  grams. 

700       " 

Yierordt. 
Pettenkofer  and  Voit. 

876  grams. 
Soo        " 

663       " 

Speck. 

770        " 

The  amounts  of  oxygen  absorbed  in  Pettenkofer  and  Voit's  experi- 
ments varied  from  594  to  1072  grams;  of  carbon  dioxid  exhaled,  from 
686  to  1285  grams. 

In  all  these  results  it  is  evident  on  examination  that  the  volume 
of  oxygen  absorbed  is  always  greater  than  the  volume  of  carbon 
dioxid  exhaled,  or,  what  amounts  to  the  same  thing,  the  weight  of  the 
oxygen  absorbed  is  always  greater  than  the  weight  of  the  oxygen 
entering  into  the  formation  of  the  carbon  dioxid  exhaled.  The  reason 
for  this  difference  between  the  amounts  of  oxygen  in  the  inspired  air 
and  in  the  C02  exhaled  is  found  in  the  fact  that  on  a  mixed  diet — one 
containing  fat — a  portion  of  the  oxygen  is  utilized  in  the  oxidation 
of  the  hydrogen  of  the  fat  with  the  formation  of  water.  Under  such 
a  diet  the  respiratory  quotient  is  always  less  than  unity,  usually  0.907. 
On  a  purely  carbohydrate  diet — one  in  which  there  is  no  surplus 
hydrogen — all  the  oxygen  will  combine  with  carbon  and  be  returned 
as  carbon  dioxid,  and  hence  the  respiratory  quotient  will  be  unity. 
The  respiratory  quotient  therefore  indicates  the  extent  to  which  the 
oxygen  absorbed  is  utilized  in  oxidizing  carbon,  on  the  one  hand,  and 
hydrogen,  on  the  other. 

Since  the  total  oxygen  absorbed  and  carbon  dioxid  discharged 
will  vary  considerably  with  the  size  of  the  animal,  it  is  customary, 
for  purposes  of  comparison,  to  reduce  all  total  results  to  the  unit  of 
body- weight  (one  kilogram)  and  to  the  unit  of  time  (one  hour). 

Respiratory  Activity. — The  activity  or  the  intensity  of  the 
respiratory  process  may  be  measured  either  by  the  oxygen  absorbed 
or  the  carbon  dioxid  discharged.  But  as  the  carbon  dioxid  is  more 
easily  estimated  than  the  oxygen,  it  is  usually  taken  as  the  index  of 
the  activity,  though  there  are  reasons  for  believing  that  it  would  be 
more  accurately  indicated  or  represented  by  the  oxygen. 

Whatever  factor  may  be  accepted  as  the  measure,  it  is  certain  that 
the  respiratory  activity  varies  in  different  tissues  in  accordance  with 
their  functional  activities,  being  least  in  bones  and  greatest  in  muscles. 
This  is  shown  by  the  relative  amounts  of  oxygen  absorbed  and  carbon 
dioxid  discharged  by  equal  amounts  of  each  of  these  and  other  tissues 
in  twenty-four  hours,  as  shown  in  the  following  table: 

QUANTITY    OF  0   AND  CG2  ABSORBED  AND  EXHALED  DURING 
TWENTY-FOUR   HOIKS,   IN  CUBIC  CENTIMETERS. 

By   100  Grams  of:  Oxygen  Absorbed.  Carbonic  Acid  Exhaled. 

Muscle, 50.8  c.c.  56.8  c.c. 

Brain,     45.8    "  42.8    " 

Kidneys,     37.0    "  15.6     " 

Spleen,    27.3    "  15.4    " 

Ti    tit  les,    18.3    "  27.5    " 

Pounded  bones     17.2    "  8.1    " 


RESPIRATION.  423 

The  total  respiratory  change  therefore  of  the  body  as  a  whole  is  the 
resultant  of  the  respiratory  changes  of  its  individual  organs  and 
tissues,  and  is  conditioned  by  all  influences  which  retard  or  hasten 
their  activity.  Among  these  influences  the  more  important  are  the 
following: 

Muscle  Activity. — As  the  muscles  constitute  a  large  part  of  the 
body,  about  40  per  cent.,  and  as  muscle-tissue  absorbs  and  discharges 
relatively  large  quantities  of  oxygen  and  carbon  dioxid,  it  is  readily 
apparent  that  an  increase  in  their  activity  would  be  followed  or  attended 
by  an  increase  in  the  respiratory  exchange.  In  passing  from  a  con- 
dition of  body  repose  to  one  of  marked  activity  there  ought  to  be  an 
increase  in  the  amount  of  oxygen  absorbed  and  C02  discharged. 
Pettenkofer  and  Voit  found  that  a  man  in  repose  who  absorbed  daily 
807.8  grams  of  oxygen  and  discharged  930  grams  C02  absorbed  during 
work  1006  grams  of  oxygen  and  discharged  1137  grams  of  C02. 
Edward  Smith,  who  estimated  only  the  C02,  found  that  a  man  in 
repose  who  discharged  carbon  dioxid  at  the  rate  of  16 1.6  c.c.  per 
minute  increased  the  amount  while  walking  at  the  rate  of  two  and 
three  miles  an  hour  to  569  c.c.  and  851  c.c.  respectively.  Similar 
results  have  been  obtained  by  other  investigators. 

Digestive  Activity. — The  activity  of  the  alimentary  canal, 
involving  contraction  of  its  muscle  coat  through  its  entire  length 
as  well  as  secretion  of  its  related  glands  called  forth  by  the  ingestion 
of  food,  materially  influences  the  absorption  of  oxygen  and  discharge 
of  carbon  dioxid,  independent  of  the  increase  due  to  the  oxidation 
of  food  materials  after  absorption.  It  was  found  that  in  a  fasting 
man  a  dose  of  sodium  sulphate  increased  the  absorption  of  oxygen  as 
much  as  17  per  cent,  and  the  discharge  of  C02  24  per  cent.  (Lowy). 
It  is  difficult  to  determine  how  much  of  the  increase  after  a  meal  is 
therefore  due  to  food  oxidation  and  how  much  to  functional  activity 
of  the  canal  itself.  The  consumption  of  nitrogenized  meals,  however, 
has  a  greater  effect  than  non-nitrogenized  meals. 

Temperature. — A  rise  in  temperature  of  the  surrounding  air 
has  as  an  effect  a  diminution  in  the  amounts  of  oxygen  consumed  and 
carbon  dioxid  discharged.  A  fall  in  temperature  has  the  opposite 
effect.  Thus  a  cat  at  a  temperature  of  — 3.20  C.  consumed  during  a 
period  of  six  hours  21.39  grams  of  oxygen  and  discharged  22  grams 
of  carbon  dioxid,  while  at  a  temperature  of  29.6 °  C.  the  correspond- 
ing amounts  for  the  same  period  of  time  were  for  oxygen  13.9  grams 
and  for  carbon  dioxid  13.12  grams.  Lavoisier  and  Sequin,  having 
reference  only  to  the  oxygen,  found  that  a  man  at  a  temperature  of 
15  °  C.  consumed  38.31  grams  of  oxygen,  while  at  a  temperature  of 
32. 8°  C.  the  corresponding  amount  was  but  35  grams.  Similar 
results  have  been  obtained  by  other  observers  with  different  animals. 
The  explanation  of  these  facts  is  to  be  found  in  the  increased  activity 
of  all  physiologic  mechanisms  coincident  with  a  fall,  and  in  the  de- 
creased activity,  coincident  with  a  rise  in  temperature.     The  lower 


424  TEXT-BOOK  OF  PHYSIOLOGY. 

temperatures  act  as  a  stimulus  to  the  peripheral  terminations  of  the 
nerve  system,  bringing  about  reflexly  increased  activity  of  the  body 
at  large.  The  muscles  especially  are  not  only  reflexly  but  volitionally 
excited  to  greater  activity.  This  leads  naturally  to  an  increase  in 
the  consumption  of  oxygen  and  in  the  production  of  carbon  dioxid 
and  in  the  evolution  of  heat. 

In  cold-blooded  animals  the  respiratory  exchange  is  influenced  in 
a  manner  the  reverse  of  that  observed  in  warm-blooded  animals. 
With  a  rise  of  external  temperature  and  a  corresponding  rise  of  body- 
temperature  the  discharge  of  carbon  dioxid  steadily  increases.  Thus 
a  frog  in  an  atmosphere  at  o°  C.  with  a  body-temperature  of  i°  C. 
discharged  per  kilogram  per  hour  4.31  c.c.  of  carbon  dioxid;  in  an 
atmosphere  of  35  °  C.  with  a  body-temperature  of  340  C.  there  was 
discharged  325  c.c.  per  kilo  per  hour.  Intermediate  temperatures 
were  attended  by  corresponding  increases  in  the  amounts  of  C02 
discharged.  The  reason  for  this  difference  in  the  two  classes  of 
animals  is  probably  to  be  found  in  the  want,  in  the  cold-blooded 
animals,  of  a  self-adjusting  heat-regulating  mechanism. 

Age. — In  early  youth,  as  a  result  partly  of  the  more  pronounced 
activity  of  the  nutritive  energies  and  partly  of  a  cutaneous  surface 
relatively  greater,  as.  compared  with  the  mass  of  the  body,  than  in 
adult  life,  the  absorption  of  oxygen  and  the  discharge  of  carbon  dioxid 
are  greater  both  absolutely  and  relatively.  Thus,  in  a  boy  of  nine 
and  a  half  years  with  a  weight  of  22  kilograms  it  was  found  that  in 
twenty-four  hours  there  was  a  discharge  of  carbon  dioxid  amounting 
to  488  grams,  or  0.92  gram  per  kilo  per  hour,  and  in  man  with  a 
weight  of  65.5  kilograms  there  was  a  discharge  of  804.72  grams,  or  0.51 
gram  per  kilo  per  hour. 

MODIFICATIONS   OF   THE   RESPIRATORY   RHYTHM. 

The  character  of  the  respiratory  movements  is  materially  modified 
by  a  change  in  the  quantitative  and  qualitative  composition  of  the 
air  and  blood  as  well  as  by  changes  of  a  pathologic  nature  of  the  res- 
piratory apparatus  itself. 

Eupnea. — So  long  as  the  air  retains  its  normal  composition  and 
the  respiratory  mechanism  its  structural  integrity,  so  long  do  the 
respiratory  movements  exhibit  a  normal  rhythm  and  frequency. 
To  the  condition  of  easy  tranquil  breathing  the  term  eupnea  is  given. 
In  this  condition  the  percentages  of  oxygen  and  carbon  dioxid  in  the 
blood  are  such  as  to  favor  at  least  the  rhythmic  discharge  of  nerve 
impulses  to  the  respiratory  muscles,  of  sufficient  energy  and  frequency 
for  the  maintenance  of  normal  respiration. 

Hyperpnea. — The  normal  rate  of  the  respiratory  movements  is 
increased  by  a  rise  in  body-temperature,  by  active  exercise,  and  by 
emotional  states.  Whatever  the  cause,  the  increase  in  rate  and  prob- 
ably in  depth  is  termed  hyperpnea. 


RESPIRATION.  425 

Febrile  states  characterized  by  a  rise  in  the  temperature  of  the 
blood  increase  considerably  the  respiratory  activity.  This  is  due  in 
all  probability  to  a  warming  of  the  respiratory  center,  in  consequence 
of  which  its  excitability  is  heightened;  for  surrounding  the  carotid 
arteries  with  warm  tubes  and  heating  the  blood  on  its  way  to  the 
medulla  has  the  same  effect.  It  is  also  possible,  however,  that  the 
high  temperature  of  febrile  conditions  may  interfere  with  the  absorb- 
ing power  of  hemoglobin,  and  thus  by  diminishing  the  quantity  of 
oxygen  absorbed  lead  to  more  frequent  respirations.  To  the  hy- 
perpnea  induced  by  heat  the  term  thermo- polypnea  is  frequently 
given. 

Muscle  activity,  especially  if  it  is  violent  and  indulged  in  by  those 
unaccustomed  to  exercise,  is  generally  followed  by  increased  rate 
and  depth  of  breathing,  and  not  infrequently  it  is  attended  with 
such  extreme  difficulty  that  the  condition  approximates  that  of  dyspnea. 
This  condition  is  attributed  to  the  production  and  discharge  into  the 
blood  of  unknown  waste  products  which  act  as  irritants  to  the  respira- 
tory center  and  thus  increase  its  activity.  As  they  apparently  can  not 
be  isolated  and  their  chemic  nature  determined,  it  is  presumable  that 
they  are  speedily  oxidized  or  reduced  in  the  blood.  Experiment 
has  shown  that  the  increase  of  carbon  dioxid  does  not  account  for  the 
increased  rate  of  breathing.  Emotional  states  temporarily  increase 
respiratory  activity.  With  their  disappearance  the  normal  condition 
returns. 

Apnea. — If  the  respiratory  movements  are  voluntarily  increased 
in  frequency  and  depth  for  a  short  time  it  will  be  found  on  cessation 
that  for  a  variable  length  of  time  the  respiratory  mechanism  remains 
in  a  condition  of  complete  rest  or  inaction.  To  this  complete  cessation 
of  activity  the  term  apnea  is  given.  The  same  phenomenon  is  wit 
nessed  in  animals  when  the  lungs  are  rapidly  inflated  with  air  by 
means  of  bellows.  At  one  time  this  was  attributed  to  an  excess  of 
oxygen  in  the  blood  (the  result  of  the  forced  ventilation  of  the  lungs), 
complete  saturation  of  the  plasma  and  hemoglobin,  in  consequence  of 
which  the  respiratory  center  remained  inactive.  This  has  been  dis- 
proved, however,  by  modern  chemic  analyses  of  the  blood.  The 
condition  is  now  attributed: 

1.  To  increased  ventilation  of  the  lungs  and  an  increased  percentage 

of  oxygen  in  the  alveoli,  as  a  result  of  which  the  normal  percentage 
of  oxygen  in  the  blood  can  be  maintained  for  a  longer  period  than 
usual. 

2.  To  a  stimulation  of   the  peripheral  terminations  of  the  pneumo- 

gastric  nerve  whereby  the  discharge  of  nerve  impulses  from  the 
respiratory  center  is  temporarily  inhibited.  Division  of  the 
pneumogastric  nerve  prevents  the  development  of  the  apneic 
condition. 
Dyspnea. — Excessive  and  laborious  respiratory  movements  con- 
stitute a  condition  known  as  dyspnea.     Movements  of  this  character 


426  TEXT-BOOK  OF  PHYSIOLOGY. 

indicate  that  the  blood  is  deficient  in  oxygen  or  overcharged  with 
carbon  dioxid.  In  either  case  the  excitability  of  the  respiratory  center 
is  abnormally  heightened.  These  conditions  of  the  blood  may  be 
caused:  (i)  By  all  those  pathologic  conditions  of  the  respiratory 
apparatus  which  limit  the  free  entrance  of  oxygen  into  and  the  free 
exit  of  carbon  dioxid  from  the  blood;  (2)  by  those  alterations  in  the 
composition  of  the  air  and  subsequently  in  the  blood  which  arise  when 
the  individual  is  confined  in  a  space  of  moderate  size  with  imperfect 
ventilation.  The  want  of  oxygen  in  the  blood  gives  rise  to  more 
forcible  inspirations;  the  presence  of  C02  in  excess,  to  more  forcible 
expirations — showing  that  the  former  condition  affects  the  inspiratory 
portion  of  the  respiratory  center,  the  latter  condition  the  expiratory 
portion.  A  deficiency  in  the  amount  or  quality  of  the  hemoglobin 
is  usually  attended  with  dyspnea. 

Asphyxia. — If  the  state  of  the  blood  observed  in  dyspnea  be  ex- 
aggerated— that  is,  if  the  decrease  in  the  percentage  of  oxygen  and 
the  increase  in  the  percentage  of  carbon  dioxid  become  more  marked 
— the  respiratory  movements  become  more  laborious.  A  continuance 
of  this  changed  composition  of  the  blood  eventuates  in  death.  Before 
this  occurs  the  individual  exhibits  a  succession  of  phenomena,  to 
the  totality  of  which  the  term  asphyxia  is  given. 

Asphyxia  may  be  caused:  (1)  By  a  sudden  interference  with  the 
entrance  of  oxygen  into  and  the  exit  of  carbon  dioxid  from  the  blood, 
as  in  drowning,  occlusion  of  the  trachea  from  any  cause,  double 
pneumothorax,  etc.  (2)  By  confinement  in  a  small  space  the  air 
of  which  speedily  undergoes  a  loss  of  oxygen  and  an  accumulation  of 
carbon  dioxid.  In  the  first  instance  death  may  occur  in  a  few  minutes; 
in  the  second  instance  it  may  be  postponed  several  hours  or  more, 
the  time  varying  with  the  size  of  the  space. 

The  succession  of  phenomena  presented  by  an  individual  in  the 
asphyxiated  condition  is  as  follows:  Increased  rate  and  depth  of  the 
respiratory  movements,  passing  rapidly  from  hyperpnea  to  dyspnea, 
with  an  active  contraction  of  all  the  muscles  concerned  in  respira- 
tion, ordinary  and  extraordinary;  a  blue  cyanosed  condition  of  the 
face  from  the  rapid  accumulation  of  carbon  dioxid  and  disappearance 
of  the  oxygen  of  the  blood;  a  diminution  in  the  depth  of  inspiration 
and  an  increase  in  the  force  and  extent  of  expiration,  followed  by 
general  convulsions;  collapse,  characterized  by  unconsciousness,  loss 
of  the  reflexes,  relaxation  of  the  muscles,  a  weak  action  of  the  heart, 
a  disappearance  of  the  pulse  and  death.  As  shown  by  observation 
of  the  circulatory  apparatus  in  artificially  induced  asphyxia,  there  is 
primarily  an  increase  in  the  activity  of  the  heart,  soon  followed  by 
retardation;  a  rise  of  blood-pressure  in  the  early  stages  and  a  fall  to 
zero  after  collapse  has  set  in.  The  retardation  and  final  cessation  of 
the  heart,  as  well  as  the  rise  of  the  blood-pressure,  are  to  be  attributed 
to  stimulation  of  the  cardio-inhibitory  and  vaso-motor  centers  from 
the  accumulation  of  the  carbon  dioxid.     With  the  exhaustion  of  the 


RESPIRATION. 


427 


nerve-centers,  there  is  a  general  relaxation  of  the  muscles  and  a  fall 
of  the  pressure. 

The  Cheyne-Stokes  Respiration. — A  modification  of  the  respir- 
atory movements  characterized  by  periods  of  rest  alternating  with 
periods  of  activity -was  described  in  1818  and  in  1854  by  the  two 
writers  whose  name  it  bears.  The  periods  of  rest  vary  in  duration 
from  twenty  to  thirty  seconds;  the  periods  of  activity  from  thirty  to 
sixty  seconds  and  may  include  from  twenty  to  thirty  respiratory 
movements. 

Each  period  of  rest  of  the  respiratory  mechanism  is  closed  by  the 
appearance  of  a  slight  shallow  respiratory  movement,  which  is  imme- 
diately followed  by  a  second,  slightly  deeper,  and  this  in  turn  by  a 


Fig.  201. — Tracing  Showing  the  Cheyne-Stokes  Form  of  Respiration. — (Hill.) 

third,  a  fourth,  a  fifth,  and  so  on,  each  becoming  deeper  than  the 
preceding  until  a  certain  maximum  is  reached,  after  which,  each 
succeeding  movement  gradually  diminishes  in  depth  until  finally  the 
movement  becomes  imperceptible  and  a  new  period  of  rest  super- 
venes. A  graphic  representation  of  the  Cheyne-Stokes  type  of  respira- 
tion is  shown  in  Fig.  201.  This  type  of  respiration  is  frequently  an 
accompaniment  of  certain  pathologic  conditions,  e.  g.,  uremic  states, 
cerebral  hemorrhage,  heart  diseases,  arteriosclerosis,  etc.,  though  no 
satisfactory  explanation  of  it  has  yet  been  presented.  A  similar 
though  far  less  marked  periodicity  in  the  respiratory  movements  is 
frequently  observed  during  normal  sleep,  especially  in  children.  A 
periodicity  can  also  be  developed  by  dividing  transversely  the  medulla 
oblongata  just  above  the  calamus  scriptorius  which  either  injures  the 
respiratory  center  or  removes  from  it  some  cerebral  influence. 


THE   NERVE   MECHANISM   OF   RESPIRATION. 

The  nerve  mechanism  by  which  the  respiratory  muscles  are  ex- 
cited to  action  is  extremely  complex  and  involves  the  action  of  both 
afferent  and  efferent  nerves  and  their  related  nerve-centers  in  the  cen- 
tral nerve  svstem.     For  the  free  introduction  of  air  into  the  lunsrs  it  is 


428  TEXT-BOOK  OF  PHYSIOLOGY. 

essential  that  the  nasal  and  laryngeal  passages  and  the  cavity  of  the 
thorax  be  simultaneously  enlarged.  The  muscles  by  which  these 
results  are  accomplished  have  already  been  mentioned  and  described. 
Their  simultaneous  and  coordinate  contraction  implies  the  coordinate 
activity  of  motor  nerves  and  their  centers;  thus,  the  nasal  and  laryn- 
geal muscles  (the  dilatator  naris  and  the  posterior  crico-arytenoid) 
involve  the  activity  of  the  facial  and  inferior  laryngeal  nerves  re- 
spectively, the  centers  of  origin  of  which  lie  in  the  gray  matter  beneath 
the  floor  of  the  fourth  ventricle;  the  diaphragm  and  intercostal  muscles 
involve  respectively  the  activity  of  the  phrenic  and  intercostal  nerves, 
the  centers  of  origin  of  which  lie  in  the  anterior  horn  of  the  gray  matter 
of  the  spinal  cord  at  a  level,  for  the  phrenic,  of  the  fourth,  fifth,  and 
sixth  cervical  nerves,  and  for  the  intercostals  at  the  level  of  the  upper 
thoracic  nerves.  Division  of  any  one  of  these  nerves  is  followed  by 
paralysis  of  its  related  muscle. 

Inspiratory  Center. — The  coordinate  contraction  of  the  inspira- 
tory muscles  implies  a  practically  simultaneous  discharge  of  nerve 
impulses  from  each  of  the  foregoing  nerve-centers,  accurately  graduated 
in  intensity  in  accordance  with  inspiratory  needs.  This  has  been 
supposed  to  necessitate  the  existence  in  the  central  nerve  system  of  a 
single  group  of  nerve-cells  from  which  nerve  impulses  are  rhythmically 
discharged  and  conducted  to  the  previously  mentioned  nerve-centers 
in  the  medulla  oblongata  and  spinal  cord,  by  which  they  are  in  turn 
excited  to  activity.  To  this  group  of  cells  the  term  "inspiratory 
center"  has  been  given. 

For  the  free  exit  of  air  from  the  lungs  it  is  not  only  essential  that 
the  air-passages  be  open,  but  that  the  air  in  the  lungs  be  compressed 
until  its  pressure  rises  above  that  of  the  atmosphere.  This  is  accom- 
plished by  the  recoil  of  the  elastic  tissue  of  the  lungs  and  thorax,  the 
return  of  the  displaced  abdominal  organs  aided  by  atmospheric  pres- 
sure, and  the  contraction  of  the  expiratory  muscles.  In  how  far 
muscle  action  is  necessary  for  expiratory  purposes  will  depend  on  the 
resistance  offered  to  the  outflow  of  air  and  on  the  degree  of  efficiency 
of  the  elastic  forces. 

Expiratory  Center. — The  simultaneous  and  coordinate  activity 
of  the  expiratory  muscles  in  impeded  expirations  also  involves  the 
action  of  motor  nerves  and  nerve-centers.  The  simultaneous  and 
coordinate  discharge  of  nerve  impulses,  also  graduated  in  intensity 
for  expiratory  needs,  apparently  implies  the  existence  in  the  central 
nerve  system  of  a  single  center  from  which  nerve  impulses  are  rhyth- 
mically discharged  which  excite  and  coordinate  the  lower  nerve-centers. 
To  this  group  of  cells  the  term  "expiratory  center"  has  been  given. 
The  two  centers  taken  together  constitute  the  so-called  "respiratory 
center." 

The  anatomic  existence,  however,  of  a  definite  group  of  cells  which 
initiates  the  respiratory  movements  has  not  as  yet  been  demonstrated. 
Nevertheless  there  is  in  the  dorsal  portion  of  the  medulla  oblongata, 


RESPIRATION.  429 

at  the  level  of  the  sensory  end-nucleus  of  the  vagus  nerve,  a  region  the 
sudden  destruction  of  which  on  one  side  is  followed  by  a  cessation  of 
respiratory  movements  on  the  corresponding  side,  though  they  con- 
tinue on  the  opposite  side,  a  fact  which  indicates  that  the  area,  though 
acting  as  a  unit,  is  bilateral.  The  bilateral  character  of  the  area  is 
also  shown  by  the  continuance  of  the  respiratory  movements  on  both 
sides  after  longitudinal  division  of  the  medulla.  Destruction  of  the 
entire  region  is  followed  by  a  complete  cessation  of  respiratory  activity 
and  death  of  the  animal.  For  this  reason  the  term  "nceud  vital" 
was  applied  to  it.  In  this  area  the  respiratory  center  was  located. 
It  has,  however,  been  shown  by  Gad  that  if  this  area  be  gradually 
destroyed  by  cauterization  the  respiratory  movements  do  not  cease, 
but  continue  until  the  cauterization  has  reached  a  point  far  forward 
in  the  formatio  reticularis,  in  which  the  respiratory  center  was  assumed 
to  lie. 

Though  its  existence  has  not  been  anatomically  determined  beyond 
question,  it  is  permissible  to  speak  of  the  central  mechanism  as  a 
"center"  located  in  the  medulla  oblongata. 

The  activity  of  the  inspiratory  center  has  long  been  described  as 
automatic  or  autochthonic  (Gad)  in  character,  expressive  of  the  idea 
that  the  rhythmic  discharge  of  nerve  impulses  is  conditioned  by  the 
composition  of  the  blood  or  lymph  by  which  it  is  surrounded,  though 
susceptible  to  the  action  of  nerve  impulses  reflected  to  it  through 
afferent  nerves.  Thus  so  long  as  the  blood  retains  its  normal  com- 
position the  inspiratory  movements  are  normal.  If,  however,  the  blood 
becomes  richer  in  oxygen  and  relatively  poorer  in  carbon  dioxid,  the  rate 
of  discharge  of  nerve  impulses  and  hence  the  inspiratory  movements,  di- 
minish until  the  condition  of  apnea  results.  If,  on  the  contrary,  the 
blood  becomes  poorer  in  oxygen  and  richer  in  carbon  dioxid,  the  reverse 
condition  obtains:  viz.,  an  increased  rate  of  discharge  of  nerve  impulses, 
increased  frequency  of  inspiration,  hyperpnea,  and  dyspnea.  This 
view  of  the  automaticity  of  the  inspiratory  center  is  supported  by  the 
fact  that  the  center  continues  more  or  less  active  after  division  of  the 
medulla  oblongata  just  posterior  to  the  corpora  quadrigemina,  of  the 
spinal  cord  at  the  level  of  the  seventh  cervical  nerve,  of  the  vagus 
nerves  and  the  posterior  roots  of  the  cervical  nerves,  channels  through 
which  the  majority  of  nerve  impulses  reach  the  center.  Whether  the 
center  would  continue  active  after  division  of  all  remaining  afferent 
nerves,  e.  g.,  trigeminal  and  glossopharyngeal,  it  is  impossible  to  state, 
since  from  the  nature  of  the  case  such  an  experiment  would  be  most 
difficult  to  perform. 

The  first  inspiration  after  birth  is  supposed  to  be  due  to  the  direct 
stimulation  of  the  respiratory  center  by  the  increase  in  the  carbon 
dioxid  present  in  the  blood,  though  it  may  be  aided  by  the  cooling 
of  the  skin  due  to  vaporization  of  the  amniotic  fluid. 

Reflex  Stimulation  of  the  Inspiratory  Center. — Whether 
the  inspiratory  center  is  automatic  in  character  or  not,  it  may  be 


430  TEXT-BOOK  OF  PHYSIOLOGY. 

influenced  directly  by  nerve  impulses  descending  from  the  brain  in 
consequence  of  volitional  acts  or  emotional  states,  and  indirectly 
by  nerve  impulses  reflected  to  it  from  the  general  periphery  through 
various  afferent  nerves,  in  consequence  of  agencies  acting  on  their 
peripheral  termination:  e.  g.,  cold  applied  to  the  skin,  irritaing  gases 
to  the  nasal  and  bronchial  mucous  membrane,  distention  and  collapse 
of  the  pulmonary  alveoli. 

Of  all  afferent  nerves,  the  vagus  appears  to  be  the  most  influential  in 
maintaining  the  rhythmic  discharge  of  nerve  impulses  from  the  inspira- 
tory center.  (Fig.  202).  Thus,  if  while  the  animal  is  breathing  regularly 
and  quietly  both  vagi  are  cut,  the  respiratory  movements  become  much 
slower,  falling  perhaps  to  one-third  their  original  number  per  minute. 
If  the  central  end  of  the  divided  vagus  be  stimulated  with  weak  faradic 
currents,  the  respiratory  movements  are  increased  in  frequency  and 
their  depth  diminished  until  the  normal  rate  is  restored.  With  the 
cessation  of  the  stimulation  the  former  condition  at  once  returns. 
This  would  indicate  that  in  the  physiologic  state  afferent  impulses 
are  ascending  the  vagus  fibers  which  influence  the  rate  of  discharge 
from  the  inspiratory  center.  If,  however,  the  stimulation  is  increased 
in  strength,  the  inspiratory  movement  gradually  so  exceeds  the  expi- 
ratory that  the  muscles  pass  into  the  tetanic  state  and  the  chest-walls 
come  to  rest  in  the  condition  of  forced  inspiration.  The  vagus  appa- 
rently contains  fibers  which  augment  the  inspiratory  movement. 
If,  on  the  other  hand,  the  central  end  of  the  divided  superior  laryngeal 
nerve  be  stimulated  with  faradic  currents,  the  opposite  effect  is  pro- 
duced: viz.,  an  excess  of  the  expiratory  over  the  inspiratory  move- 
ment until  the  chest-walls  come  to  rest  in  the  condition  of  passive 
expiration.  The  superior  laryngeal  nerve  apparently  contains  fibers 
which  inhibit  the  inspiratory  movement. 

The  same  result,  an  expiratory  standstill,  not  infrequently  follows 
strong  stimulation  of  the  divided  vagus,  and  always  after  the  admin- 
istration of  large  doses  of  chloral. 

The  vagus  apparently  contains  two  classes  of  nerve-fibers,  one  of 
which  when  stimulated  increases  the  extent  of  the  inspiratory  move- 
ment until  the  thorax  comes  to  a  standstill  in  the  state  of  forced  in- 
spiration; the  other  of  which  when  stimulated  increases  the  extent  of 
the  expiratory  movement  at  the  expense  of  the  inspiratory  until  the 
thorax  comes  to  a  standstill  in  a  state  of  deep  expiration. 

The  stimulus  adequate  to  the  excitation  of  the  nerve-fibers  in  the 
physiologic  condition  was  formerly  believed  to  be  the  chemic  action 
of  carbon  dioxid;  it  is  now  believed  to  be  a  mechanic  action,  the  result 
of  the  alternate  distention  and  collapse  of  the  walls  of  the  pulmonary 
alveoli.  Thus,  it  has  been  shown  by  Head  that  if  the  lungs  are  actively 
inflated  (positive  ventilation)  there  will  be  produced  an  inhibition 
of  the  inspiratory  and  an  augmentation  of  the  expiratory  movement 
until  the  inspiratory,  muscles  are  completely  relaxed  as  indicated  by 
the  relaxation  of  the  diaphragm,  the  movements  of  which  are  simulta- 


RESPIRATION. 


43i 


neously  recorded  (Fig.  203),  a  result  similar  in  all  respects  to  that 
produced  by  stimulation  of  the  superior  laryngeal  nerve.  On  the 
other  hand,  if  the  lungs  are  collapsed  by  the  artificial  withdrawal 
of  air  (negative  ventilation)  there  will  be  produced  an  augmentation 


med.  ob. 


Fig.  202. — Diagram  Showing  the  Relation  of  the  Pulmonary  Fibers  of  the 
Vagus  to  the  Inspiratory  Center  and  the  Connections  of  thf  Latter  with  the 
Phrenic  and  Intercostal  Nerve  Centers  and  their  Related  Muscles.  {G.  Bach- 
man.)  med.  ob.  Medulla  oblongata,  sp.c.  Spinal  cord,  p.v.r.  Pulmonary  vagus  nerve,  ex  - 
citator  and  inhibitor,  in  sp.c.  Inspiratory  center,  phr.c.  Phrenic  nerve  centers,  phr.n. 
Phrenic  nerve  int.n.c.  Intercostal  nerve  centers,  int.cn.  Intercostal  nerves,  ext.int.c.m. 
External  intercostal  muscles. 


of  the  inspiratory  and  an  inhibition  of  the  expiratory  movements  until 
the  inspiratory  muscles  are  in  a  condition  of  tetanic  contraction  as  in- 
dicated by  the  contraction  of  the  diaphragm  (Fig.  204)  and  by  the 
state  of  the  thorax  which  is  that  characteristic  of  extreme  inspiration. 


43  2 


TEXT-BOOK  OF  PHYSIOLOGY. 


Positive 
ventilation. 


a  result  similar  in  all  respects  to  that  produced  by  moderate  stimu- 
lation of  the  central  end  of  the  divided  vagus. 

A  satisfactory  explanation  of  the  action  of  the  respiratory  mechanism 
is  very  difficult  to  present.  Theories  vary  in  accordance  with  the 
estimate  of  an  investigator  as  to  the  degree  of  automaticity  of  the 
inspiratory  center,  of  the  effects  of  vagus  stimulation  and  as  to  the 

extent  to  which  the  ex- 
piratory center  is  in- 
volved with  the  activity 
of  the  inspiratory  center 
either  simultaneously  or 
successively. 

If  it  is  assumed  that 
the  inspiratory  center  is 
automatic  and  in  a  state 
of  continuous  excitation 
the  result  of  the  action 
of  carbon  dioxid  in  the 
blood  circulating  around 
it,  then  it  is  only  necessary  to  assume  the  existence,  in  the  trunk  of 
the  vagus  of  but  one  set  of  nerve-fibers,  viz.,  inhibitor  fibers,  the 
central  terminations  of  which  arborize  around  the  inspiratory  center 
and  the  function  of  which  is  to  check  or  inhibit  the  action  of  the 
inspiratory  center  and  thus  permit  of  an  expiratory  movement.  The 
inhibitor  fibers  are  supposed  to  be  stimulated  peripherally  by  the 
expansion  of  the  lungs.  With  the  recoil  of  the  lungs  the  inhibitor 
effect    gradually    dies    away,    while    the    inherent    excitation    of  ,the 


Fig.  203. — Positive  Ventilation  (Head).  Under 
the  influence  of  positive  ventilation,  the  inspiratory 
contractions  of  the  diaphragm  become  less  and  less 
till  they  disappear  completely. 


Seconds. 


Fig.  204. — Negative  Ventilation  (Head).  At  a  negative  ventilation  was  com- 
menced. The  expiratory  relaxation  of  the  diaphragm  is  seen  to  become  more  and  more 
incomplete,  until  it  finally  enters  into  continued  contraction. 


inspiratory  center  again  returns,  to  be  followed  by  another  discharge 
of  nerve  impulses  and  a  new  inspiratory  movement,  which  will  in  turn 
be  again  inhibited  as  the  inhibitor  fibers  are  stimulated  by  the  ex- 
panding   lung.     This    explanation  is  in   accordance  with  the  results 


RESPIRATION.  433 

which  follow  stimulation  of  the  superior  laryngeal  nerve  or  the  trunk 
of  the  vagus  with  strong  induced  electric  currents. 

If  it  is  assumed,  on  the  contrary,  that  the  inspiratory  center  is  not 
in  a  state  of  constant  excitation  leading  to  a  continuous  discharge 
of  nerve  impulses,  but  requires  the  arrival  of  a  stimulus  to  call  forth 
its  normal  activity,  then  this  theory  does  not  suffice,  inasmuch  as  it 
leaves  out  of  consideration  the  presence  of  nerve-fibers  in  the  vagus 
which  increase  or  augment  the  activity  of  the  inspiratory  center;  and 
that  such  fibers  are  present  is  apparently  indicated  by  the  effects  of 
stimulation  of  the  central  end  of  the  vagus  nerve  with  weak  and 
moderately  strong  induced  electric  currents  and  from  the  experiments 
of  Hering  and  Breuer,  and  later  of  Head.  These  observers  assume, 
therefore,  that  in  addition  to  the  inhibitor  fibers  there  are  also  present 
in  the  vagus,  excitator  fibers,  the  central  terminations  of  which  are  in 
relation  with  the  inspiratory  center  also  (Fig.  202; ;  and  just  as  the  inhib- 
itor fibers  are  stimulated  by  the  expansion  of  the  lungs  so  the  excitator 
fibers  are  stimulated  in  turn  by  the  recoil  of  the  lungs.  The  nerve  im- 
pulses thus  developed  ascend  to  the  inspiratory  center,  excite  it,  and  call 
forth  a  new  inspiration  sooner  than  it  would  otherwise  take  place.  Ac- 
cording to  this  view  the  respiratory  mechanism  is  self-regulative  and 
maintained  by  the  alternate  expansion  and  recoil  of  the  lungs. 

Many  experimenters,  however,  find  difficulty  in  accepting  the 
view  that  the  recoil  of  the  lungs  should  stimulate  nerve  endings  and 
hence  this  theory  has  not  received  general  acceptance. 

Another  explanation  which  is  satisfactory  in  many  respects  has  been 
presented  by  Meltzer.  This  investigator  asserts  also  the  existence  in 
the  trunk  of  the  vagus  the  two  classes  of  nerve-fibers,  the  inhibitor 
and  the  excitator;  but  that  for  some  reason  they  do  not  respond  to 
stimulation  at  the  same  time  as  shown  by  the  effects  which  follow;  the 
inhibitor  fibers  respond  first  and  the  excitator  fibers  somewhat  later. 
Therefore  when  they  are  stimulated  simultaneously  the  primary  effect 
is  an  inhibition  of  the  inspiratory  center  followed  by  an  expiratory 
movement.  The  secondary  effect  is  a  stimulation  of  the  inspiratory 
center  followed  by  a  new  inspiratory  movement.  In  this  view  ex- 
pansion of  the  lungs  stimulates  both  the  inhibitor  and  the  excitator 
fibers,  but  during  the  expansion  and  for  a  short  time  after,  the  effect 
of  the  inhibitor  stimulation,  viz.,  cessation  of  inspiration  and  the 
advent  of  expiration,  alone  manifests  itself.  With  the  cessation  of 
expiration,  the  inhibitor  stimulation  dies  away  and  the  late  effect  or 
the  long  after-effect  of  the  excitator  stimulation,  viz.,  a  new  inspiration, 
manifests  itself.  This  author  assumes  the  surface  of  the  lung  to  be 
the  peripheral  organ  of  the  respiratory,  reflexes. 

When  it  is  assumed  that  both  inspiratory  and  expiratory  centers 
cooperate  in  a  respiratory  movement,  as  they  do  in  labored  respiration 
either  simultaneously  or  successively,  the  difficulties  of  the  problem  are 
manifestly  much  greater.  In  this  case  it  may  be  supposed  that  afferent 
impulses,  developed  during  the  expansion  of  the  lung,  inhibit  the 
2S 


434  TEXT-BOOK  OF  PHYSIOLOGY. 

inspiratory  while  augmenting  the  expiratory  center,  and  that  impulses 
developed  during  the  recoil  of  the  lungs  inhibit  the  expiratory  while 
stimulating  the  inspiratory  center. 

THE  EFFECT  OF  THE  RESPIRATORY  MOVEMENTS  ON  THE  FLOW 

OF  BLOOD  THROUGH  THE  INTRA-THORACIC  VESSELS  AND 

ON  THE  ARTERIAL  PRESSURE. 

i.  On  the  Intra-thoracic  Vessels. — The  forces  which  cause  the 
air  to  flow  into  and  out  of  the  lungs  will  at  the  same  time  and  in  the 
same  way  cause  the  blood  of  the  systemic  vessels  to  flow  into,  through, 
and  out  of  the  intra-thoracic  vessels.  From  the  tendency  of  the  pul- 
monary elastic  tissue  to  recoil,  the  blood-vessels  in  the  thorax  at  the 
end  of  an  expiration  sustain  a  positive  pressure,  about  six  millimeters 
of  mercury  less,  than  that  in  the  lungs,  or,  in  other  words,  a  pressure 
negative  to  that  of  the  atmosphere  by  six  millimeters.  As  a  result 
the  blood  in  the  systemic  vessels  standing  under  atmospheric  pressure 
will  flow  steadily  toward  the  intra-thoracic  veins,  the  vense  cavse, 
and  the  right  side  of  the  heart;  i.  e.,  from  a  point  of  high  to  a  point  of 
low  pressure.  Since  during  inspiration,  with  the  increasing  tendency 
to  pulmonary  recoil,  the  positive  pressure  on  the  veins  and  heart  may 
diminish  by  thirty  millimeters  of  mercury,  the  blood  will  flow  in  in- 
creased volume  from  the  systemic  to  the  intra-thoracic  vessels.  The 
right  heart,  being  more  generally  filled  with  blood,  will  discharge  a 
larger  volume  with  each  contraction  into  the  pulmonary  artery. 

Coincident  with  these  effects  a  similar  effect  is  produced  in  the 
arterioles  and  capillaries  of  the  pulmonary  alveoli.  Situated  between 
the  two  elastic  layers  of  the  alveolar  wall,  embedded  in  a  meshwork 
of  connective  tissue,  the  pressure  to  which  they  are  subjected  at  the 
end  of  an  expiration  will  also  be  a  few  millimeters  less  than  that  of 
the  intra-pulmonary  air;  and  at  the  end  of  an  inspiration  it  will  be 
considerably  less.  With-  the  inspiration  there  will  occur  a  dilatation 
of  the  vessels,  a  larger  flow  of  blood  through  them  and  into  the  pul- 
monary veins.  The  left  heart,  being  in  consequence  more  generously 
filled  with  blood,  will  discharge  a  larger  volume  into  the  aorta  at  each 
contraction.  During  expiration  the  flow  of  blood  through  the  intra- 
thoracic vessels  will  be  diminished  for  the  reverse  reasons. 

2.  On  the  Arterial  Pressure. — An  examination  of  a  tracing  of 
the  arterial  pressure  will  show  that  it  is  characterized  by  small  un- 
dulations due  to  the  cardiac  beat  and  large  undulations  due  to  the 
respiratory  movements,  inspiration  being  accompanied  by  a  rise, 
expiration  by  a  fall  of  pressure.  These  results  are  readily  accounted 
for  by  the  difference  in  the  volume  of  blood  discharged  by  the  left 
heart  into  the  aorta  during  the  time  of  the  two  movements.  If  a 
tracing  of  the  respiratory  movements  and  of  the  blood-pressure  be 
taken  simultaneously,  it  will  be  found  that  the  rise  of  pressure  does  not 
exactly   correspond    with    inspiration,   nor  the   fall    of   pressure   with 


RESPIRATION.  435 

expiration;  for  a  certain  period  after  the  beginning  of  an  inspiration 
the  pressure  continues  to  fall,  and  for  a  certain  period  after  the  begin- 
ning of  an  expiration  the  pressure  continues  to  rise.  During  the 
remainder  of  the  period,  however,  inspiration  is  attended  by  a  rise, 
expiration  by  a  fall  of  pressure.  The  explanation  of  these  results 
lies  in  the  fact  that  at  the  beginning  of  the  inspiration,  when  the 
vessels  dilate,  the  blood-flow  momentarily  slows;  the  left  heart  con- 
tinuing to  discharge  small  volumes  into  the  aorta,  the  pressure  con- 
tinues to  fall.  So  soon  as  the  left  heart  begins  to  be  better  filled,  the 
pressure  at  once  begins  to  rise.  At  the  end  of  an  expiration  the  flow 
of  blood  into  the  left  heart  continues  and  the  pressure  rises,  but  with 
the  return  of  the  intra-thoracic  pressure  the  vessels  diminish  in  caliber, 
the  volume  of  blood  transmitted  by  them  becomes  less,  the  output 
of  the  left  heart  declines,  and  the  pressure  falls. 

The  Traube-Hering  Waves. — Under  eertain  experimental  con- 
ditions the  arterial  blood-pressure  tracing  exhibits,  in  addition  to  the 
usual  respiratory  variations,  certain  longer  rhythmic  variations  more 
or  less  wave-like  in  character,  which  are  known  as  Traube-Hering 
waves.  They  can  be  developed  on  a  blood-pressure  tracing  by  in- 
jecting magnesium  sulphate  or  morphine  into  the  circulation,  by  tying 
the  cerebral  arteries,  -etc.  These  waves  indicate  a  periodic  contrac- 
tion and  dilatation  of  the  blood-vessels  the  result  of  a  stimulation  of 
the  vaso-motor  centers. 


CHAPTER  XVI. 


ANIMAL  HEAT. 


The  chemic  changes  which  take  place  in  the  tissues  and  organs 
of  the  living  body  and  which  underlie  all  manifestations  of  life  are 
attended  by  the  evolution  of  heat.  In  consequence  of  this  each  ani- 
mal acquires  a  certain  body-temperature. 

In  man,  as  well  as  in  other  mammals  and  in  birds,  the  chemic 
changes  are  extremely  active  and  the  evolution  of  heat  very  great. 
Through  some  special  heat-regulating  mechanism,  by  which  heat- 
production  and  heat-dissipation  are  kept  in  equilibrium,  these  animals 
have  acquired  and  maintain  within  limits  a  constant  temperature  which 
is  independent  of  and  generally  above  that  of  the  surrounding  atmos- 
phere. As  the  temperature  of  these  animals  is  high  and  perceptible 
to  the  sense  of  touch,  they  were  originally  designated  "warm-blooded" 
animals.  As  this  temperature  is  constant  notwithstanding  the  great 
variations  in  external  temperature  during  the  summer  and  winter 
seasons,  they  are  more  appropriately  termed  constant-temperatured 
or  homoiothermous  animals.  The  intensity  of  the  body-temperature 
determined  by  the  insertion  of  a  thermometer  in  the  rectum  varies  in 
different  classes  of  mammals  from  37.2 °  C.  to  400  C.  The  causes  of 
this  variation  are  doubtless  connected  with  peculiarities  of  organiza- 
tion. In  birds  the  rectal  temperature  is  usually  higher,  varying  from 
40.9 °  C.  in  the  pigeon  to  44 °  C.  in  the  titmouse  and  the  swift. 

In  reptiles,  amphibians,  and  fish  chemic  changes,  as  a  rule,  are 
not  very  active  and  heat-production  relatively  slight.  As  they  are 
devoid  of  a  sufficiently  active  heat-regulating  mechanism,  the  tempera- 
ture of  the  body  is  largely  dependent  on  that  of  the  medium  in  which 
they  live,  though  it  is  always  one  or  more  degrees  above  it.  In  winter 
the  body-temperature  of  frogs,  for  example,  may  decline  as  low  as  8.90 
C,  the  temperature  of  the  surrounding  medium  being  6.7  °  C.  When 
subjected  to  temperatures  below  zero,  the  temperature  of  the  body 
may  fall  below  the  freezing-point  also,  when  the  lymph  and  fluids  of 
the  body  become  ice.  Though  apparently  dead,  the  gradual  eleva- 
tion of  the  temperature  restores  their  vitality.  In  summer-time,  on  the 
contrary,  the  body-temperature  may  attain  to  38 °  C.  Similar  varia- 
tions have  been  observed  in  other  animals.  As  the  temperature  of 
these  animals  is  low  and  perceptibly  below  that  of  our  own  bodies, 
they  were  originally  termed  "cold-blooded"  animals;  as  their  temper- 
ature is  inconstant,  varying  with  the  temperature  of  the  surrounding 
medium,  they  are  more  appropriately  termed  "variable  temperatured" 
or  poikilo-thermous  animals. 

436 


ANIMAL  HEAT.  437 

THE  TEMPERATURE  OF  THE  HUMAN  BODY. 

The  determination  of  the  temperature  of  the  human  body  under 
the  changing  conditions  of  life  is  a  matter  of  the  greatest  physiologic 
and  clinical  interest.  The  temperature  of  the  superficial  portions  of 
the  body  may  be  obtained  by  the  introduction  of  a  thermometer  into 
the  mouth,  the  rectum,  the  vagina,  or  the  axilla.  As  a  result  of  many 
observations  it  has  been  found  that  the  temperature  of  the  rectum  is, 
on  the  average,  37.2  °  C;  that  of  the  mouth,  36.8 °  C;  that  of  the 
axilla,  36.9 °  C.  Owing  to  radiation  and  conduction  the  surface  tem- 
perature is  lower  than  that  of  either  the  mouth  or  rectum,  and  varies 
to  a  slight  extent  in  different  regions  of  the  body:  e.  g.,  at  a  room- 
temperature  of  20 °  C.  the  skin  of  the  pectoral  region  has  a  temperature 
of  34.70;  that  of  the  cheek,  34.40;  that  of  the  calf,  33. 6°;  that  of  the  tip 
of  the  ear,  only  28.8 °,  etc. 

In  the  interior  of  the  body,  especially  in  organs  in  which  oxidation 
takes  place  rapidly,  and  which  at  the  same  time  are  protected  by 
their  anatomic  surroundings  from  rapid  radiation,  the  temperature 
is  higher  than  that  observed  in  the  rectum.  From  an  investigation 
of  the  temperature  of  the  blood  as  it  emerges  from  the  liver,  the  mus- 
cles, the  brain,  alimentary  canal,  etc.,  it  is  evident  that  these  organs 
have  a  higher  temperature  than  the  rectum. 

As  the  chemic  changes  underlying  physiologic  activity  vary  in 
intensity  and  extent  in  different  regions  of  the  body,  there  would  be 
marked  variations  in  their  temperature  were  it  not  that  the  blood, 
having  a  large  capacity  for  heat-absorption,  distributes  the  heat  al- 
most uniformly  to  all  portions  of  the  body,  so  that  at  a  short  distance 
beneath  the  surface  the  temperature  does  not  vary  but  within  a  few 
degrees. 

In  the  dog  the  temperature  of  the  blood  in  the  aorta  and  in  its 
principal  branches  is  approximately  38.3  °  C.  In  passing  through 
the  systemic  capillaries  the  temperature  falls  from  radiation  and  con- 
duction to  surface  temperature,  to  again  rise  as  the  venous  blood  ap- 
proaches the  deeper  regions  of  the  body.  In  the  neighborhood  of 
the  renal  veins  and  in  the  superior  vena  cava  the  temperature  is  again 
that  of  the  aorta.  In  the  portal  vein  the  temperature  rises  to  40. 2  °  C; 
in  the  hepatic  vein,  to  40. 6°  C.  In  the  right  ventricle,  owing  to  the  ad- 
mixture of  blood  from  different  localities  having  different  temperatures, 
the  temperature  falls  to  38.2 °  to  40.40.  In  passing  through  the  pul- 
monary capillaries  the  temperature  of  the  blood  again  falls,  so  that  in 
the  left  ventricle  it  will  register  from  38 °  C.  to  40. 2  °  C.  There  is  thus 
usually  a  difference  between  the  two  sides  of  the  heart  of  about  0.20  C. 

Variations  in  the  Mean  Temperature. — The  mean  tempera- 
ture of  the  human  body  for  twenty-four  hours,  which  for  the  mouth 
and  the  rectum  may  be  accepted  at  36.7  °  C.  and  37.20  C.  respectively, 
is  subject  to  variations  from  a  variety  of  circumstances,  such  as  age, 
periods  of  the  day,  food,  exercise,  etc. 


438  TEXT-BOOK  OF  PHYSIOLOGY. 

Age. — At  birth  the  temperature  of  the  infant  is  slightly  higher 
than  that  of  the  mother,  registering  in  the  rectum  about  37.5 °  C.  In 
a  few  hours  it  rapidly  declines  to  about  36.5 °,  to  be  followed  in  the 
course  of  twenty-four  hours  by  a  rise  to  the  normal  or  slightly  beyond. 
During  childhood  the  temperature  gradually  approximates  that  of 
the  adult.  In  old  age  the  temperature  rises,  as  a  rule,  and  attains  a 
maximum  at  eighty  years  of  37.4 °  C. 

Periods  of  the  Day. — The  observations  of  Jiirgensen  show  that 
there  is  a  diurnal  variation  in  the  mean  temperature  of  from  0.50  C. 
to  1.5  °  C,  the  maximum  occurring  late  in  the  afternoon,  from  5  to  7 
o'clock,  the  minimum  early  in  the  morning,  from  4  to  7  o'clock.  This 
diurnal  variation  in  the  mean  temperature  is  related  to  corresponding 
variations  in  many  other  physiologic  processes,  and  its  causes  are  to 
be  found  in  the  ordinary  habits  of  life  as  regards  the  time  of  meals, 
periods  of  exercise,  sleep,  etc. 

Food  and  Drink. — The  ingestion  of  a  hearty  meal  increases  the 
temperature  but  slightly — not  more  than  0.5 °  C.  Insufficiency  of 
food  lowers  the  temperature;  total  withdrawal  of  food,  as  in  starva- 
tion, is  followed  by  a  steady  though  slight  decline,  until  just  preceding 
the  death  of  the  animal,  when  it  falls  abruptly  to  from  6°  to  8°  C. 
Cold  drinks  lower,  hot  drinks  raise  the  temperature.  Food  and  drinks, 
however,  only  temporarily  change  the  mean  temperature,  and  after  a 
short  period  equilibrium  is  restored  through  the  activity  of  the  heat-regu- 
lating mechanism.  Alcoholic  drinks  lower  the  temperature  about 
0.5  °  C.  In  large  toxic  doses  in  persons  unaccustomed  to  their  use 
the  temperature  may  be  lowered  several  degrees.  This  is  attributed 
not  to  a  diminution  in  heat-production,  but  rather  to  an  increase  in 
heat-dissipation  (Reichert)  from  increased  action  of  the  heart,  dilata- 
tion of  the  blood-vessels  of  the  skin,  and  increased  activity  of  the  sweat- 
glands.  " 

Exercise. — The  temperature  may  be  raised  by  active  muscular 
exercise  from  i°  to  1.50  C.  as  a  result  of  increased  activity  in  chemic 
changes  in  the  muscles  themselves.  A  rise  beyond  this  point  is  pre- 
vented by  the  increased  activity  of  the  circulatory  apparatus,  the  re- 
moval of  the  heat  to  the  surface,  and  its  rapid  radiation. 

External  Temperature. — The  external  temperature  influences  but 
slightly  the  mean  temperature  of  the  human  body.  In  the  tropic  as 
well  as  in  the  arctic  regions,  notwithstanding  the  change  in  the  tem- 
perature of  the  air,  that  of  the  body  remains  almost  constant.  The 
same  is  true  for  the  seasonal  variations  in  the  temperature  of  the  tem- 
perate regions. 

THE  SOURCE  AND  TOTAL  QUANTITY  OF  HEAT  FRODUCED. 

The  Source  of  Heat. — The  immediate  source  of  the  body-heat 
is  to  be  found  in  the  chemic  changes  which  take  place  in  all  the  tissues 
and  organs  of  the  body.     Each  contraction  of  a  muscle,  each  act  of 


ANIMAL  HEAT.  439 

secretion,  each  exhibition  of  nerve-force,  is  accompanied  by  the  evo- 
lution of  heat.  The  chemic  changes  are  for  the  most  part  of  the  nature 
of  oxidations,  the  union  of  oxygen  with  the  elements,  carbon  and  hy- 
drogen, of  the  food  principles  either  before  or  after  they  have  become 
constituents  of  the  tissues.  The  ultimate  source  of  the  body-heat  is 
the  latent  or  potential  energy  in  the  food  principles,  which  was  absorbed 
from  the  sun's  energy  and  stored  up  during  the  growth  of  the  vegetable 
world.  In  the  metabolism  of  the  animal  body  the  food  principles  are 
again  reduced  through  oxidation,  directly  or  indirectly,  to  relatively 
simple  bodies,  such  as  urea,  carbon  dioxid,  and  water,  with  a  liberation 
of  a  large  portion  of  their  contained  energy  which  manifests  itself  as 
heat  and  mechanic  motion. 

The  Total  Quantity. — The  total  quantity  of  heat  liberated  in 
the  body  daily  may  be  approximately  determined  in  at  least  two  ways: 
(1)  By  determining  experimentally  the  heat  values  of  different  food 
principles  by  direct  oxidation;  (2)  by  collecting  and  measuring  with 
a  suitable  apparatus,  a  calorimeter,  the  heat  evolved  by  the  oxidation 
of  the  food  within,  and  dissipated  from,  the  body  daily. 

1.  Direct  Oxidation. — The  amount  of  heat  which  any  given  food 
principle  will  yield  can  be  determined  by  burning  a  definite  amount— 
e.  g.,  1  gram — to  carbon  dioxid  and  water  and  ascertaining  the  extent 
to  which  the  heat  thus  liberated  will  raise  the  temperature  of  a  given 
amount  of  water,  e.  g.,  1  kilogram.  The  amount  of  heat  may  be 
expressed  in  gram  or  kilogram  degrees  or  calories;  a  gram  calorie 
or  kilogram  calorie  being  the  amount  of  heat  required  to  raise  the 
temperature  of  a  gram  or  a  kilogram  (1000  grams)  of  water  i°  C. 
The  apparatus  employed  for  this  purpose  is  termed  a  calorimeter, 
which  consists  essentially  of  a  closed  chamber,  in  which  the  oxidation 
takes  place,  surrounded  by  a  water-jacket.  The  rise  in  temperature 
of  the  water  indicates  the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calor- 
imeters and  different  food  principles  of  the  same  class  vary,  though 
within  narrow  limits:  e.  g.,  1  gram  casein  yields  5.867  kilogram  calor- 
ies; 1  gram  of  lean  beef,  5,656;  1  gram  of  fat,  9.353,  9.423,  9.686  calor- 
ies; 1  gram  of  starch  or  sugar,  4. 116,  4.182,  4.479,  etc->  calories.  These 
results  are,  however  physical  values,  and  indicate  the  quantitv  of 
heat  such  quantities  of  foods  give  rise  to  when  completely  oxidized 
to  carbonic  acid  and  water.  In  the  human  body  the  carbohydrates 
and  the  fats,  with  the  exception  of  the  small  portion  which  escapes 
digestion,  are  reduced  to  carbon  dioxid  and  water,  and  hence  prac- 
tically liberate  as  much  heat  as  they  do  when  oxidized  outside  the  body. 
The  proteids,  however,  are  only  reduced  to  the  stage  of  urea.  As  this 
compound  is  capable  of  further  reduction  in  the  calorimeter  to  carbon 
dioxid  and  water,  with  the  liberation  of  heat,  the  quantity  of  heat  it 
contains  must  therefore  be  deducted  from  the  physical  heat  value  of  the 
proteid.  According  to  Rubner,  1  gram  of  urea  will  yield  2.523  kilogram 
calories.     As  about  one-third  of  a  gram  of  urea  results  from  the  oxi- 


44o  TEXT-BOOK  OF  PHYSIOLOGY. 

dation  of  i  gram  of  proteid,  the  amount  of  heat  to  be  deducted  from 
the  heat  value  of  the  proteid  is  -J  of  2.523,  or  0.841  calories.  It  has 
also  been  shown  by  the  same  investigator  that  some  of  the  ingested 
proteid  is  found  in  the  feces,  the  heat  value  of  which  must  also  be  deter- 
mined and  deducted.  This  having  been  done,  the  physiologic  heat 
value  becomes  4.124  calories. 

The  following  estimates  give  approximately  the  number  of  kilo- 
gram calories  which  should  be  liberated  within  the  body  when  the 
proteid  is  burned  to  the  stage  of  urea,  and  the  fat  and  carbohydrate 
to  the  stage  of  carbon  dioxid  and  water : 

1  gram  of  proteid 4.124  calories 

1       "         fat 9.353       " 

1       "         carbohydrate   4. 116       " 

The  total  number  of  kilogram  calories  yielded  by  the  various  diet 
scales  can  be  readily  determined  by  multiplying  the  quantities  of  the 
food  principles  consumed  by  the  foregoing  factors.  The  diet  scale 
of  Yierordt,  for  example,  yields  the  following: 

120  grams  of  proteid   494. 88  calories 

90      "  fat 841.77       " 

330      "  starch 135S.28 

Total, 2694.93       " 

The  total  calories  obtained  from  other  diet  scales  would  be  as  follows : 
Ranke's,  2335;  Voit's,  3387;  Moleschott's,  2984;  Atwater's,  3331; 
Hultgren's,  3436.  These  numbers  indicate  theoretically  the  total 
heat-production  in  the  body  daily. 

2.  Calorimetric  Measurements.— -By  this  method  the  heat  dissipated 
from  the  body  of  an  animal  is  directly  collected  and  measured,  and 
the  amount  so  obtained  is  taken  as  a  measure  of  the  heat  evolved  by 
the  oxidation  of  the  food.  A  calorimeter  is  therefore  an  apparatus 
for  the  direct  estimation  of  the  quantity  of  heat  dissipated  from  the 
body  in  a  given  time.  The  substance  employed  for  collecting  and 
measuring  the  heat  is  either  water  or  air.  The  calorimeters  in  general 
use  consist  essentially  of  two  metallic  boxes  placed  one  within  the  other, 
though  separated  by  a  space  sufficiently  large  to  hold  a  definite  amount 
of  water  (Fig.  205).  The  animal  is  placed  in  the  inner  box,  which 
is  also  provided  with  tubes  for  the  entrance  of  fresh  and  the  exit  of 
expired  air.  The  heat  radiated  is  absorbed  by  the  water  and  its  tem- 
perature raised.  To  prevent  loss  by  radiation  and  to  render  it  inde- 
pendent of  changes  in  the  surrounding  temperature  the  calorimeter  is 
surrounded  by  a  poorly  conducting  material,  such  as  wool.  The 
temperature  of  the  animal  is  taken  at  the  beginning  and  the  end  of  the 
experiment.  If  the  temperature  of  the  animal  remains  the  same  at 
the  end  of  the  experiment,  then  the  heat  absorbed  by  the  water  repre- 
sents the  amount  produced  by  the  animal.  If,  on  the  contrary,  the 
temperature  of  the  animal  rises  or  falls,  the  number  of  calories  so  re- 


ANIMAL  HEAT. 


441 


tained  or  lost  must  be  added  to  or  subtracted  from  the  amount  ab- 
sorbed by  the  calorimeter.  In  the  determination  of  the  absolute 
amount  of  heat  retained  or  lost  by  the  animal  above  or  below  the  initial 
temperature,  as  well  as  that  absorbed  by  the  materials  of  the  appara- 
tus in  these  various  instances,  the  water  equivalent  of  the  tissues  of  the 
animal  and  the  materials  of  the  calorimeter  must  be  obtained,  and  then 
added  to  or  subtracted  from,  as  the  case  may  be,  the  amount  of  water 
in  the  calorimeter,  and  the  amount  thus  obtained  multiplied  by  its 
rise  in  temperature.  In  properly  conducted  experiments  in  which 
the  sources  of  error  are  reduced  to  a  minimum  there  is  a  very  close 
correspondence  between  the  total  physiologic  heat  value  of  the  food 
and  the  amount  col- 
lected by  the  calor- 
imeter. Thus,  in  an 
experiment  detailed 
by  Rubner,  a  dog  was 
given  during  twelve 
days  228.06  grams  of 
proteid  and  340.4 
grams  of  fat  the 
physical  heat  value  of 
which  was  estimated 
at  4419  calories.  The 
urine  and  feces  during 
this  period  were  col- 
lected and  their  heat 
value  determined, 
which  amounted  to 
305  calories.  The 
heat  which  theoretic- 
ally should  have  been 
produced  was  41 19 
calories.     During  the 

experiment  the  calorimeter  actually  absorbed  3958  calories,  a  differ- 
ence between  the  theoretic  and  experimental  results  of  156  calories; 
thus  of  the  total  energy  liberated  96  per  cent,  appeared  as  heat. 

Calorimetric  experiments  on  man  corresponding  to  those  made 
by  Rubner  on  dogs  have  not  been  successful,  owing  purely  to  tech- 
nical difficulties.  Various  attempts  have  been  made,  however,  to 
determine  the  daily  heat-dissipation.  Liebermeister  immersed  a 
man  in  a  bath  with  a  temperature  lower  than  that  of  the  man's  body. 
From  the  rise  in  temperature  of  the  water  it  was  calculated  that  the 
man  produced  daily  3525  calories.  Leyden  placed  the  leg  alone  of 
a  man  in  a  calorimeter.  In  one  hour  6  calories  were  absorbed.  As- 
suming that  the  total  superficial  area  of  the  body  was  fifteen  times  that 
of  the  leg,  he  calculated,  taking  into  consideration  various  sources  o'f 
error,  that  the  entire  body  would  produce  daily  2376  calories.     Ott, 


Fig.  205.  —  Water  Calorimeter  of  Dulong.  D 
and  D'.  Tubes  for  the  entrance  and  exit  of  air.  T  and  T'. 
Thermometers  for  ascertaining  the  temperature  of  the 
water.  S.  A  mechanic  contrivance  for  stirring  the  water 
for  the  purpose  of  distributing  the  absorbed  heat  uni- 
formly. To  prevent  the  escape  of  heat  with  the  expired 
air,  the  tube  D'  is  wound  many  times  in  the  water-space 
beneath  the  animal  cage. 


442  TEXT-BOOK  OF  PHYSIOLOGY. 

employing  a  water  calorimeter,  found  that  the  body  of  a  man  produced 
103  calories  during  an  afternoon,  or  at  the  rate  of  2472  calories  daily. 
These  and  similar  experiments,  while  not  free  from  many  objections, 
furnish  results  which  indicate  that  the  heat  dissipated  from  the  body 
approximates  the  physiologic  heat  values  of  the  foods. 

HEAT-DISSIPATION  AND  REGULATION  OF  THE  TEMPERATURE. 

Heat-dissipation. — From  the  preceding  statements  it  is  evident 
that  the  body  is  continually  liberating  heat  in  amounts  daily  far  in 
excess  of  that  necessary  for  the  maintenance  of  the  body-temperature. 
Should  this  heat  be  retained,  the  temperature  of  the  body  would  be 
raised  at  the  end  of  twenty-four  hours  an  additional  18 °  or  200  C. — 
a  temperature  far  in  excess  of  that  compatible  with  the  maintenance 
of  physiologic  processes.  That  the  body  may  be  kept  at  the  mean 
temperature  of  3 7. 8°  C.  it  is  essential  that  the  heat  liberated  be  dissi- 
pated as  fast  as  it  is  produced,  or  to  state  the  problem  in  another  way, 
the  heat  dissipated  by  the  body  must  be  replaced  by  an  equal  amount 
liberated,  if  equilibrium  of  temperature  is  to  be  maintained.  The 
dissipation  of  the  heat  is  accomplished  in  several  ways:  (1)  In  warm- 
ing the  food  and  drink  to  the  temperature  of  the  body.  (2)  In  warm- 
ing the  inspired  air  to  the  same  temperature.  (3)  In  the  evaporation 
of  water  from  the  lungs.  (4)  In  evaporating  water  from  the  skin. 
(5)  In  radiation  and  conduction  from  the  skin.  The  quantities  of 
heat  lost  to  the  body  by  these  different  processes  it  is  difficult  for  ob- 
vious reasons  to  accurately  determine,  and  the  estimates  usually  given 
must  be  regarded  only  as  approximative. 

Assuming  2500  calories  to  be  an  average  of  heat  liberated  during 
a  day  of  repose,  the  losses,  in  the  ways  stated  above,  may  be  given  as 
follows : 

1.  In  Warming  Food  and  Drink. — The  average  temperature  of  food 

and  drink  is  about  12  °  C;  the  amount  of  both  together  is  about 
3  kilograms;  the  specific  heat  of  food  about  0.8  that  of  water. 
The  absorption  of  body-heat  therefore  by  the  food  amounts 
approximately  to  3X0.8X250  C.  =  60  calories  =  2.8  per  cent.  With 
the  removal  of  the  end-products  of  the  foods  and  drink  from  the 
body  an  equal  amount  of  heat  is  carried  out. 

2.  In  Warming  the  Inspired  Air. — The  average  temperature  of  the 

air  is  120  C.;  the  amount  of  inspired  air,  about  15  kilograms;  the 
specific  heat  of  air,  0.26.  The  absorption  of  body-heat  by  the  air 
until  it  attains  the  temperature  of  the  body  will  therefore  amount 
to  15  X  0.26  X  25°=  97.5  calories  =  3.8  per  cent.  The  expired  air 
removes  from  the  body  a  corresponding  amount. 

3.  In   the  Evaporation  0}    Water  from  the  Lungs. — The  quantity  of 

water  evaporated  from  the  lungs  may  be  estimated  at  400  grams; 
as  each  gram  requires  for  its  evaporation  0.582  calorie,  the 
quantity  of  heat  lost  by  this  channel  would  be  400X0.582  = 
232.8  calories  =  9.4  per  cent. 


ANIMAL  HEAT.  443 

4.  /;/  the  Evaporation  of  Water  from  the  Skin. — The  quantity  of  water 

evaporated  from  the  skin  may  be  estimated  at  660  grams,  caus- 
ing a  loss  of  heat  by  this  channel  of  660  X  0.582  =  384.1  calories  = 
15.3  per  cent. 

5.  In   Radiation    and   Conduction   from   the   Skin. — The   amount   of 

heat  lost  by  this  process  can  be  indirectly  determined  only  by 
subtracting  the  total  amount  lost  by  the  above-mentioned  channels 
from  the  total  amount  produced.  Thus,  2500  —  774.4=  1725.6 
calories  =  69  per  cent,  would  represent  the  average  amount  lost  by 
radiation  and  conduction. 
Regulation  of  the  Mean  Temperature. — In  order  that  the 
mean  temperature  of  the  body  may  remain  practically  constant,  the 
heat  dissipated  must  be  exactly  balanced  by  the  heat  liberated.  Should 
there  be  any  want  of  correspondence  between  the  two  processes,  there 
would  arise  either  an  increase  or  a  decrease  in  the  mean  temperature. 
As  both  heat-production  and  heat-dissipation  are  variable  factors, 
dependent  on  a  variety  of  internal  and  external  conditions,  their 
adjustment  is  accomplished  by  a  complex  self-regulating  mechanism 
involving  muscular,  vascular,  and  secretory  elements,  coordinated  by 
the  nerve  system.  Heat-production  varies  in  intensity  and  amount, 
in  accordance  with  a  number  of  conditions,  but  principally  with 
variations  in  physiologic  activity,  the  quantity  and  quality  of  the 
food,  and  changes  in  the  external  temperature.  All  physiologic  and 
especially  muscle  activity  is  attended  by  chemic  changes  and  the 
evolution  of  heat.  The  greater  the  activity,  the  larger  is  the  volume 
of  heat.  Foods  have  different  physiologic  heat  values.  If  the  food 
consumed  contains  much  potential  energy  and  the  quantity  con- 
sumed be  larger  than  the  average  daily  requirements,  there  will  be 
an  increase  in  heat-production.  A  lowering  of  the  external  tem- 
perature, as  in  the  winter  season,  leads  to  increased  heat-production 
through  stimulation  of  the  nerve-centers.  When  all  these  conditions, 
increased  muscular  activity,  increased  amount  of  food  of  high  poten- 
tial energy,  and  a  low  external  temperature  coexist,  heat-production 
attains  its  maximum,  amounting  to  as  much  as  4726  calories  daily 
(Hultgren). 

Heat-dissipation  varies  in  rapidity  in  accordance  with  variations 
of  a  number  of  factors,  but  principally  with  variations  in  the  external 
temperature  and  the  activity  of  the  perspiratory  apparatus.  The 
heat  is  dissipated  mainly  by  the  skin,  69  per  cent.,  in  consequence  of 
radiation  and  conduction  and  by  the  evaporation  of  the  sweat.  The 
loss  by  this  channel  as  well  as  from  the  lungs  is  dependent  for  the 
most  part  on  a  difference  of  temperature  of  the  surrounding  air  and 
of  the  body.  If  the  surrounding  temperature  is  high,  there  is  an 
increase  in  the  activity  of  both  the  circulatory  and  respiratory  mechan- 
isms, brought  about  by  the  central  nervous  system.  In  addition  to 
an  increased  action  of  the  heart,  the  blood-vessels  of  the  skin  dilate, 
and  deliver  to  the  surface  a  larger  volume  of  blood  in  a  given  time, 


444  TEXT-BOOK  OF  PHYSIOLOGY. 

thus  increasing  the  conditions  favorable  to  radiation.  The  sweat- 
glands  at  the  same  time  are  stimulated  to  increased  activity,  and 
in  consequence  of  the  additional  volumes  of  blood  brought  to  the  skin 
a  larger  amount  of  sweat  is  secreted,  which  speedily  undergoes  evap- 
oration. As  each  gram  of  water  for  its  evaporation  requires  0.582  of 
a  calorie,  it  is  evident  that  increased  secretion  of  sweat  favors  heat- 
dissipation.  The  nerve-centers  influencing  the  activity  of  the  sweat- 
glands  may  be  stimulated  not  only  reflexly,  but  directly  by  an  excess 
of  heat  in  the  blood.  If,  however,  the  atmosphere  itself  possesses  a 
high  percentage  of  moisture,  evaporation  from  the  body  is  much 
diminished  and  the  value  of  sweating  as  a  means  of  lowering  the 
body-temperature  is  much  impaired.  Evaporation  is  hastened  by 
air  in  motion.  Hastened  respiratory  movements  and  the  dilatation 
of  blood-vessels  of  the  respiratory  surface  also  increase  the  evaporation 
of  water  from  the  lungs  and  thus  occasion  a  greater  loss  of  heat. 

If  the  external  temperature  falls  there  is  a  decrease  in  the  physio- 
logic activity  of  the  skin  from  a  contraction  of  the  blood-vessels,  a 
diminution  of  the  blood-supply,  and  a  cessation  in  the  secretion  of 
sweat.  The  blood,  being  prevented  from  coming  to  the  surface,  is 
retained  in  the  deeper  portion  of  the  body,  and  in  consequence  the 
conditions  for  radiation  are  diminished.  These  variations  in  the 
cutaneous  circulation  in  response  to  variations  in  the  external  tem- 
perature are  brought  about  by  the  vaso- motor  nerve  mechanism;  and 
as  they  take  place  with  extreme  promptness  heat-dissipation  and  heat- 
production  are  quickly  adjusted  and  the  mean  temperature  maintained. 

Radiation  from  the  skin  is  modified  to  some  extent  by  clothing. 
An  excess  of  clothing  diminishes,  a  diminution  of  clothing  increases 
radiation.  The  quality  of  clothing  is  also  an  important  factor.  Wool 
is  a  poor  conductor  of  heat  but  a  good  absorber  and  retainer  of  moisture, 
and  hence  is  adapted  for  cold  weather.  Linen  and  cotton  possess  the 
opposite  qualities,  and  hence  are  adapted  for  warm  weather.  Radia- 
tion from  the  skin  is  somewhat  interfered  with  by  subcutaneous  fat, 
the  extent  of  the  interference  being  dependent  on  its  amount. 

The  foregoing  estimates  as  to  the  amounts  of  heat  produced  have 
reference  only  to  the  body  in  repose.  When  the  body  passes  into  a 
state  of  muscle  activity,  there  is  at  once  a  notable  increase  in  heat- 
production  in  consequence  of  the  increase  in  the  activity  of  the  chemic 
changes  which  underlie  body  activity,  as  shown  by  the  increase  in  the 
consumption  of  oxygen  and  the  production  of  carbon  dioxid.  Not 
all  of  the  potential  energy  set  free,  however,  appears  as  heat;  for, 
if  the  muscles  are  engaged  in  doing  work  a  part  of  the  energy  which 
would  otherwise  manifest  itself  as  heat  is  converted  into  mechanic 
motion.  From  the  work  done  during  a  period  of  eight  hours  it  has 
been  estimated  that  about  500  calories  are  so  transformed  or  utilized. 
Hirn  calculated  from  an  average  of  five  experiments  that  a  man 
weighing  67  kilos  in  repose  produced  154-4  calories  per  hour  and 
absorbed  30.7  grams  of  oxygen  per  hour;  but  when  engaged  in  active 


ANIMAL  HEAT.  445 

muscle  movements  produced  271.2  calories  and  absorbed  119.84 
grams  of  oxygen  per  hour.  The  increase  in  heat-production  per  hour 
during  activity  was  thus  almost  doubled,  though  the  sum  total  pro- 
duced daily  in  which  there  was  a  working  period  of  eight  or  ten  hours 
was  only  about  one-third  more  than  during  a  day  of  repose.  During 
sleep  there  is  a  greatly  diminished  heat-production,  not  more  than 
40  calories  per  hour  being  produced.  The  preceding  data  may  be 
tabulated  as  follows  (Martin) : 

Day  of  Rest.  Day  of  Work. 


Heat  units  (calor-  \  Rest  16  hrs.    Sleep  S  hrs.     Rest  S  hrs.    Work  8  hrs.     Sleep  8  hrs. 
ies)  produced ..  /  2470.4  320  I235-2  2169.6  320 

2790.4  3724-S 


CHAPTER  XVII. 
SECRETION. 

Secretion. — Secretion  is  a  term  applied  to  a  process  by  which  a 
portion  of  the  constituents  of  the  blood  are  separated  from  the  blood- 
stream, by  the  activities  of  the  endothelial  cells  of  the  capillary  wall,  as 
the  blood  flows  through  the  capillary  blood-vessels.     In  this  process  the 
endothelial  cell  is  aided  by  the  physical  forces,  diffusion,  osmosis  and  fil- 
tration.    The  materials  thus  separated  are  collectively  termed  lymph. 
This  separated  or  secreted  material  may  be  utilized  in  several  ways: 
i.  For  the  repair  of  the  tissues,  for  growth,  for  the  liberation  of  energy. 
2.  For  the  elaboration  or  production  by  specialized  organs  of  a  variety 
of  complex  fluids  of  widely   different   application.     The   fluids 
thus  formed  are  utilized  for  the  most  part  to  meet  some  special 
need  of  the  body.     All  such  fluids  are  termed  secretions. 

All  secretions  are  products  of  the  activities  of  epithelial  cells 
covering  a  flat,  or  lining  a  more  or  less  complexly  involuted, 
membrane  which  in  each  instance  may  be  termed  a  secretor  organ. 
As  the  fluids  are  poured  out  on  the  surface  of  the  body,  they  have 
been  termed  external  secretions:  e.  g.,  mucus,  saliva,  gastric  juice, 
milk,  sebaceous  matter,  etc.  Within  recent  years  it  has  been  demon- 
strated that  the  epithelium  of  certain  organs  and  particularly  of  those 
which  do  not  possess  a  duct,  such  as  the  thyroid,  thymus,  adrenals, 
hypophysis,  etc.,  also  produces  certain  specific  constituents  which  are 
reabsorbed  into  the  blood,  and  which  in  some  unknown  but  yet  favor- 
able way  influence  the  general  nutrition.  To  such  products  of  these 
organs  the  term  internal  secretions  has  been  given. 

The  blood,  in  addition  to  its  nutritive  constituents,  contains  a 
number  of  principles,  derived  from  the  tissues,  which  are  to  be  re- 
garded as  waste  products,  the  outcome  of  the  katabolic  activity  of 
the  tissues  and  of  no  further  use  to  the  body.  If  retained,  they  would 
seriously  if  not  fatally  interfere  with  the  normal  physiologic  activities 
of  the  different  tissues.  They  are  therefore  removed  by  specialized 
organs  after  their  separation  from  the  blood-stream.  The  waste 
products  in  solution  thus  removed  are  not  capable  of  being  utilized  for 
any  special  purpose,  and  arc  therefore  termed  excretions:  e.  g.,  urine, 
perspiration,  etc.  Excretion,  however,  is  performed  by  the  activities 
of  epithelial  cells  aided  by  the  physical  forces  of  diffusion,  osmosis 
and  nitration;  and  though  a  distinction  is  made  between  the  two 
classes  of  fluids,  no  sharp  line  can  be  drawn  between  the  cell  processes 
which  take  place  in  secretor  and  excretor  organs. 

446 


SECRETION.  447 

All  secretor  organs  may  be  divided  into — 
i.  Epithelial. 

2.  Reticular  and  vascular,  the  latter  term  indicating  merely  their  re- 
lation to  blood-vessels. 

The  Epithelial  Secretor  Organs. — The  epithelial  secretor  or- 
gan consists  primarily  of  a  thin  delicate  homogeneous  membrane,  one 
side  of  which  is  covered  with  a  layer  of  epithelial  cells  and  the  other 
side  of  which  is  closely  invested  by  a  network  of  capillary  blood- 
vessels, lymph- vessels  and  nerves.  Though  the  epithelial  cells  have  a 
general  histologic  resemblance  one  to  another,  their  physiologic  function 
varies  in  different  situations,  in  accordance  probably  with  their  ulti- 
mate chemic  structure,  a  fact  which  determines  the  difference  in  the 
character  of  the  secretions. 

The  epithelial  secretor  organs  may  consist  of  a  single  layer  of  cells 
or  a  group  of  cells,  and  may  be  subdivided  into — 
i.  Secreting  membranes. 
2.   Secreting  glands. 

The  secreting  membranes  are  the  mucous  membranes  lining 
the  gastro-intestinal,  the  pulmonary,  and  the  genito-urinary  tracts, 
and  the  serous  membranes  lining  closed  cavities,  such  as  the  pleural, 
pericardial,  peritoneal,  and  synovial  membranes. 

The  mucous  membranes  are  soft  and  velvety  in  character  and 
are  composed  of  a  condensed  connective  tissue  forming  a  basement 
membrane  beneath  which  is  a  layer  of  blood-vessels  and  muscle-fibers, 
and  on  which  is  a  layer  of  epithelium,  the  histologic  as  well  as  physio- 
logic characters  of  which  vary  in  different  situations.  The  mucus 
secreted  by  the  various  epithelial  forms  will  very  naturally  possess  a 
somewhat  different  composition,  according  to  the  locality  in  which  it 
is  formed.  In  a  general  way  it  may  be  said  that  mucus  is  a  pale, 
semitransparent,  alkaline  fluid,  containing  leukocytes  and  epithelial 
cells.  It  is  composed  chemically  of  water,  mineral  salts  and  an  al- 
buminoid body,  mucin,  to  the  presence  of  which  it  owes  its  viscidity. 
Much  of  the  mucus  is  secreted  by  the  goblet  cells  on  the  surface  of  the 
mucous  membranes.  The  principal  varieties  of  mucus  are  the  nasal, 
bronchial,  vaginal,  urinary,  gastro-intestinal. 

The  serous  membranes  are  composed  of  thin  membrane  formed 
by  a  condensation  of  connective  tissue  and  covered  by  a  single  layer 
of  large,  flat,  nucleated  cells  with  irregular  margins.  These  mem- 
branes enclose  what  are  practically  large  lymph  sacs  or  spaces,  and 
the  fluid  they  contain  resembles  lymph  in  all  respects  and  is  prac- 
tically identical  with  it.  It  serves  to  diminish  friction  when  the  viscera 
they  enclose  move  over  one  another.  The  most  important  of  the  serous 
membranes  are  the  pleural,  pericardial,  and  peritoneal. 

The  synovial  membranes  in  and  around  joints  resemble  serous 
membranes.  The  cells  covering  them,  however,  secrete  a  clear, 
colorless  fluid  resembling  lymph,  but  differing  from  it  in  containing 
a  mucin-like  substance,  a  nucleo-albumin,  which  imparts  to  it  con- 


44S  TEXT-BOOK  OF  PHYSIOLOGY. 

siderable  viscidity.  This  synovial  fluid  serves  to  diminish  friction 
between  the  opposing  surfaces  of  the  bones  as  they  glide  over  one 
another  during  movement. 

Other  secretions,  such  as  the  aqueous  and  vitreous  humors  of  the 
eve,  the  fluid  of  the  internal  ear,  the  cerebrospinal  fluid,  etc.,  will  be 
considered  in  connection  with  the  organs  with  which  they  are  associated, 
as  have  been  the  digestive  secretions. 

The  secreting  glands  are  formed  of  the  same  histologic  elements 
as  the  secreting  membranes.  They  are  formed  by  an  involution  of 
the  mucous  membrane  or  skin  the  epithelium  of  which  is  variously 
modified  structurally  and  functionally  in  the  various  situations  in 
which  they  are  formed.  Like  the  membranes  themselves,  the  glands 
are  invested  by  capillary  blood-vessels  and  supplied  with  lymph- vessels 
and  nerves,  of  which  the  latter  are  in  direct  connection  with  the  blood- 
vessels and  epithelial  cells.  The  interior  of  each  gland  is  in  com- 
munication with  the  free  surface  by  one  or  more  passageways  known 
as  ducts. 

These  glands  may  be  classified  according  as  the  involution  is 
cylindrical  or  dilated  as — 

i.  Tubular.  The  tubular  glands  may  be  simple — e.  g.,  sweat- 
glands,  intestinal  glands,  fundus  glands  of  the  stomach;  or  compound 
— e.  g.,  kidney,  testicle,  salivary,  and  lachrymal  glands. 

2.  Alveolar.  The  alveolar  glands  may  also  be  simple — e.  g., 
the  sebaceous  glands,  the  ovarian  follicles,  meibomian  glands;  or 
compound,  as  the  mammary  glands  and  salivary  glands. 

For  the  production  of  a  secretion  it  is  necessary  that  the  plasma 
of  the  blood,  the  common  material,  be  delivered  to  the  lymph-spaces 
with  which  the  epithelial  cells  are  in  close  relation.  The  processes 
involved  in  the  passage  of  the  plasma  across  the  capillary  wall  have 
already  been  considered  in  connection  with  the  production  of  lymph. 
They  include  the  physical  processes,  diffusion,  filtration,  and  osmosis, 
combined  with  a  secretory  activity  of  the  cells  of  the  capillary  wall. 
The  question  as  to  which  of  these  processes  is  the  more  active  is  yet 
a  subject  of  investigation. 

As  the  chemic  composition  and  the  chemic  features  of  the  organic 
constituents  of  all  secretions  have  been  demonstrated  to  be  the  out- 
come of  metabolic  processes  going  on  within  the  epithelial  cells,  it 
must  be  assumed  at  least  that  these  differences  are  correlated  with 
differences  in  the  histologic  features  and  molecular  structure  of  the 
epithelium.  The  discharge  of  the  secretion  is,  as  a  rule,  intermittent; 
that  is,  there  are  periods  of  activity  on  the  part  of  the  gland  followed 
by  periods  of  inactivity  or  rest.  In  rest  more  especially  the  epithelial 
cells,  after  the  assimilation  of  lymph,  accumulate  within  themselves 
such  characteristic  products  as  globules  of  mucin,  granules  which 
apparently  are  the  antecedents  of  the  digestive  enzymes,  granules 
of  glycogen,  globules  of  fat,  sugar,  and  proteid,  as  in  the  case  of  the 
mammary  gland.     In  how  far  all  these  compounds  are  the  result  of 


SECRETION.  449 

secretor  activity  or  of  a  cell  degeneration  and  disintegration  it  is 
impossible  to  state  in  the  light  of  present  knowledge.  During  the 
period  of  gland  rest  the  blood-supply  to  the  gland  is  merely  sufficient 
for  nutritive  purposes.  When  the  occasion  arises  for  gland  activity, 
the  blood-vessels,  under  the  influence  of  the  vaso- motor  nerves,  dilate 
and  deliver  to  the  gland  an  amount  of  blood  far  beyond  that  required 
for  nutritive  purposes.  As  a  result  the  gland  becomes  red  and  vas- 
cular and  the  blood  emerging  by  the  veins  frequently  retains  its  cus- 
tomary arterial  color.  The  increased  blood-supply  favors  a  rapid 
transudation  of  water  and  salts  into  the  lymph-spaces  from  which  they 
are  speedily  absorbed  and  transmitted  by  the  epithelial  cells  into  the 
interior  of  the  gland  lumen.  Coincident  with  the  passage  of  water 
through  the  cell,  the  organic  constituents  are  extruded  from  the  end  of 
the  cell  bordering  the  lumen  to  become  dissolved,  or  in  the  case  of  fat 
to  be  suspended,  in  the  water.  The  secretion  thus  formed  accumulates 
and  with  .the  rise  of  pressure  which  inevitably  follows  it  at  once  passes 
into  the  ducts  to  be  discharged  on  the  surface  of  the  mucous  membrane 
or  skin,  as  the  case  may  be. 

Influence  of  the  Nerve  System. — The  activity  of  every  gland 
is  controlled  by  nerve-centers  situated  in  the  central  nerve  system. 
These  centers  may  be  excited  to  activity  either  by  impressions  made 
on  the  peripheral  terminations  or  by  emotional  states,  or,  possibly,  by 
changes  in  the  composition  of  the  blood  itself.  As  a  rule,  all  normal 
secretion  is  a  reflex  act  involving  the  usual  mechanism:  viz.,  a  sentient 
surface  (skin,  mucous  membrane,  or  sense-organ),  an  afferent  nerve,  an 
emissive  cell  from  which  emerges  an  efferent  nerve  to  be  distributed 
to  a  responisve  organ,  the  gland  epithelium. 

For  the  production  of  the  secretion  by  the  epithelial  cell  it  is  believed 
by  some  experimenters  that  two  physiologically  distinct,  efferent 
nerve-fibers  are  involved — one  stimulating  the  production  of  the  organic 
constituents  {trophic  nerves),  the  other  stimulating  the  secretion  of 
water  and  inorganic  salts  {secretor  nerves).  The  evidence  for  the 
influence  of  the  nerve  system  on  secretion  and  the  mode  of  connection 
of  the  nerve-fibers  with  the  gland-cells  have  been  alluded  to  (page  163) 
and  will  again  be  in  subsequent  chapters. 

The  reticular  and  vascular  glands,  though  not  possessing  any 
common  histologic  features,  are  grouped  together  merely  for  con- 
venience, and  will  be  considered  in  a  separate  chapter  in  connection 
with  the  problems  of  internal  secretion. 

MAMMARY   GLANDS. 

The  mammary  glands,  which  secrete  the  milk,  are  two  more  or 
less  hemispheric  organs  situated  in  the  human  female  on  the  anterior 
surface  of  the  thorax.  Though  rudimentary  in  childhood,  they 
gradually  increase  in  size  as  puberty  approaches.  The  gland  pre- 
sents at  its  convexity  a  small  conical  eminence  termed  the  mammilla 
29 


45° 


TEXT-BOOK  OF  PHYSIOLOGY. 


or  nipple,  surrounded  by  a  circular  area  of  pigmented  skin,  the  areola. 
The  gland  proper  is  covered  by  a  layer  of  adipose  tissue  anteriorly 
and  is  attached  posteriorly  to  the  pectoral  muscles  by  a  network  of 
fibrous  tissue. 

During  utero-gestation  the  mammary  glands  become  larger, 
firmer,  and  more  lobulated;  the  areola  darkens  and  the  blood-vessels, 
especiallv  the  veins,  become  more  prominent.  At  the  period  of 
lactation  the  gland  is  the  seat  of  active  histologic  and  physiologic 
changes  correlated  with  the  production  of  milk.  At  the  close  of 
lactation  these  activities  cease,  the  glands  diminish  in  size,  undergo 

involution,  and  gradually  return 
to  their  former  non-secreting 
condition. 

Structure  of  the  Mammary 

Gland. — Each  mammary  gland 

p||  consists    of    an    aggregation    of 

|||  some    15  or  20  irregular  pyram- 

llfj'j  idal    lobes,    each    of    which    is 


(-a 


m 


i^ii'i 


'  1 


m 


Fig.  206. — Mammary  Gland.  i. 
Lactiferous  ducts.  2.  Lobuli  of  the 
mammary  gland. 


Fig.  207. — Acini  of  the  Mammary 
Gland  of  a  Sheep  During  Lactation. 
a.  Membrana  propria.  b.  Secretory 
epithelium. 


surrounded  by  a  framework  of  fibrous  tissue.  •  This  tissue  affords 
support  for  blood-vessels,  lymphatics,  and  nerves.  Each  lobe  is  pro- 
vided with  a  single  excretory  duct,  the  lactiferous  duct,  which  as  it 
approaches  the  areola  expands  into  a  fusiform  ampulla  or  reservoir. 
At  the  base  of  the  nipple  the  ampullae  contract  to  form  some  16  or  18 
narrow  ducts,  which,  ascending  the  nipple,  open  by  constricted  orifices 
0.5  mm.  in  diameter  on  its  apex  (Fig.  206). 

On  tracing  the  lactiferous  duct  into  a  lobe,  it  is  found  to  divide 
and  subdivide  into  a  number  of  branches,  which  pass  into  smaller 
masses — the  lobules.  The  lobule  in  turn  is  composed  of  a  large 
number  of  tubular  acini  or  alveoli,  the  final  terminations  of  the  lobu- 
lar ducts.  Each  acinus  consists  of  a  basement  membrane  lined  by  a 
single  layer  of  low  cuboidal  epithelial  cells  (Fig.  207).  Externally 
the  acinus  is  surrounded  by  blood-vessels,  nerves,  and  lymphatics. 


SECRETION.  451 


MILK. 


Milk  as  obtained  during  active  lactation  is  an  opaque  bluish- 
white  fluid,  almost  inodorous,  with  a  sweet  taste,  an  alkaline  reaction, 
and  a  specific  gravity  of  from  1.025  to  1.040.  Examined  micro- 
scopically, it  is  seen  to  consist  of  a  clear  fluid,  the  milk  plasma,  hold- 
ing in  suspension  an  enormous  number  of  small,  highly  refractive 
oil-globules,  which  measure  on  the  average  about  T u  \  0  u  of  an  inch 
in  diameter.  It  has  been  asserted  by  some  observers  that  each  globule 
is  surrounded  by  a  thin  proteid  envelope  which  enables  it  to  maintain 
the  discrete  form.     This,  however,  is  at  present  disbelieved. 

The  quantity  of  milk  secreted  daily  by  the  human  female  averages 
about  1200  c.c. 

Chemic  analysis  has  shown  that  the  milk  of  all  the  mammalia 
consists  of  all  the  different  classes  of  nutritive  principles,  though  .in 
different  proportions,  which  are  necessary  to  the  growth  and  devel- 
opment of  the  body.  The  only  exception  appears  to  be  an  insuffi- 
cient amount  of  iron  for  the  formation  of  the  coloring-matter  of  the 
blood,  the  hemoglobin. 

Caseinogen  is  the  chief  proteid  constituent  of  milk.  Associated 
with  it,  however,  are  two  other  proteids,  lactalbumin  and  lactoglobulin, 
both  of  which  are  present  in  but  small  quantity.  When  milk  is  treated 
with  acetic  acid,  sodium  chlorid,  or  magnesium  sulphate  to  saturation, 
the  caseinogen  is  precipitated  as  such,  and  after  the  removal  of  the 
fat  with  which  it  is  entangled  may  be  collected  by  appropriate  chemic 
methods.  On  the  addition  of  rennet  prepared  from  the  mucous 
membrane  of  the  calf's  stomach,  which  contains  the  enzyme  rennin 
or  pexin,  the  caseinogen  undergoes  cleavage  into  an  insoluble  proteid, 
casein  or  tyrein,  and  a  small  quantity  of  a  new  soluble  proteid.  To 
this  process  the  term  coagulation  has  been  given.  The  presence  of 
calcium  phosphate  appears  to  be  essential  to  this  process,  inasmuch 
as  it  does  not  take  place  if  the  milk  be  completely  decalcified  by  the 
addition  of  potassium  oxalate.  x\fter  coagulation,  the  more  or  less 
solid  mass  of  milk  separates  into  a  liquid  portion,  the  serum,  and  a 
solid  portion,- the  coagulum.  The  former,  generally  termed  whey, 
consists  of  water,  salts,  lactalbumin,  sugar;  the  latter,  the  curd,  con- 
sists of  the  casein  and  entangled  fat.  Boiling  the  milk  retards  and 
even  prevents  the  coagulation  by  rennet,  owing  to  the  precipitation 
of  the  calcium  phosphate.  When  milk  is  taken  into  the  stomach,  it 
is  probable  that  the  rennin  coagules  the  caseinogen  in  a  manner  similar 
to,  if  not  identical  with,  this  process,  which  appears  to  be  essential  to 
the  normal  digestion  of  the  milk. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures. 
It  is  a  compound  of  olein,  palmitin,  and  stearin  with  small  quantities 
of  butyrin  and  caproin.  The  melting-point  of  butter  varies  between 
31  °  and  340  C.  When  milk  is  allowed  to  stand  for  some  time,  the 
fat-globules  rise  to  the  surface  and  form  a  thick  lavcr  known  as  cream. 


452 


TEXT-BOOK  OF  PHYSIOLOGY. 


Churning  the  milk  or  cream  causes  the  fat-globules  to  run  together 
and  form  a  coherent  mass  termed  butter. 

Lactose  is  the  particular  form  of  sugar  characteristic  of  milk. 
It  belongs  to  the  saccharose  group  and  has  the  following  composition: 
CI2H22OXI.  Though  incapable  of  undergoing  fermentation  by  the 
action  of  the  yeast  plant,  it  is  readily  reduced  by  the  Bacillus  acidi 
lactici  to  lactic  acid  and  carbon  dioxid,  the  former  of  which  imparts 
to  milk  an  acid  reaction  and  a  sour  taste.  With  the  accumulation  of 
the  lactic  acid  the  caseinogen  is  precipitated  as  a  more  or  less  consistent 
mass. 

The  inorganic  salts  of  milk  are  chiefly  potassium,  sodium, 
calcium,  and  magnesium  phosphates  and  chlorids.  Iron  is  also 
present  in  small  amount.  The  following  table  of  Bunge  gives  the 
quantitative  amounts  of  these  constituents  in  both  human  and  cow's 
milk: 


In  iooo  Parts. 

Potas- 
sium. 

Sodium. 

Calcium. 

Magne- 
sium. 

Iron 
Oxid. 

Phos- 
phoric 
Acid. 

Chlorin. 

0.78 
1.76 

0.25 
1. 11 

°-33 
1-59 

0.06 
0.21 

0.0036 
0.0030 

0.47 
1.97 

°-43 
1.69 

Cow's  milk, 

Mechanism  of  Milk  Secretion. — During  the  time  of  lactation 
the  mammary  gland  exhibits  periods  of  secretory  activity  which 
alternate  with  periods  of  repose.  Coincidently  with  these  periods 
certain  histologic  changes  take  place  in  the  secreting  epithelium 
At  the  close  of  a  period  of  active  secretion  and  after  the  discharge  of 
the  milk  each  acinus  presents  the  following  features:  The  epithelial 
cells  are  short,  cubical,  nucleated,  and  border  a  relatively  wide  lumen, 
in  which  is  found  a  variable  quantity  of  milk.  After  the  gland  has 
rested  for  some  time  active  metabolism  again  begins.  The  cells  grow 
and  elongate;  the  nucleus  divides  into  two  or  three  new  nuclei;  con- 
striction takes  place  and  the  inner  portion  is  detached  and  discharged 
into  the  lumen  of  the  acinus.  During  the  time  these  changes  are 
taking  place  oil-globules  make  their  appearance  in  the  cell  protoplasm, 
some  of  which  are  discharged  separately  into  the  lumen,  while  others 
remain  for  a  time  associated  with  the  detached  portion  of  the  cell 
(Fig.  208).  From  these  histologic  changes  it  is  inferred  that  the  case- 
inogen and  fat  are  products  of  the  metabolism  of  the  cell  protoplasm 
and  not  derived  directly  through  the  lymph  from  the  blood.  The 
lactose  apparently  has  a  similar  origin,  as  appears  from  the  fact  that 
it  is  not  found  either  in  the  blood  or  any  other  tissue,  and  that  it  is 
formed  independently  of  carbohydrate  food.  The  water,  and  especially 
the  inorganic  salts,  are  the  result  of  secretory  activity  rather  than  of 
rl illusion  and  filtration.  This  is  rendered  probable  from  the  fact  that 
the  proportions  of  the  inorganic  salts  of  milk  are  more  closely  allied 
to  those  of  the  tissues  of  the  new-born  child  than  to  blood.     With  the 


SECRETION.  453 

passage  of  the  water  and  salts  into  the  lumen  of  the  acinus  the  proteids 
undergo  disintegration  and  solution  and  the  liquid'  assumes  the  charac- 
teristics of  milk. 

The  discharge  of  milk  is  occasioned  by  the  suction  efforts  on  the 
part  of  the  child,  aided  by  atmospheric  pressure  and  the  contractions 
of  the  non-striated  muscle-fibers  of  the  lactiferous  ducts. 

Influence  of  the  Nerve  System. — Judging  from  analogy,  it  is 
probable  that  the  secretion  of  milk  is  regulated  by  influences  emanating 
from  the  nerve  system,  though  the  exact  nerve-channels  for  the  trans- 
mission of  such  influences  have  not  been 
determined    experimentally.       Various    at- 
tempts   have    been    made   to   isolate   and 
study  these  nerves,  but  the  results  are  in- 
conclusive.     It  is  well  known  that  emo- 
tional   states    on  the  part  of   the   mother 
modify  the  quantity  as  well  as  quality  of 
milk,  indicating  a  connection  between  the 
gland-cells  and  the  central  organs  of  the 
nerve  system.      Nerve  terminals  have  been 
discovered    in    and    around    the  epithelial  Fig.  208.— Section  of  the 

cells — a  fact  which  supports  this  view.  Mammary  Gland  of  a  Cat 

Colostrum.— Within  a  day  or  two  after      IN  THE  Early  Stages  of  Lac- 

J  _..  tation.     A.  Cavitv  of  alveoli 

parturition  the  alveoli  become  filled  with  a  filled  with  granule's  and  glob- 
fluid  which  in  some  respects  resembles  ules  of  fat-  *>  2>  3-  Epithe- 
milk  and  which  has  been  termed  colostrum.  l£Ziti™-(YeoTS  °f  ^ 
This  is  a  watery  fluid  containing  disinte- 
grated epithelial  cells,  fat-globules,  as  well  as  colostrum  corpuscles, 
which  are  probably  emigrated  leukocytes.  Colostrum  is  distinguished 
from  milk  in  being  richer  in  sugar  and  inorganic  salts.  It  is  said  to 
possess  constituents  which  act  as  a  laxative  to  the  young  child. 

THE   LIVER. 

The  liver  is  a  large  gland  situated  in  the  upper  and  right  side 
of  the  abdominal  cavity,  where  it  is  held  in  position  largely  by  liga- 
ments formed  by  reduplications  of  the  peritoneal  investment.  In 
the  adult  it  weighs,  freed  of  blood,  from  1300  to  1700  grams.  The 
liver  is  connected  with  the  duodenal  portion  of  the  intestine  by  the 
hepatic  duct.  It  receives  blood  both  from  the  hepatic  artery  and  from 
the  portal  vein,  and  in  this  respect  differs  from  all  other  glands  in 
the  body.  The  epithelial  structures  of  the  liver  are  inclosed  by  a 
firm  fibrous  membrane,  known  as  Glisson's  capsule.  At  the  trans- 
verse fissure  it  invests  and  follows  the  blood-vessels,  which  there 
enter,  in  all  their  ramifications  through  the  gland. 

Structure  of  the  Liver. — The  liver  is  composed  of  an  enormous 
number  of  small  masses,  rounded,  ovoid,  or  polygonal  in  shape,  called 
lobules,  measuring  about  one  millimeter  in  diameter  and  separated 


454 


TEXT-BOOK  OF  PHYSIOLOGY. 


from  one  another  by  a  narrow  space  in  which  are  to  be  found  blood- 
vessels, lymphatics,  hepatic  ducts,  supported  by  connective  tissue. 
In  the  pig  this  space  and  its  contained  elements  is  quite  distinct, 
sharply  marking  out  the  border  of  the  lobule  (Fig.  209).  This  is  not 
so  apparent  in  man.  Each  lobule  is  made  up  of  irregular  or  polygonal 
shaped  cells  measuring  about  30  to  40  micromillimeters  in  diameter. 
These  cells  are  arranged  in  a  radial  manner  from  the  center  to  the 
circumference  of  the  lobule  (Fig.  210).  Each  cell  possesses  one  and 
at  times  two  nuclei.  There  is  no  evidence  for  the  existence  of  a  distinct 
cell-wall.  The  cell  protoplasm  frequently  contains  globules  of  fat, 
granules  of  a  proteid  nature,  granules  of  glycogen,  pigment  material, 

etc.  The  appearance 
presented  by  the  cell  will 
vary  considerably,  ac- 
cording to  the  time  it  is 
observed.  Thus  there 
may  be  a  complete  ab- 
sence of  these  constitu- 
ents, when  the  cell  may 
present  a  series  of  vacu- 
oles separated  by  bands 
of  protoplasm.  The 
cells  are  the  secreting 
structures  of  the  liver, 
and  hence  are  in  close 
relation  with  capillary 
blood-vessels,  lymphatic 
spaces,  nerves,  and  ir- 
regular channels  or  pas- 
sageways. The  latter 
running  between  the  epithelial  cells  may  be  compared  to  the  lumen  of 
other  secreting  glands. 

Blood-vessels  and  Their  Distribution.— The  blood-vessels 
which  are  in  relation  with  the  liver  are: 

1.  The  portal  vein. 

2.  The  hepatic  artery. 

3.  The  hepatic  vein. 

The  portal  vein  and  the  hepatic  artery  enter  the  liver  at  the  trans- 
verse fissure.  After  penetrating  its  substance  they  divide  and  sub- 
divide into  smaller  and  smaller  branches,  which  ultimately  occupy 
the  space  between  the  lobules,  completely  surrounding  and  limiting 
them.  From  their  situation  they  are  termed  interlobular  veins  and 
arteries. 

The  interlobular  veins  give  off  small  capillary  vessels  which  pene- 
trate the  lobule  at  all  points  of  its  surface.  These  capillaries,  though 
frequently  anastomosing,  form  a  radial  meshwork  which  converges 
toward  the  center  of  the  lobule.      In  the   meshes  of  this  plexus  are 


i:^i 


Fig.  209. — Section  of  Liver  of  Pig,  showing 
very  Diagrammatically  the  Lobules,  a.  Interlobu- 
lar connective  tissue,  b,  c.  Branches  of  portal  vein 
and  of  hepatic  artery,  d.  Bile- ducts,  e.  Intralobular 
vein. — (Pier  sol.) 


SECRETION. 


455 


■  Trabecule  of 
hepatic  cells. 


Central  vein. 


found,  arranged  in  a  corresponding  radial  manner,  the  liver  cells. 
The  interlobular  arteries  are  distributed  to  the  walls  of  the  portal 
vein,  to  the  connective  tissue,  and  finally  terminate  in  the  portal  vein 
capillaries.  The  intralobular  capillaries  thus  receive  and  transmit 
blood  which  is  an  admixture  of  both  arterial  and  venous  blood.  In 
the  center  of  each  lobule  there  is  a  large  vein,  formed  by  the  union  of 
the  intralobular  capillaries,  known  as  the  intralobular  vein,  which 
collects  all  the  blood  of  the  lobule  and  transmits  it  through  the  lobule 
to  an  underlying  or  sublobular  vein  (Fig.  211).     These  latter  vessels, 

uniting  and  reuniting,  ultimately 
form  the  hepatic  vein,  which  empties 
the  blood  into  the  inferior  vena  cava. 
Bile  Capillaries  and  Hepatic 
Ducts. — The  bile  capillaries  are 
narrow  channels  which  penetrate 
the  lobule  in  all  directions  and  are 
generally  found  running  along  the 
sides  of  the  cells.  These  channels, 
which  are  devoid  of  walls,  receive 
from  the  cells  some  of  the  products 
of  their  secretory  activity,  and  hence 
are  comparable  to  the  lumen  of  the 
alveoli  of  other  secreting  glands. 
At  the  periphery  of  the  lobules  the 
bile  capillaries  communicate  with 
larger  channels  which  are  the  begin- 
nings of  the  hepatic  or  bile-ducts 
lying  in  the  interlobular  spaces.  The 
interlobular  bile-ducts  possess  a  dis- 
tinct wall  lined  by  flattened  epithe- 
lium. There  is,  however,  no  distinct 
line  of  demarcation  between  the  cells 
of  the  interlobular  ducts  and  the 
secreting  cells  of  the  liver  proper,  as 
the  two  blend  insensibly,  the  one  into  the  other.  As  the  hepatic  ducts 
increase  in  size  they  gradually  acquire  the  structure  characteristic  of 
the  main  hepatic  duct:  viz.,  a  mucous,  a  muscle,  and  a  fibrous  coat. 

Nerves. — Experimental  investigations  have  demonstrated  that 
the  liver  is  supplied  with  nerves  derived  from  the  central  nerve  system. 
The  route  of  these  nerves  is  probably  by  way  of  the  splanchnics 
and  the  vagi.  Many  of  the  nerves  which  enter  the  liver  are  vaso-motor 
in  function;  as  to  whether  others  are  secretory  in  character  is  yet  a 
subject  of  investigation.  It  has  been  asserted  that  nerve  terminals 
have  been  demonstrated  running  between  the  cells  and  even  pene- 
trating their  substance.  This  fact  would  indicate  that  the  metabolic 
processes  of  the  liver  are  under  the  control  of  the  central  nerve  system. 
Functions   of   the  Liver. — The  anatomic  and  histologic  pecul- 


Interlobular  vein.    Hepatic  duct. 

Fig.  210. —Scheme  of  a  Hepatic 
Lobule,  represented  in  Transverse 
Section  below  and,  by  Partial  Lev- 
eling, in  Longitudinal  Section 
Above.  In  the  left  half  the  blood- 
vessels are  drawn;  in  the  right  half 
only  the  cords  of  hepatic  cells.  X  2°- 
—(Stohr.) 


456  TEXT-BOOK  OF  PHYSIOLOGY. 

iarities  of  the  liver  would  indicate  that  it  has  a  variety  of  relations  to 
the  general  processes  of  the  body.     Experimental  investigation  has 
brought   some   of  these   relations  to   light.     Though   its   physiologic 
actions  are  not  yet  wholly  understood,  it  may  be  said  that  it — 
i.  Secretes  bile. 

2.  Produces  and  stores  glycogen. 

3.  Assists  in  the  formation  of  urea. 

Secretion  of  Bile. — The  physical  properties  and  chemic  com- 
position of  the  bile  have  already  been  considered  (page  202).  The 
characteristic  salts  of  the  bile,  sodium  glycochlate  and  taurocholate, 
do  not  pre-exist  in  the  blood,  and  therefore  must  be  formed  by  the  liver 
cells  out  of  materials  derived  from  the  blood  of  the  intralobular  capil- 


Fxg.  211. — Transverse  Section  of  a  Single  Hepatic  Lobule,  i.  Intralobular 
vein,  cut  across.  2,  2,  2,  2.  Afferent  branches  of  the  intralobular  vein.  3,  3,  3,  3,  3,  3, 
3,  3,  3.  Interlobular  branches  of  the  portal  vein,  with  its  capillary  branches,  forming  the 
lobular  plexus,  extending  to  the  radicles  of  the  intralobular  vein. — (Sappey.) 

larics.  The  antecedents  of  the  bile  salts,  glycocoll  and  taurin,  are 
crystallizable  nitrogenized  compounds,  and  known  chemically  as 
amido-acetic  and  amido-ethylsulphonic  acids.  Their  chemic  composi- 
tion indicates  that  they  are  derivatives  of  the  proteids  or  the  albu- 
minoids, though  the  intermediate  stages  in  their  production  are  un- 
known. The  origin  of  the  cholalic  acid  with  which  they  are  com- 
bined  is  equally  obscure.  The  bile  salts  as  they  are  found  in  the  bile 
are  produced  in  the  liver  cells  by  metabolic  activity. 

The  primary  coloring-matter  of  the  bile,  bilirubin,  has  been  shown 
to  be  a  derivative  of  hematin,  a  product  of  the  disintegration  of  hemo- 
globin. It  is  supposed  that  the  liver  cells  bring  about  this  change 
by  combining  water  with  hematin,  with  the  abstraction  of  iron.  The 
prod  ml  thus  formed  is  bilirubin,  which  is  excreted,  while  the  iron  is 
for  1  he  mosl  pari  retained. 


SECRETION.  457 

Cholesterin  is  a  waste  product  derived  largely  from  the  nerve- 
tissue.  It  is  brought  to  the  liver  and  simply  excreted  by  the  cells. 
The  remaining  constituents  of  the  bile,  water  and  inorganic  salts,  are 
secreted  here  as  in  all  other  glands. 

When  once  formed,  the  liver  cells  discharge  these  various  com- 
pounds into  the  channels  by  which  they  are  surrounded;  they  then 
pass  into  the  open  mouths  of  the  bile-ducts  at  the  periphery  of  the 
lobules.  Under  the  increasing  pressure  which  arises  from  the  secre- 
tion and  accumulation  of  bile,  this  fluid  flows  from  the  smaller  into  the 
larger  bile-ducts,  and  finally  is  emptied  either  directly  into  the  in- 
testine or  into  the  gall-bladder,  where  it  is  stored  until  required  for 
digestive  purposes.  The  secretion  of  bile,  as  observed  by  means  of 
a  biliary  fistula,  is  continuous  and  not  intermittent,  though  the  rate  of 
flow  is  subject  to  considerable  variation. 

The  liver  cells,  as  far  as  the  secretion  of  bile  is  concerned,  appear 
to  be  independent  of  the  nerve  system.  Their  activity,  however, 
is  stimulated  by  the  increased  blood-supply  which  arises  during  di- 
gestion in  consequence  of  the  dilatation  of  the  intestinal  vessels, 
since  it  is  at  this  period  that  the  rate  of  discharge  is  the  greatest. 
The  same  results  have  been  shown  by  experiment.  Thus,  division 
of  the  splanchnic  nerves  is  followed  by  an  increased  discharge  of 
bile,  apparently  due  to  the  dilatation  of  the  portal  vessels;  stimula- 
tion of  their  peripheral  ends  is  followed  by  a  decreased  discharge  of 
bile  in  consequence  of  the  contraction  of  the  portal  vessels.  The 
bile  salts  appear  to  be  the  most  efficient  stimulants  to  the  activity  of 
the  liver  cells,  for  their  administration  and  absorption  is  followed  by 
an  increase  not  only  in  the  amount  of  water,  but  of  the  inorganic 
salts  and  other  solid  constituents  as  well. 

The  flow  of  bile  from  the  bile  capillaries  to  the  main  hepatic 
duct,  though  primarily  dependent  on  differences  of  pressure,  is  aided 
by  the  contraction  of  the  muscular  walls  of  the  bile-ducts  and  the 
inspiratory  movements  of  the  diaphragm.  Any  obstacle  to  the  dis- 
charge of  bile  leads  to  its  accumulation,  a  rise  of  pressure  beyond 
that  of  the  capillary  blood-vessels,  and  a  reabsorption  by  the  lymph- 
vessels  of  the  bile  constituents.  After  their  discharge  into  the  blood 
from  the  thoracic  duct  these  constituents  are  deposited  in  part  in 
various  tissues,  giving  rise  to  the  phenomena  of  jaundice,  and  in  part 
are  eliminated  in  the  urine. 

The  Production  of  Glycogen. — In  1857  Bernard  discovered  the 
fact  that  the  liver  normally  during  life  produces  a  sugar-forming 
substance,  analogous  in  its  chemic  composition  to  starch,  to  which  he 
gave  the  name  glycogen.  This  substance  can  be  obtained  by  the 
following  method:  Small  pieces  of  the  liver  of  an  animal  recently 
killed,  preferably  after  a  meal  rich  in  carbohydrates,  are  placed  in 
acidulated  boiling  water  for  a  few  minutes;  then  rubbed  up  in  a 
mortar  with  sand,  again  boiled,  after  which  the  proteids  are  removed 
by  filtration.     The  filtrate  thus  obtained  is  opalescent  and  resembles 


458  TEXT-BOOK  OF  PHYSIOLOGY. 

a  solution  of  starch.  The  glycogen  may  be  precipitated  from  this 
solution  with  alcohol  as  a  white  amorphous  powder,  soluble  in  water. 
Chemic  analysis  shows  that  it  consists  of  C6HioO-,  or  a  multiple  of  it. 

When  either  the  original  solution  obtained  by  boiling  or  a  solution 
of  this  amorphous  powder  is  treated  with  iodin,  it  strikes  a  port-wine 
color.  When  digested  with  saliva,  pancreatic  juice,  or  boiled  with 
dilute  acids,  the  solution  becomes  clear,  and  testing  with  Fehling's 
solution  reveals  the  presence  of  sugar. 

If  the  liver  be  allowed  to  remain  in  the  body  of  an  animal  for  a 
period  of  twenty-four  hours  before  the  decoction  is  made  as  above 
described,  it  will  be  found  that  the  solution  contains  only  a  small 
amount  of  glycogen  but  a  relatively  large  amount  of  sugar.  The 
inference  drawn  is  that  after  death  the  glycogen  is  transformed  by 
some  agent,  possibly  a  ferment,  into  sugar  (dextrose). 

The  presence  of  glycogen  in  the  liver  cells  can  be  shown  micro- 
scopically in  the  form  of  discrete  hyaline  and  refractive  granules. 
As  they  are  soluble  in  water  they  can  be  readily  dissolved  out  from 
the  cells,  leaving  small  vacuoles  separated  from  one  another  by  strands 
of  cell  substance.  The  amount  of  glycogen  in  a  well-fed  animal  varies 
from  1.5  to  4  per  cent,  of  the  total  weight  of  the  liver.  The  production 
of  glycogen  is  dependent  very  largely  on  the  consumption  of  car- 
bohydrates, the  greater  the  amount  of  sugar  and  starch  in  the  food, 
the  greater  being  the  production  of  glycogen.  On  a  pure  proteid  diet 
it  is  still  produced,  though  in  small  amounts. 

Glycogen  is  also  found  in  muscles,  placenta  and  embryonic  tissues 
generally.  Muscles  contain  from  0.5  per  cent,  to  1  per  cent,  and  as 
they  amount  to  a.bout  40  per  cent,  of  the  weight  of  the  body,  70  kilo- 
grams, they  contain  from  140  to  280  grams  of  glycogen.  During 
periods  of  prolonged  activity  of  the  muscles  the  percentage  of  glycogen 
rapidly  diminishes,  a  fact  'which  leads  to  the  inference  that  it  is  the 
source  in  large  part  of  the  energy  expended  by  the  muscle.  During 
rest  the  percentage  of  glycogen  again  rapidly  increases  until  the 
normal  percentage  is  regained.  The  source  of  the  muscle  glycogen 
is  undoubtedly  the  liver  in  which  it  is  temporarily  stored. 

The  facts  connected  with  the  formation  of  glycogen,  as  well  as  its 
disposition  as  at  present  generally  accepted,  may  be  stated  as  follows: 
The  dextrose  into  which  the  carbohydrates  arc  converted  by  the 
action  of  the  digestive  fluids  is  absorbed  into  the  blood  of  the  portal 
vein  and  carried  direct  into  the  liver,  where  by  the  action  of  the  cells 
it  is  abstracted,  dehydrated,  and  temporarily  deposited  under  the 
form  of  the  non-diffusible  body  glycogen.  At  a  subsequent  period  and 
in  proportion  to  the  needs  of  the  system  the  liver  cells,  through  the 
agency  of  a  ferment,  transform  the  glycogen  into  dextrose,  return  it  to 
the  circulation,  by  which  it  is  transported  to  the  systemic  capillaries, 
where  it  disappears.  The  blood  of  the  hepatic  vein  therefore  contains 
more  sugar  than  the  blood  of  any  other  part  of  the  body,  and  the 
blood  of  l he  a ri cries  more  than  the  blood  of  the  other  veins.     Should 


SECRETION.  459 

there  be  a  failure  on  the  part  of  the  liver  cells  to  abstract  the  sugar, 
it  would  pass  through  the  liver  into  the  general  circulation,  from  which 
it  would  be  eliminated  by  the  kidneys.  The  final  fate  of  the  sugar 
is  uncertain.  It  is,  however,  probable  that  after  its  delivery  to  the 
muscles,  for  example,  it  may  be  directly  oxidized,  or  stored  as  glycogen 
or  possibly  utilized  in  the  formation  of  living  material.  Ultimately, 
however,  through  oxidation  it  yields  heat,  and  contributes  to  the  pro- 
duction of  muscle  energy. 

In  opposition  to  this  view,  Dr.  Pavy,  after  years  of  accurate  ex- 
perimentation, states  that  the  blood  on  the  cardiac  side  of  the  liver 
never  under  normal  circumstances  contains  a  larger  percentage  of 
sugar  than  is  to  be  found  in  any  part  of  the  circulation,  except  in  the 
portal  vein.  He  states  that  glycogen  is  never  reconverted  into  sugar, 
and  denies  that  the  liver  produces  sugar,  to  be  discharged  into  the 
blood;  that  the  function  of  the  liver  is  merely  to  arrest  the  passage  of 
sugar,  and  so  to  shield  the  general  circulation  from  an  excess;  that  the 
sugar  which  arises  in  the  liver  after  death  is  a  post-mortem  product 
and  not  an  illustration  of  what  takes  place  during  life.  Dr.  Pavy, 
having  apparently  demonstrated  the  glucoside  constitution  of  proteid 
material  in  general,  accounts  for  the  presence  of  glycogen  in  muscles 
and  other  tissues  on  the  assumption  that  during  the  cleavage  of  the 
proteid  molecule  the  carbohydrate  element  is  set  free  and  temporarily 
stored  as  glycogen.  He  thus  accounts  for  the  production  of  sugar 
in  the  body,  even  in  the  absence  of  all  sugar  and  starch  from  the  food. 
Pavy  believes  that  the  glycogen  produced  in  the  liver  is  utilized  in  the 
formation  of  fat  and  the  synthesis  of  complex  proteids  necessary  to 
the  construction  of  the  tissues. 

The  Influence  of  the  Nerve  System. — The  results  of  various  ex- 
perimental investigations  indicate  that  the  production  of  sugar  from 
the  glycogen  in  the  liver  is  influenced  by  the  activities  of  the  nerve  sys- 
tem. It  was  discovered  by  Bernard  that  puncture  of  the  floor  of  the 
fourth  ventricle,  at  a  point  between  the  acoustic  and  vagus  nerves,  near 
the  middle  line,  is  followed  within  an  hour  or  two  by  the  appearance  of 
sugar  in  the  urine,  which  lasted  for  from  five  to  six  hours  in  the  rab- 
bit and  from  two  to  three  or  even  seven  in  the  dog.  For  this  reason 
Bernard  gave  to  this  area  the  name  of  "diabetic  area." 

Coincident  with  the  appearance  of  sugar  in  the  urine  (glycosuria) 
there  is  an  increase  in  the  percentage  of  sugar  in  the  blood  (hyper- 
glycemia). The  liver  at  the  same  time  contains  a  higher  percentage 
of  sugar  than  normally.  Apparently  the  initial  step  in  this  series  of 
phenomena  is  an  increased  conversion  of  glycogen  into  sugar.  This 
supposition  receives  support  from  the  fact  that  the  degree  of  the  hyper- 
glycemia, and  the  subsequent  glycosuria,  will  depend  on  the  amount  of 
glycogen  previously  in  the  liver.  If  the  animal  has  been  well  fed  on 
carbohydrates,  the  resulting  glycosuria  will  be  pronounced;  if,  on  the 
contrary,  it  has  been  allowed  to  fast  for  several  days,  the  glycosuria 
will  be  slight. 


460  TEXT-BOOK  OF  PHYSIOLOGY. 

Assuming  that  the  nerve-cells  which  constitute  the  diabetic  area 
influence  the  conversion  of  glycogen  into  sugar,  the  question  arises  as 
to  whether  the  puncture  destroys  the  nerve-cells,  or  whether  it  stimulates 
them  to  increased  activity.  The  results  of  experiment  lead  to  the 
latter  supposition.  Thus  if  the  vagus  nerve  is  divided  in  the  neck  and 
its  central  end  stimulated  there  is  developed  a  glycosuria.  Stimulation 
of  other  sensor  nerves  has  a  similar  effect.  As  stimulation  of  the  vagus 
has  the  same  effect  as  the  puncture,  the  inference  is  that  the  center  is 
normally  excited  to  physiologic  activity  by  impulses  reflected  from 
some  surface  or  organ  in  the  peripheral  distribution  of  this  nerve. 

If  the  nerve-cells  in  the  diabetic  area  regulate  the  production  of 
sugar  in  the  liver,  the  further  question  arises  as  to  the  pathway  through 
which  the  nerve  impulses  emanating  from  them  reach  the  liver,  whether 
by  way  of  the  vagi  or  by  way  of  the  spinal  cord  and  splanchnic  nerves. 
That  it  is  not  by  way  of  the  vagi  is  shown  by  the  fact  that  the  glyco- 
suria established  by  the  puncture  does  not  disappear  when  they  are 
divided;  that  it  is  by  way  of  the  spinal  cord,  as  far  at  least  as  the 
first  dorsal  nerve,  and  subsequently  the  splanchnic  nerves,  is  indicated 
by  the  fact  that  a  cross- section  of  the  spinal  cord  above  this  level,  destruc- 
tion of  the  upper  three  dorsal  roots  as  well  as  division  of  the  splanchnic 
nerves  prevents  the  development  of  the  glycosuria  which  follows  punc- 
ture of  the  medulla.  Though  stimulation  of  the  upper  dorsal  (pre- 
ganglionic) nerve-fibers  gives  rise  to  glycosuria,  yet,  contrary  to 
expectation,  stimulation  of  the  splanchnic  (post-ganglionic)  nerve- 
fibers  does  not  have  the  same  effect.  This  may  be  due,  however,  to 
changes  in  the  relation  of  the  capillary  blood-vessels  to  the  liver  cells 
or  to  the  character  of  the  stimulus  employed. 

A  further  question  arises  as  to  whether  the  nerve  impulses  which 
pass  from  the  diabetic  center  to  the  liver  are  vaso-motor  in  character 
and  exerting  their  effect  on  the  blood-vessels,  or  whether  they  are  secretor 
in  character  and  exerting  their  effect  on  the  liver  cell.  Bernard  was 
of  the  opinion  that  they  are  vaso-motor  in  character  and  that  the  dia- 
betic area  was  a  part  of  the  general  vaso-motor  center.  More  recent 
investigators  are  of  the  opinion  that  they  are  secretor  in  character,  for 
the  reason  that  whether  the  blood-pressure  rises  from  a  stimulation 
of  the  central  end  of  the  divided  vagus,  or  falls  from  a  stimulation 
of  the  depressor  nerve,  in  each  instance  there  follows  a  glycosuria. 

If  the  production  of  sugar  in  the  liver  is  a  reflex  act  as  Bernard 
supposed,  taking  place  through  a  mechanism  consisting  of  an  afferent 
pathway,  the  vagus  nerve,  and  an  efferent  pathway  consisting  of  the 
spinal  cord  and  splanchnic  nerves,  the  question  arises  as  to  the  seat 
of  action  of  the  stimulus.  This  Bernard  located  in  the  lungs,  for  the 
reason  that  though  division  of  the  vagus  in  the  neck  checks  the  produc- 
tion of  the  sugar,  division  below  the  origin  of  the  pulmonary  branches 
had  no  such  effect. 

Diabetes.-  Diabetes  is  a  chronic  disease  characterized  by  the 
appearance  of  sugar  in  the  urine  in  variable  amounts.     This  patho- 


SECRETION.  461 

logic  condition  has  usually  been  associated  with  derangements  of  the 
glycogen  function  of  the  liver,  though  doubtless  derangements  of 
other  organic  functions  will  produce  the  same  condition.  At  the 
present  time  it  is  believed  that  the  excretion  of  sugar  by  the  kidneys 
depends  on  two  causes:  (1)  An  ineffectual  abstraction  and  storage 
of  sugar  due  to  some  impairment  in  the  activity  of  the  liver  cells;  (2) 
a  rapid  cleavage  of  the  proteid  constituents  of  the  tissues,  in  consequence 
of  some  profound  alteration  in  the  nutritive  process,  whereby  their 
glucose  radicals  are  liberated  in  unusual  amounts.  The  physiologic 
mechanism  by  which  the  normal  metabolism  of  the  carbohydrates 
is  regulated  is  unknown.  That  it  is  complex  in  character  is  shown 
by  the  phenomena  which  follow  not  only  puncture  of  the  medulla, 
but  also  removal  of  the  pancreas  and  the  administration  of  various 
toxic  agents. 

Removal  of  the  pancreas  from  the  body  of  a  dog  or  other  animal 
is  at  once  followed  by  a  rise  in  the  percentage  of  sugar  in  the  blood 
and  its  elimination  by  the  kidneys.  In  a  short  time  acetone,  aceto- 
acetic  and  oxybutyric  acids  make  their  appearance,  attended  by  the 
usual  symptoms  characteristic  of  glycosuria  in  man.  The  quantity 
of  sugar  excreted  and  the  gravity  of  the  attendant  symptoms  may  be 
much  diminished  by  allowing  a  portion  of  the  gland  to  remain  in  situ. 
even  though  its  capacity  for  the  production  of  pancreatic  juice  is  en- 
tirely abolished.  Transplantation  of  the  pancreas  to  the  subcutaneous 
tissue  or  to  the  abdominal  cavity  will  practically  prevent  the  glycosuria. 
The  explanations  which  have  been  offered  as  to  the  manner  in  which 
the  pancreatic  tissue  prevents  and  its  absence  gives  rise  to  the  ex- 
cretion of  sugar  are  purely  hypothetical.  It  has  been  claimed  by  some 
investigators  that  the  pancreas  secretes  a  specific  material,  which  enters 
the  blood  and  promotes  oxidation  of  the  sugar.  In  the  absence  of  this 
material  the  sugar  accumulates,  and  is  finally  eliminated  by  the  kidneys. 
Since  the  discovery  of  the  islands  of  Langerhans  it  has  been  suggested 
by  some  investigators  that  the  production  of  the  material  which  regu- 
lates carbohydrate  metabolism  should  be  attributed  to  them  rather  than 
to  the  pancreas  as  a  whole.  The  sugar  excreted  doubtless  in  part 
comes  from  the  glycogen  of  the  liver,  as  this  disappears  in  a  short  time. 
But  as  sugar  continues  to  be  excreted,  even  though  all  carbohvdrates 
be  withdrawn  from  the  food,  the  conclusion  is  justifiable  that  it  arises 
in  consequence  of  increased  proteid  metabolism.  This  supposition  is 
strengthened  by  the  fact  that  the  quantity  of  urea  excreted  rises  and 
falls  with  the  quantity  of  sugar  excreted. 

Phloridzin,  a  glucoside  obtained  from  the  root  bark  of  the  cherry 
and  plum  tree,  gives  rise  to  the  appearance  of  sugar  in  the  urine,  in 
amounts  beyond  that  which  might  come  from  the  glucose  normally 
present  in  the  blood  or  from  the  glycogen  of  the  liver.  As  there  is  a 
concomitant  increase  in  the  amount  of  urea  excreted,  the  supposition 
is  that  phloridzin  increases  proteid  metabolism. 

Curara,  in  doses  sufficient  to  paralyze  the  muscles,  also  gives  rise 


462  TEXT-BOOK  OF  PHYSIOLOGY. 

to  the  appearance  of  sugar  in  the  urine.  This  is  not  due,  however, 
to  an  increased  production  on  the  part  of  the  liver,  but  rather  to  a 
want  of  consumption  on  the  part  of  the  muscles,  due  to  their  inac- 
tivity. The  accumulation  of  the  sugar  in  the  blood  which  takes  place 
for  this  reason  leads  very  promptly  to  its  removal  by  the  kidneys. 

The  Formation  of  Urea. — It  is  now  generally  believed  that  the 
liver  is  the  most  active  of  all  the  organs  which  may  be  engaged  in  the 
production  of  urea.  This  belief  is  based  on  numerous  physiologic 
and  pathologic  data.  The  compounds  out  of  which  the  hepatic  cells 
construct  urea  have  been  for  chemic  reasons  asserted  to  be  the  am- 
monium salts,  e.  g.,  the  carbonate,  lactate,  carbamate,  which  are 
constantlv  present  in  the  blood.  These  salts,  which  result  from 
proteid  metabolism,  may  be  absorbed  from  the  tissues  or  from  the  in- 
testines carried  to  the  liver,  and  there  synthesized  to  urea.  This 
supposition  is  supported  by  an  experiment  as  follows:  The  liver  of 
an  animal  recently  living  is  removed  from  the  body  and  its  vessels 
perfused  continuously  with  blood  (the  urea  content  of  which  is  known) 
containing  the  ammonium  salts.  An  analysis  of  this  blood  shows, 
after  a  time,  a  diminution  of  these  salts,  and  a  large  increase  in  the 
amount  of  the  urea.  After  the  establishment  of  an  Eck  fistula 
(the  union  of  the  portal  vein  with  the  ascending  vena  cave  whereby 
the  liver  is  largely  excluded  from  acting  on  products  absorbed  from 
the  intestines)  there  is  a  marked  diminution  in  the  production  of  urea 
while  the  ammonia  content  of  the  urine  largely  increases.  The  leucin 
and  tvrosin  which  result  from  the  prolonged  action  of  pancreatic  juice 
on  hemi-peptone  are  also  capable  of  being  converted  to  urea  by  the 
hepatic  cells,  and  in  all  probability  are  so  disposed  of.  Destructive 
diseases  of  the  liver — e.  g.,  acute  yellow  atrophy,  suppuration,  cirrhosis 
—largely  diminish  the  production  of  urea,  but  increase  the  quantities 
of  the  ammonium  salts  in  the  urine.  The  same  is  true  when  the  liver 
cells  are  destroyed  during  acute  phosphorus-poisoning. 

VASCULAR  OR  DUCTLESS  GLANDS. 

INTERNAL   SECRETIONS. 

The  metabolism  of  the  body  generally,  as  well  as  that  of  individual 
organs,  has  been  shown  to  be  related  not  only  to  the  physiologic  ac- 
tivity of  such  organs  as  the  liver  and  pancreas,  but  also  to  the  activity 
of  the  so-called  vascular  or  ductless  glands.  The  influence  of  the 
pancreas  in  regulating  the  production  of  glycogen  by  the  liver,  and 
the  influence  of  the  liver  in  the  maintenance  of  the  general  metabo- 
lism through  the  production  of  glycogen  and  the  formation  of  urea, 
are  now  established  facts.  That  the  vascular  or  duct  less  glands  to 
an  equal  extent,  though  perhaps  in  a  different  way,  assist  in  the  main- 
tenance of  physiologic  processes,  appears  certain  from  the  results 
of  animal  experimentation.     The  explanation  given  for  the  influence 


SECRETION. 


463 


of  these  glands  is  that  they  produce  specific  substances,  which  are 
poured  into  the  blood  or  lymph  and  carried  direct  to  the  tissues,  to 
the  activities  of  which  they  appear  to  be  essential;  for  without  these 
substances  the  nutrition  of  the  tissues  declines  and  in  a  short  time 
a  fatal  termination  ensues. 

Inasmuch  as  these  partly  unknown  substances  are  formed  by  cell 
activity  and  are  poured  into  the  interstices  of  the  tissues,  they  have 
been  termed  "internal  secretions."  Though  the  term  internal  secre- 
tions is  applicable  to  all  substances  which  arise  in  consequence  of 
tissue  metabolism,  and  which,  after  being  poured  into  the  blood, 
influence  in  varying  degrees 
and  ways  physiologic  proc- 
esses, yet  the  term  in  this 
connection  will  be  applied 
only  to  the  secretions  of  the 
thyroid  gland,  hypophysis 
cerebri,  and  adrenal  bodies. 

Thyroid  Gland.— The 
thyroid  gland  or  body  con- 
sists of  two  lobes  situated  on 
the  lateral  aspect  of  the  upper 
part  of  the  trachea  (Fig.  212). 
Each  lobe  is  pyriform  in 
shape,  the  base  being  directed 
downward  and  on  a  level  with 
the  fifth  or  sixth  tracheal  ring. 
The  lobe  is  about  50  mm.  in 
length,  20  mm.  in  breadth, 
and  25  mm.  in  thickness.     As 

a  rule,  the  lobes  are  united  by  a  narrow  band  or  isthmus  of  the  same 
tissue.  The  gland  is  reddish  in  color,  and  abundantly  supplied  with 
blood-vessels  and  lymphatics. 

Microscopic  examination  shows  that  the  thyroid  consists  of  an 
enormous  number  of  closed  sacs  or  vesicles,  variable  in  size,  the  largest 
not  measuring  more  than  0.1  mm.  in  diameter  (Fig.  213).  Each  sac  is 
composed  of  a  thin  homogeneous  membrane  lined  by  cuboid  epithe- 
lium. The  interior  of  the  sac  in  adult  life  contains  a  transparent, 
viscid,  fluid  containing  albumin  and  termed  "colloid"  substance. 
Externally,  the  sacs  are  surrounded  by  a  plexus  of  capillarv  blood- 
vessels and  lymphatics.  The  individual  sacs  are  united  and  sup- 
ported by  connective  tissue,  which  forms,  in  addition,  a  covering  for 
the  entire  gland. 

Function  of  the  Thyroid. — The  knowledge  at  present  possessed 
as  to  the  function  of  the  thyroid  gland,  especially  in  mammals,  is  the 
outcome  of  a  study  of  the  effects  Avhich  follow  its  arrested  development 
in  the  child,  its  degeneration  in  the  adult,  and  its  extirpation  in  the 
human  being  as  well  as  in  animals.     The  results,  however,  which  fol- 


Fig.  212. — View  of  Thyroid  Body.  i. 
Thyr°id  isthmus.  2.  Median  portion  of  crico- 
thyroid membrane.  3.  Crico-thyroid  muscle. 
4.  Lateral  lobe  of  thyroid  body. — {After  Morris.) 


464 


TEXT-BOOK  OF  PHYSIOLOGY. 


low  its  extirpation  are  not  always  uniform  in  all  animals,  though  suf- 
ficient reasons  for  the  lack  of  uniformity  can  not  always  be  assigned. 
Cretinism,  a  condition  characterized  by  a  want  of  physical  and 
mental  development,  is  associated  with,  if  not  directly  dependent  on, 
a  congenital  absence  of  the  thyroid,  or  its  arrested  development  dur- 
ing the  early  years  of  childhood. 

Myxedema,  a  condition  of  the  skin  in  which  there  is  a  hyperplasia 
of  the  connective  tissue,  of  an  embryonic  type,  rich  in  mucin,  is  gener- 
allv  regarded  as  one  of  the  effects  of  degenerative  processes  in  the 

thyroid.  Partly  in 
consequence  of  this 
change  in  the  skin  the 
face  becomes  broader, 
swollen,  and  flattened, 
giving  rise  to  a  loss  of 
expression.  At  the 
same  time  the  mind 
becomes  dull,  clouded, 
even  approximat- 
ing the  idiotic  type. 
This  supposed  infil- 
tration of  the  skin 
with  mucin  was 
termed  myxedema  by 
Ord,  who  at  the  same 
time  associated  it  with 
a  change  in  the  struc- 
ture of  the  thyroid  as 
a  result  of  which  it 
became  functionally 
useless. 

Extirpation  of  the 
thyroid,  for  relief  from  symptoms  due  to  grave  pathologic  changes, 
has  been  followed  in  human  beings  by  symptoms  similar  to  those  of 
myxedema.  To  this  condition  the  terms  operative  myxedema  and 
cachexia  strumipriva  have  been  applied. 

After  the  publication  of  the  history  of  the  myxedema  which  fol- 
lowed surgical  removal  of  the  thyroid,  Schiff,  in  1887,  repeated  his 
earlier  experiments  on  dogs,  and  found  again  that  removal  of  the  thyroid 
was  speedily  followed  by  tremors,  convulsions,  and  death.  Similar 
experiments  were  made  by  Horsley  on  monkeys,  with  results  which 
resembled  those  characteristic  of  myxedema.  Among  the  symptoms 
which  developed  within  a  few  days  after  the  removal  of  the  gland  may 
be  mentioned  loss  of  appetite;  fibrillar  contractions  of  muscles;  trem- 
ors and  spasms;  mucinoid  degeneration  of  the  skin,  giving  rise  to 
puffiness  of  the  eyelids  and  face  and  to  a  swollen  condition  of  the 
abdomen;   hebetude   of   mind,   frequently  terminating   in   idiocy;   fall 


Fig  213. — A  Lobule  from  a  Thin  Section  of  the 
Thyroid  Gland  of  an  Adult  Man.  i.  Colloid  sub- 
stance. 2.  Epithelium.  3.  Tangential  section  of  a  tubule, 
the  epithelium  viewed  from  the  surface.  4.  Tubule  in 
transverse  section.     5.   Connective  tissue. — (Stohr.) 


SECRETION.  465 

of  blood-pressure;  dyspnea;  albuminuria;  atrophy  of  the  tissues, 
followed  by  death  of  the  animal  in  the  course  of  from  five  to  eight 
weeks.  The  complexus  of  symptoms  observed  in  monkeys  was 
divided  by  Horsley  into  three  stages:  viz.,  the  neurotic,  the  mucinoid, 
and  the  atrophic. 

It  is  evident  that  the  pressure  of  the  thyroid  is  essential  to  the 
normal  activity  of  the  tissues  generally.  As  to  the  manner  in  which 
it  exerts  its  favorable  influence,  there  is  some  difference  of  opinion. 
The  view  that  the  gland  removes  from  the  blood  certain  toxic  bodies, 
rendering  them  innocuous  and  thus  preserving  the  body  from  a  species 
of  auto-intoxication,  is  gradually  yielding  to  the  more  probable  view 
that  the  epithelium  is  engaged  in  the  secretion  of  a  specific  material, 
which  finds  its  way  into  the  blood  or  lymph  and  in  some  unknown 
way  influences  favorably  tissue  metabolism.  This  view  of  the  function 
of  the  thyroid  is  supported  by  the  fact  that  successful  grafting  of  a 
portion  of  the  thyroid  beneath  the  skin  or  in  the  abdominal  cavity  will 
prevent  the  usual  symptoms  which  follow  thyroidectomy.  The  same 
result  is  obtained  by  the  intravenous  injection  of  thyroid  juice  or  by 
the  administration  of  the  raw  gland.  It  was  shown  by  Murray  that 
myxedematous  patients  could  be  benefited,  and  even  cured,  by  feeding 
them  with  fresh  thyroids  or  with  the  dry  extract. 

The  chemic  features  of  the  material  secreted  and  obtained  from 
the  structures  of  the  thyroid  indicate  that  it  is  a  complex  proteid  con- 
taining iodin,  which,  under  the  influence  of  various  reagents,  under- 
goes cleavage,  giving  rise  to  a  non-proteid  residue,  which  carries  with 
it  the  iodin  and  phosphorus.  The  amount  of  iodin  in  the  thyroid 
varies  from  0.33  to  1  milligram  for  each  gram  of  tissue.  To  this  com- 
pound the  term  thyroiodhi  has  been  given.  The  administration  of  this 
compound  produces  effects  similar  to  those  which  folloAv  the  therapeu- 
tic administration  of  the  fresh  thyroid  itself;  viz.,  a  diminution  of  all 
myxedematous  symptoms.  In  normal  states  of  the  body,  thyroiodin 
influences  very  actively  the  general  metabolism.  It  gives  rise  to  a  de- 
composition of  fats  and  proteids  and  to  a  decline  in  body-weight. 
In  large  doses  it  may  produce  toxic  symptoms,  e.  g.,  increased  cardiac 
action,  vertigo,  and  glycosuria. 

The  conclusions  as  to  the  functions  of  the  thyroid  gland  which 
have  been  drawn  from  the  results  that  have  followed  its  removal  from 
animals  by  surgical  procedures,  have  been  made  questionable,  since 
the  discovery  of  the  parathyroid  glands  and  a  study  of  the  phenomena 
which  follow  when  they  alone  are  removed.  From  their  situation  and 
close  relationship  to  the  thyroid  gland  it  is  generally  accepted,  that  in 
the  earlier  experiments,  especially  those  made  on  cats  and  dogs,  and 
some  other  carnivorous  animals,  both  sets  of  glands  were  removed 
and  hence  the  symptoms  which  developed  after  the  removal  of  the 
thyroids  were  due  to  the  loss  of  function  not  only  of  the  thyroid  but 
of  the  parathyroids  as  well. 

The  Parathyroids. — The  parathyroids  are  small  bodies,  usually 
3° 


466  TEXT-BOOK  OF  PHYSIOLOGY. 

four  in  number,  two  on  each  side.  They  are  divided  into  superior 
and  inferior.  The  superior  are  situated  internally  and  on  the  poste- 
rior surface  in  close  relation  to,  and  frequently  imbedded  in,  the  sub- 
stance of  the  thyroid;  the  inferior  are  situated  externally,  sometimes 
in  contact  with,  and  at  other  times  removed  a  variable  distance  from 
the  thyroid.  Microscopically  the  parathyroids  consist  of  thick  cords 
of  epithelial  cells  separated  by  septa  of  fine  connective  tissue  and  sur- 
rounded by  capillary  blood-vessels.  Chemic  analysis  shows  that  they 
also  contain  iodin  in  combination  with  some  organic  compound. 

Effects  of  Parathyroid  Removal. — The  surgical  removal  of  the 
parathyroids  is  followed  in  the  course  of  from  two  to  five  days  by  the 
death  of  the  animal  preceded  in  most  instances  by  a  series  of  symp- 
toms which  are  embraced  under  the  general  term  "tetany."  These 
symptoms  are  fibrillary  contractions  of  muscles,  tremors,  spasmodic 
contractions  and  paralyses  of  groups  of  muscles  and  not  infrequently 
convulsive  seizures  and  coma.  During  the  convulsion  there  is  an 
acceleration  of  the  heart-beat,  and  increase  in  the  respiratory  move- 
ments which  frequently  become  dyspneic  in  character.  There  is  also 
a  loss  of  appetite,  nausea,  mucous  vomiting,  and  diarrhea.  Death 
may  occur  during  a  convulsion  or  from  coma.     (Morat  and  Doyon.) 

These  results  for  the  most  part  occur  only  when  all  the  parathy- 
roids are  removed.  It  is  asserted  that  even  if  one  gland  is  retained 
the  animal  does  not  die.  The  above  described  symptoms  may  mani- 
fest themselves,  however,  but  they  are  slight  in  degree. 

Vincent  and  Jolly  have  recently  published  the  results  of  a  series 
of  experiments  which  seem  to  negative  to  some  extent  the  preceding 
statements.  These  experimenters  state  that  while  it  is  true,  that,  as  a 
rule,  the  removal  of  both  thyroids  and  parathyroids  in  the  carnivora 
is  a  fatal  operation,  there  are  nevertheless  many  exceptions;  and  in 
the  mammalia  generally,  e.g.,  cats,  dogs,  foxes,  guinea-pigs,  rats  and 
monkeys,  the  exception  becomes  the  rule  as  more  than  51  per  cent,  of 
animals  survived  the  operation  for  a  prolonged  period  and  of  these  68 
per  cent,  showed  no  specific  symptoms  of  any  kind.  From  the  con- 
tradictory observations  it  is  evident  that  the  subject  needs  further  in- 
vestigation. 

The  Pituitary  Body. — This  is  a  small  body  lodged  in  the  sella 
turcica  of  the  sphenoid  bone.  It  consists  of  an  anterior  lobe,  somewhat 
red  in  color,  and  a  posterior  lobe,  yellowish-gray  in  color.  (Fig.  214.) 
The  former  is  much  the  larger  and  partly  embraces  the  latter.  The 
anterior  lobe  is  developed  from  an  invagination  of  the  epiblast  of  the 
mouth  cavity,  and  consists  of  distinct  gland  tissue.  The  posterior 
lobe  is  an  outgrowth  from  the  brainj  and  is  connected  with  the  in- 
fundibulum  by  a  short  stalk.  It  has  been  suggested  that  the  term 
infundibular  body  be  reserved  for  the  posterior  lobe,  and  the  term 
hypophysis  cerebri  for  the  anterior  lobe.  This  distinction  appears 
to  be  desirable,  inasmuch  as  in  their  origin  and  structure  they  are 
separate  and  distinct  bodies. 


SECRETION. 


467 


Removal  of  the  hypophysis  cerebri,  or  the  pituitary  body,  is  always 
followed  by  a  fatal  result,  preceded  by  symptoms  not  unlike  those 
which  follow  removal  of  the  thyroid:  viz.,  anorexia,  tremors,  spasms, 
etc.  Degeneration  of  the  pituitary  body  has  been  found  in  connection 
with  a  hypertrophic  condition  of  the  bones  of  the  face  and  extremities 
to  which  the  term  acromegalia  has  been  given. 

Intravenous  injection  of  an  extract  of  the  pituitary  increases 
the  force  of  the  heart-beat  without  any  change  in  its  frequency,  and 
causes  a  rise  of  blood-pressure  from  a  stimula- 
tion of  the  arterioles  (Schafer  and  Oliver).  The 
material  secreted  by  the  pituitary  has  not  been 
isolated,  hence  its  chemic  features  are  unknown. 
After  its  formation  it  probably  passes  through  a 
system  of  ducts  into  the  cerebrospinal  fluid,  after 
which  it  influences  the  metabolism  of  the  nerve 
and  osseous  tissues  as  well  as  the  force  of  the 
heart-muscle. 

An  extract  of  the  anterior  lobe  itself  exerts  no 
appreciable  effect  on  the  blood-pressure  or  on 
the  rate  of  the  heart-beat,  nor  does  it  influence 
the  circulatory  and  respiratory  organs.  An  ex- 
tract of  the  infundibular  body  intravenously 
injected,  however,  gives  rise  to  increased  blood- 
pressure  and  to  a  slowing  of  the  heart-beat 
(Howell). 

Adrenal  Bodies,  or  Suprarenal  Capsules. 
— These  are  two  flattened  bodies,  somewhat 
crescentic  or  triangular  in  shape,  situated  each 
upon  the  upper  extremity  of  the  corresponding 
kidney,  and  held  in  place  by  connective  tissue. 
They  measure  about  40  mm.  in  height,  30  mm. 
in  breadth,  and  from  6  to  8  mm.  in  thickness. 
The  weight  of  each  is  about  4  gm. 

Function  of  the  Adrenal  Bodies. — It  was  observed  by  Addison 
that  a  profound  disturbance  of  the  nutrition,  characterized  by  a  bronze- 
like discoloration  of  the  skin  and  of  the  mucous  membranes  of  the 
mouth,  extreme  muscular  weakness,  and  profound  anemia,  was  asso- 
ciated with,  if  not  dependent  on,  pathologic  conditions  of  the  suprarenal 
glands.  In  the  progress  of  the  disease  the  asthenia  gradually  increases, 
the  heart  becomes  weak,  the  pulse  small,  soft,  and  feeble,  indicating  a 
general  loss  of  tone  of  the  muscular  and  vascular  apparatus.  Death 
ensues  from  paralysis  of  the  respiratory  muscles.  The  essential  nature 
of  the  lesion  which  gives  rise  to  these  symptoms  has  not  been  deter- 
mined. 

Removal  of  these  bodies  from  various  animals  is  invariably  and 
in  a  short  time  followed  by  death,  preceded  by  some  of  the  symptoms 
characteristic    of    Addison's   disease.     Their   development,    however, 


Fig.  214. — Sagittal 
Section*  of  the  Pitu- 
itary Body  and  In- 
fundibulum  with  ad- 
JOINING Part  of  Third 
Ventricle,  a.  Ante- 
rior lobe.  a'.  A  projec- 
tion from  it  toward  the 
front  of  the  infundibu- 
lum.  b.  Posterior  lobe 
connected  by  a  stalk 
with  the  infundibulum, 
i.  I.e.  Lamina  cinerea. 
0.  Right  optic  nerve. 
ch.  Sectionofoptic 
chiasm,  r.o.  Recess  of 
ventricle  above  the 
chiasma.  cm.  Corpus 
mammillare.-(Sc/',"<:,(7rtf, 
from  Qua  in.) 


468  TEXT-BOOK  OF  PHYSIOLOGY. 

is  more  acute.  From  the  fact  that  animals  so  promptly  die  after  ex- 
tirpation of  these  bodies,  and  the  further  fact  that  the  blood  of  such 
animals  is  toxic  to  the  subjects  of  recent  extirpation,  but  not  to  normal 
animals,  the  conclusion  was  drawn  that  the  function  of  the  adrenal 
bodies  is  to  remove  from  the  blood  some  toxic  product  of  muscle 
metabolism.  Its  accumulation  after  extirpation  gives  rise  to  death 
through  auto-intoxication. 

On  the  supposition  that  the  adrenals  might  secrete  and  pour  into 
the  blood  a  specific  material  which  favorably  influences  general 
metabolism,  Schafer  and  Oliver  injected  hypodermi'cally  glycerin 
and  water  extracts,  and  observed  at  once  an  increased  activity  of  the 
heart-beats  and  of  the  respiratory  movements.  The  effects,  however, 
were  only  transitory.  When  these  extracts  are  injected  into  the  veins 
directly,  there  follows  in  a  short  time  a  cessation  of  the  auricular 
contraction  of  the  heart,  though  the  ventricular  contraction  continues 
with  an  independent  rhythm.  If  the  vagi  are  cut  previous  to  the 
injection  or  if  the  inhibition  is  removed  by  atropin,  the  rapidity  and 
vigor  of  both  auricles  and  ventricles  are  increased.  Whether  the 
inhibitory  influence  is  removed  or  not,  there  is  a  marked  increase  in 
the  blood-pressure,  though  it  is  greater  in  the  former  instance.  This 
is  attributed  to  a  direct  stimulation  and  contraction  of  the  muscle- 
fibers  of  the  arterioles  themselves,  and  not  to  vaso-motor  influences, 
as  it  occurs  also  after  division  of  the  cord  and  destruction  of  the  bulb. 
The  contraction  of  the  arterioles  is  quite  general,  as  shown  by  plethys- 
mographic  studies  of  the  limbs,  spleen,  kidney,  etc.  Applied  locally 
to  the  mucous  membranes,  adrenal  extract  produces  contraction  of 
the  blood-vessels  and  pallor.  The  skeletal  muscles  are  affected  by 
the  extract  very  much  as  they  are  by  veratrin.  The  duration  of  a 
single  contraction  is  very  much  prolonged,  especially  in  the  phase  of 
relaxation  or  of  decreasing  energy. 

It  is  evident  from  these  experiments  that  the  adrenal  bodies  are 
engaged  in  elaborating  and  pouring  into  the  blood  a  specific  material 
which  stimulates  to  increased  activity  the  muscle-fibers  of  the  heart 
and  arteries,  and  thus  assist  in  maintaining  the  normal  blood-pres- 
sure as  well  as  the  tonicity  of  the  skeletal  muscles.  An  alkaloid al 
substance  was  isolated  by  Abel  from  extracts  of  this  gland,  to  which 
the  term  epinephrin  was  given.  A  crystallizable  substance  was  iso- 
lated first  by  Takamine  and  later  by  Aldrich,  to  which  the  term  adren- 
alin was  given.  Both  substances  are  apparently  equally  efficacious 
in  causing  contraction  of  the  blood-vessels  and  in  raising  the  blood- 
pressure.  The  question  as  to  which  of  these  two  substances  represents 
the  active  principle  of  the  gland  is  as  yet  a  subject  of  discussion. 

The  Spleen. — The  spleen  is  a  soft  bluish-red  organ,  oval  in  shape, 
from  twelve  to  fifteen  centimeters  long  by  eight  broad  and  four  thick. 
It  is  situated  in  the  left  hypochondrium  between  the  stomach  and  the 
diaphragm.  In  this  situation  it  is  held  in  position  by  a  fold  of  the  perit- 
oneum which  passes  from  the  upper  border  to  the  diaphragm. 


SECRETION. 


469 


Structure. — A  section  of  the  spleen  shows  that  it  consists  of  con- 
nective tissue,  blood-vessels,  lymph-corpuscles,  and  lymphoid  tissue. 
The  surface  of  the  spleen  is  covered  by  a  capsule  composed  of  dense 
fibrous  tissue,  from  the  inner  surface  of  which  septa  or  trabecular 
pass  inward  toward  the  center  of  the  organ.  In  their  course  they  give 
off  a  series  of  processes  which  unite  freely,  forming  a  spongy  connective- 
tissue  framework.  The  capsule  and  the  main  trabecular  in  some 
animals  contain  numerous  non-striated  muscle-fibers.  In  man  they 
are  relatively  few  in  number.  The  blood-vessels  which  enter  the  spleen 
are  supported  by  the  connective-tissue  septa.  As  they  pass  toward  the 
center  of  the  organ  they  divide  very  rapidly  and  soon  diminish  in  size. 
In  their  course  small  branches  are  given  off,  which  penetrate  the  inter- 
trabecular  tissue  and  be- 
come encased  with  spheric 
or  cylindric  masses  of 
adenoid  tissue  known  as 
Malpighian  corpuscles. 
These  corpuscles  are  com- 
posed largely  of  leukocytes. 
In  some  animals  the  leuko- 
cytes, instead  of  being  ar- 
ranged in  masses,  are  dis- 
tributed along  the  walls  of 
the  artery  as  a  continuous 
layer.  Within  the  corpus- 
cles the  arteries  pass  into 
capillaries;  whether  the 
artery  passes  directly  to  the 
splenic  pulp  or  indirectly 
by  way  of  the  corpuscles, 
its  ultimate  branches  ter- 
minate in  capillaries  which 

open  into  the  spaces  of  the  splenic  pulp.  From  these  spaces  a  net- 
work of  venules  gathers  the  blood  and  transmits  it  to  the  veins.  It  is 
a  disputed  question  as  to  whether  the  spaces  are  lined  by  epithelium, 
thus  forming  a  continuous  blood  channel,  or  whether  they  are  wanting 
in  this  histologic  element. 

The  Splenic  Pulp. — The  spaces  of  the  connective-tissue  frame- 
work are  filled  with  a  dark  red  semifluid  mass  known  as  the  splenic 
pulp.  When  microscopically  examined,  the  pulp  presents  a  fine 
loose  network  of  adenoid  tissue,  large  numbers  of  leukocytes  or  lymph- 
corpuscles,  red  corpuscles  in  various  stages  of  disintegration,  and 
pigment  granules.  Chemic  analysis  reveals  the  presence  of  a  number 
of  nitrogen-holding  bodies,  e.  g.,  leucin,  tyrosin,  xanthin,  uric  acid; 
organic  acids,  e.  g.,  acetic,  lactic,  succinic  acids;  pigments  containing 
iron,  and  inorganic  salts. 

The   Functions   of   the   Spleen. — Notwithstanding   all    the   ex- 


Fig.  215. — Malpighian  Corpuscle  of  a  Cat's 
Spleen  Injected,  a.  Artery,  b.  Meshes  of  the 
pulp  injected,  c.  The  artery  of  the  corpuscle  rami- 
fying in  the  lymphatic  tissue  composing  it. 


47° 


TEXT-BOOK  OF  PHYSIOLOGY. 


periments  which  have  been  made  to  determine  the  functions  of  the 
spleen,  it  can  not  be  said  that  any  very  definite  results  have  been 
obtained.  The  fact  that  the  spleen  can  be  removed  from  the  body 
of  an  animal  without  appreciably  interfering  with  the  normal  metabo- 
lism would  indicate  that  its  function  is  not  very  important.  The 
chief  changes  observed  after  such  a  procedure  are  an  enlargement 
of  the  lymphatic  glands  and  an  increase  in  the  activity  of  the  red 
marrow  of  the  bones.  The  presence  of  large  numbers  of  leukocytes 
in  the  splenic  pulp  and  in  the  blood  of  the  splenic  vein  suggested 
the  idea  that  the  spleen  is  engaged  in  the  production  of  leukocytes, 
and  to  this  extent  contributes  to  the  formation  of  blood.     The  presence 

of  disintegrated  red  blood- 
corpuscles  has  suggested 
the  view  that  the  spleen 
exerts  a  destructive  action 
on  functionally  useless  red 
corpuscles.  These  and 
other  theories  as  to  splenic 
functions  have  been  offered 
by  different  observers,  but 
all  are  lacking  positive  con- 
firmation. 

Volume  Variations  of 
the  Spleen. — It  was  shown 
some  years  since  by  Roy, 
with  the  aid  of  the  plethys- 
mograph,  that  the  spleen 
undergoes  rhythmic  varia- 
tions in  volume  from  mo- 
ment to  moment.  In  the 
cat  and  in  the  dog  the  diminution  in  the  volume  (the  systole)  and  the 
increase  in  volume  (the  diastole)  together  occupied  about  one  minute. 
This  fact  was  determined  by  withdrawing  the  spleen  through 
an  opening  in  the  abdominal  wall  and  enclosing  it  in  a  box  with 
rigid  walls,  the  interior  of  which  was  connected  with  a  piston  record- 
ing apparatus.  The  system  being  filled  with  oil,  each  variation 
in  volume  was  attended  by  a  to-and-fro  displacement  and  a  cor- 
responding movement  of  the  recording  lever.  The  special  form  of 
plethysmograph  used  for  this  purpose  is  known  as  the  oncometer  or 
bulk  measurer,  and  the  recording  apparatus  as  the  oncograph  (Fig. 
216  and  Fig.  222;. 

The  cause  of  these  variations  in  volume  Roy  attributed  to  a  rhyth- 
mic contractility  of  the  non-striated  muscle-fibers  in  the  capsule  and 
trabecular,  and  not  to  changes  in  the  arterial  blood-pressure,  as  the 
curve  of  the  pressure  taken  simultaneously  remained  practically 
uniform.  The  effect  of  the  rhythmic  contractions  of  the  splenic 
muscle  tissue  is  to  force  the  blood   through   the  organ,  a  condition 


Fig.  216. — Spleen  Oncometer  Laid  Open. 


SECRETION.  471 

necessitated  perhaps  by  the  pressure  relations  within,  though  what 
function  is  thereby  fulfilled  is  not  apparent. 

It  was  subsequently  shown  by  Schafer  and  Moore  that  the  splenic 
volume  is  extremely  responsive  to  all  fluctuations  of  the  arterial  blood- 
pressure;  that  though  the  spleen  may  passively  expand  and  recoil  in 
response  to  the  rise  and  fall  of  the  blood-pressure,  nevertheless  the 
reverse  conditions  may  obtain:  viz.,  that  the  splenic  volume  may 
diminish  as  the  pressure  rises,  if  the  splenic  arterioles  contract  simul- 
taneously with  the  contraction  of  the  arterioles  generally.  On  the 
contrary,  the  splenic  volume  may  increase  coincident  with  a  dilatation 
of  the  splenic  and  systemic  arterioles.  In  addition  to  the  rhythmic 
variations,  the  spleen  steadily  increases  in  volume  for  a  period  of  five 
hours  after  digestion,  and  then  gradually  returns  to  its  former  con- 
dition. 

Influence  of  the  Nerve  System. — The  nerves  which  supply  the 
vascular  and  visceral  muscles  in  the  spleen  are  derived  directly 
from  the  semilunar  ganglion  (post-ganglionic  fibers)  and  pass  to  it 
in  company  with  the  splenic  artery.  The  nerve-cells  from  which 
they  arise  are  in  physiologic  relation  with  nerve-fibers  (pre-ganglionic 
fibers)  which  emerge  from  the  spinal  cord  in  the  anterior  roots  of 
the  third  thoracic  to  the  first  lumbar  nerves  inclusive,  though  they 
are  found  most  abundantly  in  the  sixth,  seventh,  and  eighth  thoracic 
nerves.     Their  center  of  origin  is  in  the  medulla  oblongata. 

Stimulation  of  the  nerves  in  any  part  of  their  course  gives  rise 
to  a  diminution  in  splenic  volume;  division  of  the  nerves  is  followed 
by  an  increase  in  the  volume.  In  asphyxia  the  spleen  is  small  and 
contracted,  a  condition  attributed  to  a  stimulation  of  the  centers  in 
the  medulla  by  the  venosity  of  the  blood. 

The  musculature  of  the  spleen  may  also  be  excited  to  contraction 
by  reflex  influences,  as  shown  by  the  fact  that  stimulation  of  the  central 
end  of  a  sensory  nerve  is  attended  by  a  diminution  of  volume. 

Inasmuch  as  the  excised  spleen  will  continue  to  exhibit  variations 
in  volume  when  perfused  with  blood,  it  would  appear  that  it  possess 
some  mechanism  independent  to  some  extent  of  the  nerve  system. 


CHAPTER  XVIII. 

EXCRETION. 

As  stated  in  the  preceding  chapter,  the  term  excretion  is  limited  to 
the  process  by  which  the  end-products  of  tissue  metabolism  are  re- 
moved from  the  body,  the  nature  of  the  process,  however,  differing 
in  no  essential  particulars  from  that  underlying  the  process  of  secre- 
tion. The  histologic  structures  involved  and  the  forces  at  work 
being  of  the  same  general  character,  it  is  impossible  to  draw  any 
sharp  line  of  distinction  between  them.  As  a  general  fact  it  may 
be  stated  that  in  their  composition  all  the  characteristic  ingredients 
of  the  excretions  are  incapable  either  of  entering  into  the  formation 
of  tissue  or  of  undergoing  oxidation  for  the  purpose  of  heat-production. 
As  the  retention  of  these  end-products  in  the  body  would  exert  a 
deleterious  influence  on  normal  metabolism,  their  prompt  removal 
becomes  essential  to  the  maintenance  of  physiologic  activity.  The 
principal  excretions  of  the  body — urine,  perspiration,  and  bile — are 
complex  fluids  in  which,  with  the  exception  of  those  given  off  in  the 
lungs,  are  to  be  found  in  varying  proportions  the  chief  end-products 
of  metabolism. 

THE  URINE. 

Normal  urine  has  a  pale  yellow  or  amber  color,  an  aromatic 
odor,  an  acid  reaction,  and  a  specific  gravity  of  1.020.  As  a  rule,  it 
is  perfectly  transparent,  though  its  transparency  may  be  diminished 
from  the  presence  of  mucus,  calcium  and  magnesium  phosphates,  and 
mixed  urates. 

The  color,  which  varies  within  physiologic  limits  from  a  pale 
yellow  to  a  reddish-brown,  is  due  to  the  presence  of  the  coloring- 
matters  urobilin,  urochrome,  and  uroerylhrin,  all  of  which  are  de- 
rivatives from  the  bile  pigments  absorbed  from  the  liver  or  the  alimen- 
tary canal. 

The  reaction  of  the  urine  is  acid,  owing  to  the  presence  of  the  acid 
phosphates  of  sodium  and  calcium.  The  degree  of  acidity,  however, 
varies  at  different  periods  of  the  day.  Urine  passed  in  the  morning 
is  strongly  acid,  while  that  passed  during  and  after  digestion,  espe- 
cially if  the  food  be  largely  vegetable  in  character  and  rich  in  alkaline 
salts,  is  either  neutral  or  alkaline  in  reaction.  The  diminished  acidity 
after  meals  is  attributed  to  the  formation  of  hydrochloric  acid  by  the 
gastric  glands  and  the  consequent  liberation  of  bases  which  are  ex- 
creted   in   the   urine.     The   phosphoric   acid    which   enters  into  com- 

472 


EXCRETION.  473 

bination  with  sodium  and  potassium  bases  is  a  product  of  tissue 
metabolism. 

The  specific  gravity  is  about  1.020,  though  it  varies  from  1.015 
to  1.025.  It  will  diminish,  other  things  being  equal,  with  increased 
consumption  of  water  and  diminished  activity  of  the  skin;  it  will  be 
increased  of  course  by  the  opposite  conditions. 

The  quantity  of  urine  excreted  in  twenty-four  hours  varies  from 
1200  to  1700  c.c.  Amounts  both  above  and  below  these  are  fre- 
quently passed  from  a  variety  of  causes. 

The  odor  of  the  urine  is  characteristic  and  due  to  the  presence  of 
aromatic  compounds. 

COMPOSITION  OF  URINE. 

Water, 1500.00  c.c. 

Total  solids, 72.00  grams. 

Urea, 33-iS       " 

,  Uric  acid  (urates)  , 0.55       " 

Hippuric  acid  (hippurates),  .' 0.40       " 

Kreatinin,  xanthin,  hypoxanthin,  guanin,  ammo-  \  « 

nium  salts,  pigment,  etc. 
Inorganic  salts:  sodium  and  potassium  sulphates,  ] 
phosphates,  and  chlorids;  magnesium  and  cal-  | 
cium  phosphates,  \      27.00       " 

Organic  salts:  lactates,  acetates,  formates  in  small  | 
amounts, 

Sugar, a  trace 

Gases,  nitrogen,  and  carbonic  acid. 

The  estimation  of  total  urinary  solids  in  any  given  sample  of 
urine  is  frequently  a  matter  of  clinical  interest.  This  may  approxi- 
mately be  attained  by  multiplying  the  last  two  figures  of  the  specific 
gravity  by  the  coefficient  of  Haeser  or  Christison,  2.33.  The  result 
expresses  the  total  solids  in  1000  parts:  e.  g.,  urine  with  a  specific 
gravity  of  1.020  would  contain  20  X  2.33,  or  46.60  grams  of  solid 
matter  per  1000  c.c.  If  the  amount  passed  in  twenty-four  hours  be 
1500  c.c,  the  total  solids  would  amount  to  69.9  grams. 

The  Water  of  the  Urine. — The  amount  of  urinary  water  and  its 
ratio  to  the  solid  constituents  will  vary  with  the  amount  consumed 
and  the  activity  of  the  skin  and  lungs.  In  summer  the  foods,  liquid 
and  solid,  remaining  the  same,  the  quantity  of  water  in  the  urine  is 
diminished  in  consequence  of  increased  activity  of  skin  and  lungs 
and  the  ratio  of  water  to  solids  decreased.  In  winter  the  reverse 
conditions  obtain.  The  food  remaining  the  same,  the  consumption 
of  large  quantities  of  water  hastens  at  least  the  removal  of  end-products 
from  the  tissues  and  thus  increases  the  urinary  solids. 

Urea  is  the  most  abundant  of  the  organic  constituents  of  the 
urine  and  is  present  to  the  extent  of  from  2  to  3  per  cent.  It  is  a 
colorless  neutral  substance,  crystallizing  under  varying  conditions 
in  long  silky  needles  or  in  rhombic  prisms.  It  is  soluble  in  water 
and  alcohol.  It  is  composed  of  CON,H4.  When  subjected  to  pro- 
longed   boiling,    it    combines   with    water,    "iving   rise   to   ammonium 


474  TEXT-BOOK  OF  PHYSIOLOGY. 

carbonate.  The  presence  of  Micrococcus  urea  in  urine  will  also 
convert  the  urea,  by  combining  it  with  two  molecules  of  water,  into 
ammonium  carbonate,  CON2H4  +  2H20  =  (NH4)2C03. 

The  average  amount  of  urea  excreted  daily  varies  from  30  to  34 
grams.  As  urea  is  now  known  to  be  the  principal  end-product  of 
proteid  metabolism  within  the  body,  it  is  evident  that  the  quantity 
produced  and  eliminated  in  the  twenty-four  hours  will  depend  on  the 
quantity  of  proteid  food  consumed  and  on  the  extent  to  which  the 
proteid  constituents  of  the  tissues  are  metabolized.  In  the  condition  of 
nutritive  equilibrium,  when  the  proteid  ingested  is  100  grams  and  the 
urea  egested  31.5  grams,  it  is  difficult  to  state  the  percentage  of  urea 
which  is  derived  from  the  metabolism  of  the  proteid  food  (circulating 
proteid)  and  that  derived  from  the  metabolism  of  the  proteids  of  the 
tissues  (organ  proteid).  In  this  condition,  however,  it  is  found  that 
if  the  proteid  consumed  is  varied  within  limits  above  or  below  the 
standard  amount  of  100  grams,  the  quantity  of  urea  excreted  rises  and 
falls  in  practically  the  same  ratio,  indicating  apparently  that  the 
production  of  urea  is  directly  dependent  on  the  proteid  supply.  On 
the  contrary,  it  has  been  observed  in  human  beings  in  the  fasting 
condition  that  for  a  period  of  ten  days  there  is  a  daily  excretion  of 
about  21  grams  of  urea,  equivalent  to  about  70  grams  of  proteid. 
Again,  contrary  to  former  views,  the  metabolism  of  proteid  and  the 
production  of  urea  are  practically  independent  of  muscular  work. 
Even  after  severe  labor  extending  over  a  period  of  some  hours  there 
is  no  noticeable  increase  in  the  urea  eliminated. 

Seat  of  Urea  Formation. — It  is  quite  certain  in  the  light  of  present 
knowledge  that  urea  is  partly  formed  in  the  liver  by  the  action  of 
the  cells  out  of  cleavage  products  of  proteid  metabolism.  The  par- 
ticular compounds  out  of  which  the  cells  synthetize  urea  are  the 
ammonium  salts,  especially  the  carbamate  and  carbonate.  The 
experimental  reasons  for  this  view  have  already  been  stated  on  page 
462. 

Uric  acid  is  one  of  the  constant  ingredients  of  the  urine.  It  is  a 
crystalline  nitrogen-holding  body  closely  resembling  urea,  its  formula 
being  CSH4N403.  The  total  quantity  excreted  daily  varies  from  0.2 
to  1  gram.  It  is  doubtful  if  uric  acid  exists  in  a  free  state  in  the 
urine,  the  indications  being  that  it  is  combined  with  sodium  and 
potassium  in  the  form  of  a  quadriurate.  The  urates  are  frequently 
deposited  when  in  excess  from  the  urine  as  a  brick-red  sediment, 
the  color  being  due  to  their  combination  with  the  coloring-matter 
uroerythrin.  When  pure,  uric  acid  crystallizes  in  the  rhombic  form, 
though  it  assumes  a  variety  of  forms.  Uric  acid  was  long  regarded 
as  a  product  of  general  proteid  metabolism  and  for  chemic  reasons 
an  antecedent  of  urea.  This  view  has  been  abandoned.  At  present 
it  is  believed  that  it  is  a  cleavage  product  of  nuclein,  a  constituent 
of  all  cell  nuclei.  In  the  metabolism  of  nuclein  a  proteid  and  nucleic 
acid  are  formed,  from  the  latter  of  which  uric  acid  is  derived.     Nu- 


EXCRETION.  475 

cleic  acid  when  decomposed  yields  a  series  of  bases,  such  as  xanthin, 
hypoxanthin,  adenin,  guanin,  etc.  Because  of  the  fact  that  these 
bodies  can  also  be  obtained  from  a  synthetized  body  termed  purin 
they  are  known  collectively  as  the  purin  bases.  Though  there  is  a 
close  relationship  between  uric  acid  and  the  purin  bases,  it  has  been 
impossible  to  experimentally  derive  one  from  the  other.  When  hy- 
poxanthin, however,  is  given  internally  it  is  oxidized  and  converted 
into  uric  acid.  It  is  extremely  probable,  therefore,  that  uric  acid  is 
an  oxidation  product  of  one  or  more  of  the  purin  bases. 

It  is  probable,  however,  that  not  all  of  the  uric  acid  eliminated  is 
derived  from  the  nuclein  of  tissue-cells  and  their  decomposition 
products,  the  purin  bases.  Some  of  it  is  undoubtedly  derived  from 
the  nucleins  contained  in  foods.  The  uric  acid  eliminated  is  there- 
fore partly  endogenous  and  partly  exogenous  in  origin. 

Xanthin,  hypoxanthin,  guanin,  etc.,  are  also  found  in  urine 
in  small  but  variable  amounts.  They  are  nitrogenized  compounds 
derived  mainly  from  the  metabolism  of  the  nuclein  bodies. 

Kreatinin  is  a  crystalline  nitrogenous  compound  closely  resem- 
bling kreatin,  one  of  the  constituents  of  muscular  tissue.  The  amount 
excreted  daily  is  about  i  gram.  Though  kreatinin  may  arise  in  conse- 
quence of  proteid  metabolism,  it  is  probable  that  it  is  largely  derived 
from  a  transformation  of  the  kreatin  contained  in  the  meat  consumed 
as  food. 

Hippuric  acid  in  combination  with  sodium  and  potassium  is 
very  generally  present  in  urine,  though  in  small  amounts.  It  is  more 
abundant  in  the  urine  of  the  herbivora  than  the  carnivora.  In  man 
the  amount  excreted  daily  is  about  0.7  gram,  though  the  amount  may 
be  raised  by  a  diet  of  asparagus,  plums,  cranberries,  etc.,  and  by  the 
administration  of  benzoic  and  cinnamic  acids.  There  is  evidence  that 
hippuric  acid  is  formed  in  the  kidney  from  benzoic  acid,  its  pre- 
cursors, or  related  bodies.  Various  compounds  of  this  class  are  found 
in  vegetable  foods,  a  fact  which  may  account  for  the  increase  in  the 
excretion  of  hippuric  acid  on  a  vegetable  diet. 

Leucin,  tyrosin,  phenol,  cystin,  indoxyl,  skatoxyl,  are  found 
in  small  amounts  even  under  normal  conditions.  They  arise  from 
putrefactive  change  in  the  intestine. 

Inorganic  Salts. — Sodium  and  potassium  phosphates,  known 
as  the  alkaline  phosphates,  are  found  in  both  blood  and  urine.  The 
total  quantity  excreted  daily  is  about  4  grams.  Calcium  and  mag- 
nesium phosphates,  known  as  the  earthy  phosphates,  are  present  to 
the  extent  of  1  gram.  Though  insoluble  in  water,  they  are  held  in 
solution  in  the  urine  by  its  acid  constituents.  If  the  urine  be  rendered 
alkaline,  they  are  at  once  precipitated.  Sodium  and  potassium 
sulphates  are  also  present  to  the  extent  of  about  2  grams.  The  phos- 
phoric and  sulphuric  acids  which  are  combined  with  these  bases  enter 
the  body  for  the  most  part  in  the  foods,  though  there  is  evidence  that 
they  also  arise  by  oxidation  in  consequence  of  the  metabolism  of  proteids 


476 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  contain  phosphorus  and  sulphur.  Sodium  chlorid  is  the  most 
abundant  of  the  inorganic  salts.  It  is  derived  mainly  from  the  food. 
The  amount  excreted  is  about  15  grams  in  twenty-four  hours. 

THE  KIDNEYS. 

The  kidneys  are  the  organs  engaged  in  the  excretion  of  the  urinary 
constituents  from  the  blood.     They  resemble  a  bean  in  shape,  are 

from  10  to  12  centi- 
meters in  length,  2  in 
breadth,  and  weigh 
from  144  to  170 
grams.  They  are  situ- 
ated in  the  lumbar 
region,  one  on  each 
side  of  the  vertebral 
column  behind  the 
peritoneum,  and  ex- 
tend from  the  eleventh 
rib  to  the  crest  of  the 
ilium.  The  anterior 
surface  is  convex,  the 
posterior  surface  con- 
cave. The  latter  pre- 
sents a  deep  notch — 
the  hilum.  The  kid- 
ney is  surrounded  by 
a  thin  smooth  mem- 
brane composed  of 
white  fibrous  and 
yellow  elastic  tissue; 
though  it  is  attached 
to  the  surface  of  the 
kidney  by  minute 
processes  of  connec- 
tive tissue,  it  can  very 
readily  be  torn  away. 
The  substance  of  the 
kidney  is  dense  but 
friable. 

Upon  making  a 
longitudinal  section 
of  the  kidney  it  will 
be  observed  that  the 
hilum  extends  into  the  interior  of  the  organ  and  expands  to  form  a 
cavity  known  as  the  sinus,  in  which  are  found  the  blood-vessels,  nerves, 
and  duct  (Fig.  217).  This  cavity  is  mainly  occupied  by  the  upper 
part  of  the  renal  duct,  the  ureter,  the  interior  of  which  is  termed  the 


Fig.  217. -—Longitudinal  Section  through  the 
Kidney,  the  Pelvis  of  the  Kidney,  and  a  Number 
of  Renal  Calyces.  A.  Branch  of  the  renal  artery. 
U.  Ureter.  C.  Renal  calyx,  i.  Cortex,  i'.  Medullary 
rays.  i".  Labyrinth,  or  cortex  proper.  2.  Medulla. 
2'.  Papillary  portion  of  medulla,  or  medulla  proper. 
2".  Border  layer  of  the  medulla.  3,  3.  Transverse  sec- 
tion through  the  axes  of  the  tubules  of  the  border  layer. 
4.  Fat  of  the  renal  sinus.  5,  5.  Arterial  branches. 
*.  Transversely  coursing  medulla  rays. — {Tyson,  after 
Henle.) 


EXCRETION.  477 

pelvis.  The  ureter  divides  into  several  portions  which  terminate  in 
small  caps  of  calyces  which  receive  the  apices  of  the  pyramids.  The 
parenchyma  of  the  kidney  consists  of  two  portions:  viz. — 
i.  An  internal  or  medullary  portion,  consisting  of  a  series  of  pyramids 
or  cones,  some  twelve  or  fifteen  in  number,  which  present  a  dis- 
tinctly striated  appearance. 
2.  An  external  or  cortical  portion,  half  an  inch  in  thickness  and  dis- 
tinctly friable  in  character. 
The  Histology  of  the  Kidney. — The  kidney  is  composed  of  a 
connective-tissue  framework  supporting  secreting  tubules,  blood- 
vessels, lymphatics,  and  nerves,  all  of  which  are  directly  connected 
with  the  removal  of  the  urinary  constituents  from  the  blood.  The 
kidney  is  structurally  a  compound  tubular  gland.  If  the  apex  of 
each  pyramid  be  examined  with  a  lens,  it  will  present  a  number  of 
small  orifices  which  may  be  regarded  as  the  beginnings  of  the  urinifer- 
ous  tubules.  From  this  point  the  tubules  pass  outward  in  a  straight 
but  somewhat  diverging  manner  toward  the  cortex,  giving  off  at 
acute  angles  a  number  of  branches  (Fig.  218).  From  the  apex  to  the 
base  of  the  pyramids  they  are  known  as  the  tubules  of  Bellini.  In 
the  cortical  portion  of  the  kidney  the  tubule  becomes  enlarged  and 
twisted,  and,  after  pursuing  an  extremely  convoluted  course,  turns 
backward  into  the  medullary  portion  for  some  distance,  forming  the 
ascending  limb  of  Henle's  loop;  it  then  turns  upon  itself,  forming  the 
descending  limb  of  the  loop,  reenters  the  cortex,  again  expands  and 
becomes  convoluted,  and  finally  terminates  in  an  ovoid  enlargement 
known  as  Midler's  or  Bowman's  capsule,  in  which  is  contained  a 
small  tuft  of  blood-vessels — the  glomerulus.  Each  tubule  consists  of 
a  basement  membrane  lined  throughout  its  entire  extent  by  epithelial 
cells.  The  epithelium  as  well  as  the  tubule  vary  in  shape  and  size  in 
different  parts  of  its  course.  In  the  capsule  the  epithelium  is  flattened, 
lining  not  only  the  inner  surface  of  the  capsule  but  reflected  over 
the  blood-vessels  as  well.  This  is  known  as  the  glomerular  epi- 
thelium. In  the  convoluted  portions  of  the  tubules  the  epithelium 
is  cuboidal,  granular,  and  somewhat  striated;  in  Henle's  loop  it  is 
more  or  less  flattened. 

The  Blood-vessels  of  the  Kidney. — The  renal  artery  enters  the 
kidney  at  the  hilum  behind  the  ureter;  it  soon  divides  into  several 
large  branches  which  penetrate  the  substance  of  the  kidney  between 
the  pyramids  and  pass  outward  into  the  cortex.  At  the  base  of  the 
pyramids  branches  of  the  arteries  form  an  anastomosing  plexus. 
From  this  plexus  vessels  are  given  off,  some  of  which  follow  the  straight 
tubules  toward  the  apex  of  the  pyramids,  vasa  recta,  while  others 
enter  the  cortex  and  pass  to  its  surface  (Fig.  218).  In  the  course  of 
the  latter  small  branches  are  given  off,  each  of  which  soon  divides  and 
subdivides  to  form  a  ball  of  capillary  vessels  known  as  the  glomer- 
ulus. These  capillaries,  however,  do  not  anastomose,  but  soon  re- 
unite to  form  an  efferent  vessel  the  caliber  of  which  is  less  than  that 


478 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  the  afferent  artery.  In  consequence  of  this,  there  is  a  greater  re- 
sistance to  the  outflow  of  blood  than  to  the  inflow,  and  therefore  a 
higher  blood-pressure  in  the  glomerulus  than  in  capillaries  generally. 
The  relation  of  the  glomerulus  to  the  tubule  is  important  from  a 


Lobule. 


Lobule. 


Tunica  albuginea. 


Renal  corpuscle. --___ 


Intarcalated 
piece. 


Thick..  _•___ 


Thin -  ■'-] 

division  of  thi 
loop  of  Henk 


Collecting 
tubule. 


Papillary  rluct 


Papilla. 


Interlobular 
artery. 
—  Interlobular 
vein. 


Arciform  artery 


Arcitorm  vein. 


-•Interlobar  artery. 
..Interlobar  vein. 


I  p,.  218.     Scheme  of  the  Course  of  the  Uriniferous  Tubules  and  the  Renal 

Vessels. 


EXCRETION. 


479 


physiologic  point  of  view.  As  stated  above,  the  glomerulus  is  re- 
ceived into  and  surrounded  by  the  terminal  expansion  or  capsule  of 
the  tubule.  This  capsule,  formed  by  an  indentation  of  the  terminal 
portion  of  the  tubule,  consists  of  two  walls,  an  outer  one  consisting 
of  an  extremely  thin  basement  membrane,  covered  by  flattened  epi- 
thelial cells,  and  an  inner  one  consisting  apparently  only  of  flattened 
epithelium  which  is  reflected  over  and  closely  invests  the  glomerular 
blood-vessels  (Fig.  219).  The  blood  is  thus  separated  from  the  interior 
of  the  capsule  by  the  epithelial  wall  of  the  capillary  and  the  epithelium 
of  the  reflected  wall  of  the 
capsule.  During  the  periods 
of  secretory  activity  the  blood- 
vessels of  the  glomerulus  are 
filled  with  blood  to  such  an 
extent  that  the  sac  cavity  is 
almost  obliterated.  After  its 
exit  from  the  capsule  the  effer- 
ent vessel  of  the  glomerulus 
soon  again  divides  and  sub- 
divides to  form  an  elaborate 
capillary  plexus  which  sur- 
rounds and  closely  invests  the 
convoluted  tubules.  From 
this  plexus  as  well  as  from  the 
plexus  which  surrounds  the 
straight  tubules  veins  arise 
which  pass  toward  and  empty 
into  veins  at  the  base  of  the 
pyramids.  The  renal  vein 
formed  by  the  union  of  these 
latter  veins  emerges  from  the 
kidney  at  the  hilum  and  finally  empties  into  the  vena  cava  inferior. 

The  nerves  of  the  kidney  are  derived  from  the  renal  plexus  and 
follow  the  course  of  the  blood-vessels  to  their  termination. 

The  Renal  Duct. — The  excretory  duct  of  the  kidnev,  the  ureter, 
is  a  musculo-membranous  tube  about  5  mm.  in  diameter  when  dis- 
tended, 30  cm.  in  length,  and  extends  from  the  hilum  to  the  base  of 
the  bladder.  The  upper  extremity  is  expanded  and  within  the  renal 
sinus  becomes  irregularly  branched,  giving  rise  to  a  number  of  short 
tubes,  called  calyces,  each  of  which  embraces  the  apex  of  a  Malpighian 
pyramid.  The  interior  of  the  expanded  portion  of  the  ureter  is 
known  as  the  pelvis.  The  wall  of  the  ureter  consists  of  a  mucous 
membrane,  a  muscle  coat,  and  an  external  fibrous  investment. 

MECHANISM  OF  URINE  SECRETION. 

The  secretion  of  urine  is  a  complex  process  and  susceptible  of 
several  interpretations.     It  was  originally  inferred  by  Bowman  that, 


Fig.  219. — Scheme  of  the  Renal  or  Mal- 
pighian Corpuscle,  i.  Interlobular  artery. 
2.  Afferent  vessel.  3.  Efferent  vessel.  4.  Outer 
wall.  5.  Inner  wall.  6.  Glomerulus.  7.  Neck 
of  tubule. — (Stohr.) 


4So  TEXT-BOOK  OF  PHYSIOLOGY. 

as  the  kidney  presents  anatomically  an  apparatus  for  nitration,  the 
capsule  with  its  enclosed  glomerulus,  and  an  apparatus  for  secretion, 
the  epithelium  of  the  urinary  tubules,  the  elimination  of  the  urinary 
constituents  from  the  blood  is  accomplished  by  the  two  processes  of 
filtration  and  secretion;  that  the  water  and  highly  diffusible  inorganic 
salts  simply  pass  by  diffusion,  under  pressure,  though  the  walls  of 
the  glomerular  capillaries,  while  the  organic  constituents  are  removed 
by  the  epithelium  lining  the  tubules. 

Influenced  largely  by  the  facts  of  blood-pressure  Ludwig  advanced 
the  view  that  the  factors  concerned  in  the  secretion  of  urine  were 
purely  physical;  that  in  consequence  of  the  high  pressure  in  the  vessels 
of  the  glomeruli,  due  to  the  resistance  offered  by  the  smaller  efferent 
vessel,  all  the  urinary  constituents  were  filtered  off  in  a  state  of  extreme 
dilution.  In  order  to  account  for  the  higher  percentage  of  the  organic 
constituents  in  the  urine,  it  was  assumed  that  as  the  dilute  urine  passed 
through  the  tubules  the  water  was  partly  reabsorbed,  passing  by 
diffusion  into  the  lymph  and  blood  until  the  urine  acquired  its  normal 
characteristics.  In  support  of  this  view,  a  large  number  of  facts 
relating  to  the  influence  of  an  increase  and  a  decrease  of  pressure  in 
the  blood-vessels  of  the  glomeruli,  the  velocity  of  the  blood-stream, 
etc.,  in  determining  the  rate  of  urinary  flow  were  adduced,  all  of  which 
apparently  indicated  that  the  former  stood  to  the  latter  in  the  relation 
of  cause  and  effect,  and  that  the  formation  of  urine  was  accomplished 
entirely  by  physical  forces. 

The  progress  of  physiologic  investigation,  however,  has  thrown 
some  doubt  on  the  validity  of  this  physical  interpretation,  and  has 
rather  served  to  support  the  view  of  Bowman  that  the  organic  con- 
stituents at  least  are  removed  from  the  blood  by  a  process  of  selection 
on  the  part  of  the  epithelium  of  the  convoluted  part  of  the  urinary 
tubules;  in  other  words,  that  the  secretion  of  urine  is  physiologic 
rather  than  physical.  Heidenhain  has  brought  forward  a  series  of 
facts  which  support  this  view.  As  evidence  that  the  cells  possess  a 
selective  power,  he  presents  the  following  experiment:  The  spinal 
cord  of  an  animal  is  divided  in  the  neck  for  the  purpose  of  lowering  the 
blood-pressure  in  the  kidney  below  the  pressure  at  which  the  urine  is 
secreted;  a  solution  of  incligo-carmine  is  injected  into  the  blood- 
vessels; after  the  lapse  of  ten  minutes  the  animal  is  killed,  the  blood- 
vessels washed  out  with  alcohol  for  the  purpose  of  precipitating  the 
indigo-carmine  in  situ.  Section  of  the  kidney  shows  a  uniform  blue 
stain  of  the  cortex  alone.  Microscopic  examination  reveals  the  fact 
that  the  blue  stain  is  due  to  the  deposition  of  the  pigment  in  the  lumen 
and  in  the  lumen  border  of  the  cells  of  the  convoluted  tubules  and 
the  ascending  limb  of  Henle's  loop;  while  the  epithelium  of  Bow- 
man's capsule  as  well  as  the  glomerular  epithelium  present  no  evi- 
dence of  pigmentation. 

Nussbaum  attempted  to  establish  the  secretory  power  of  the  epi- 
thelium in  another  way.     in  the  frog  the  kidney  receives  blood  from 


EXCRETION.  481 

two  sources:  the  glomeruli  receive  their  blood  from  the  renal  artery, 
the  tubules  from  the  capillaries  formed  by  the  anastomosis  of  branches 
of  the  efferent  vessel  of  the  glomerulus  and  the  branches  of  the  renal 
portal  vein.  Nussbaum  believed  that  by  ligating  the  renal  artery 
all  glomerular  activity  could  be  abolished  and  the  part  played  by  the 
epithelium  could  be  established.  After  so  doing  the  flow  of  urine  was 
at  once  checked;  the  injection  of  urea  at  once  reestablished  it.  This 
fact  was  taken  as  a  proof  that  the  tubular  epithelium  not  only  ex- 
creted urea,  but  water  and  perhaps  other  constituents  as  well.  It 
was  also  found  that  sugar,  peptones,  carmine,  etc.,  which  are  always 
eliminated  from  the  blood  under  normal  conditions,  are  not  removed 
after  ligation  of  the  renal  artery.  It  was  concluded  from  these  ex- 
periments that  the  secreting  structures  of  the  kidney  consist  of  two 
distinct  systems,  the  glomerular  and  the  tubular;  the  former  secreting 
water,  salts,  sugar,  peptone,  etc. ;  the  latter  urea,  uric  acid,  etc.  These 
and  similar  facts  indicate  that  the  renal  epithelium  possesses  a  secretory 
rather  than  an  absorptive  function.  Heidenhain  and  those  who 
agree  with  him  assert  that  even  the  water  and  inorganic  salts  which 
pass  through  the  glomerular  epithelium  do  so  in  consequence  of  cell 
selection  and  cell  activity;  that  the  entire  process  is  one  of  secretion, 
though  conditioned  by  blood-pressure,  blood  velocity,  etc. 

Influence  of  Blood-pressure. — Whether  the  elimination  of  the 
urinary  constituents  is  entirely  secretory  (physiologic)  in  character 
or  not  there  can  be  no  doubt  that  the  whole  process  is  largely  deter- 
mined by  the  pressure  and  velocity  of  the  blood  in  the  glomerular 
capillaries,  or,  to  state  it  more  accurately,  on  the  difference  of  pres- 
sure between  the  blood  in  the  capillaries  and  the  urine  in  the  capsules. 
As  a  rule,  this  latter  pressure  is  at  a  minimum.  If  the  urine  should 
accumulate  in  the  ureter  and  tubules  either  from  ligation  or  mechan- 
ical obstruction  until  its  pressure  approximates  that  of  the  blood,  the 
secretion  would  be  diminished  if  not  abolished.  It  is  difficult  to 
determine  the  average  pressure  or  velocity  of  the  blood  in  the  glomers 
ular  capillaries,  though  they  both  must  be  greater  than  in  capillarie- 
in  other  parts  of  the  body,  from  the  fact  that  the  efferent  vessel  is 
narrower  than  the  afferent,  and  therefore  offers  great  resistance  to  the 
outflow  of  blood,  a  condition  most  favorable  to  the  production  of  a 
high  pressure  in  the  glomerulus. 

The  pressure  of  the  blood  in  the  glomeruli  may  be  raised  and  the 
velocity  increased : 

1.  By  an  increase  in  blood-pressure  generallv. 

2.  By  an  increase  in  the  pressure  of  the  renal  artery  alone. 

The  first  condition  may  be  brought  about  by  an  increase  in  either 
the  force  or  frequency  of  the  heart's  action  or  by  a  contraction  of  the 
arterioles  of  vascular  areas  in  any  or  all  parts  of  the  body,  excepting, 
of  course,  the  renal  vascular  area.  The  second  condition  is  brought 
about  by  a  dilatation  of  the  renal  artery  alone  and  possibly  by  a  con- 
traction of  the  efferent  vessels  of  the  crlomeruli. 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  pressure  of  the  blood  in  the  glomeruli  may  be  diminished 
and  the  velocity  decreased: 

i.  Bv  a  decrease  in  the  blood-pressure  generally. 
2.  By  a  decrease  in  the  pressure  of  the  renal  artery  alone. 

The  first  condition  is  brought  about  by  a  decrease  in  either  the 
force  or  frequency  of  the  heart's  action  or  by  a  dilatation  of  the  arteri- 
oles of  large  vascular  areas  in  any  or  all  parts  of  the  body.  The 
second  condition  is  brought  about  by  contraction  of  the  renal  artery 
alone  and  possibly  by  a  dilatation  of  the  efferent  vessels  of  the  glom- 
eruli.    The   effect   of  the   contraction   and   relaxation   of   either   the 

afferent  or  efferent  vessels  on  the 
pressure  within  the  glomerulus 
is  shown  in  Figure  220. 

Coincident  with  the  rise  and 
fall  of  pressure  in  the  glomerular 
capillaries  there  is  a  rise  and  fall 
in  the  rate  of  urinary  flow.  Thus 
it  has  been  found  that  an  in- 
crease in  the  aortic  pressure  from 
127  to  142  mm.  of  mercury,  by 
ligation  of  the  carotid,  femoral, 
and  vertebral  arteries,  increased 
the  rate  of  urinary  flow  from 
8.7  grams  in  thirty  minutes  to 
21.2  grams.  On  the  contrary,  a 
decrease  in  aortic  pressure  below 
40  mm.  of  mercury  caused  by 
division  of  the  spinal  cord  is  fol- 
lowed by  a  total  abolition  of  the 
urinary  flow.  These  facts  serve 
to  indicate  the  dependence  of 
the  secretion  on  blood-pressure. 
That  there  is  an  increase  in 


Bowm 
Caps 


r  ^=^ 


Fig.  220. — To  Illustrate  the  Effect 
of  Active  Changes  in  the  Vasa  Affer- 
entia  and  efferentia  on  the  pressure 
in  the  Glomerular  Capillaries.  A.  Re- 
nal arteries.  G.  Glomerular  capillaries. 
C.  Tubular  capillaries.  V.  Vein.  The  short 
thick  lines  represent  the  vasa  afferentia  and 
efferentia.  The  continuous  heavy  line  repre- 
sents the  mean  average  pressure.  If  the  vas 
afferens  dilates  and  the  vas  efferens  contracts 
separately  or  conjointly,  the  pressure  will  rise, 
as  indicated  by  the  upper  dotted  line.  If  the 
vas  afferens  contracts  and  the  vas  efferens 
dilates  separately  or  conjointly,  the  pressure 
will  fall,  as  indicated  by  the  lower  dotted 
line. — {After  Moral.) 

the  volume  of  the  blood  flowing  through  the  kidney  during  its  functional 
activity  is  apparent  from  inspection.  It  is  enlarged,  swollen,  and  red 
in  color.  The  blood  in  the  renal  vein  is  bright  red  in  color  and  con- 
tains more  oxygen  and  less  carbon  dioxid  than  venous  blood  generally. 
During  the  intervals  of  activity  the  kidney  diminishes  in  size,  is  pale 
in  color  and  the  blood  of  the  renal  vein  dark  and  venous  in  character. 
These  variations  in  the  volume  of  the  kidney  have  also  been  experi- 
mentally determined  and  registered  by  means  of  the  oncometer  and 
.  oncograph  devised  by  Roy. 

The  oncometer  consists  of  a  metallic  box  (Fig.  221)  composed 
of  halves  which  open  and  close  by  means  of  a  hinge.  It  is  connected 
with  a  recording  apparatus,  the  oncograph  (Fig.  222),  through  the 
tube  T.  The  kidney,  withdrawn  from  the  body,  is  placed  within  the 
oncometer.     Through  an  opening  in  the  side  pass  the  artery,  vein, 


EXCRETION. 


483 


and  ureter.  Between  the  kidney  and  the  wall  of  the  capsule  there  is 
placed  a  thin  membrane.  Oil  is  then  poured  through  the  side  tube 
I  until  the  space  between  the  capsule  and  the  kidney,  as  well  as  the 
tube  leading  to  the  chamber  of  the  oncograph,  are  completely  filled. 
When  the  tube  I  is  closed,  the  conditions  are  such  that  all  variations 
in  the  volume  of  the  kidney  are  taken  up  and  reproduced  by  the 
recording  lever  attached  to  the  piston  of  the  oncograph.     A  curve 


-  -,Fig.  221. — Oncometer.  K.  Kidney;  the  thick  line  is  the  metallic  capsule.  h.  Hinge. 
I.  .Tube  for  filling  apparatus.  T.  Tube  to  connect  with  T/;  a,  v,  u.  Artery,  vein, 
ureter. — (Stirling,  after  Roy.) 

Fig.  222. — Oxcograph.     C.   Chamber  filled  with  oil,  communicating  by  T,  with 
T.     p.  Piston.     /.  Writing-lever. — (Stirling,  after  Roy.) 

of  the  variations  in  the  volume  of  the  kidney  is  shown  in  Figure  223 
taken  simultaneously  with  the  curve  of  the  blood-pressure.  An 
examination  of  this  curve  shows  that  the  volume-changes  coincide 
with  changes  in  the  blood-pressure,  exhibiting  not  only  the  respiratorv 
but  also  the  cardiac  undulations. 

Influence  of  the  Nerve  System. — The  influence  of  the  nerve 


B.P. 


Fig.  223. — B.  P.  Blood-pressure  curve.  K.  Curve  of  the  volume  of  the  kidnev. 
T.  Time  curve;  intervals  indicate  a  quarter  of  a  minute.  A.  Abscissa. — (Stirling,  after 
Roy.) 

system  in  regulating  the  blood-supply  to  the  kidney  is  evident  from 
the  results  of  experimentation.  If  the  nerves  which  accompanv 
the  renal  artery  into  the  kidney  are  divided,  the  artery  at  once  dilates, 
the  kidney  enlarges,  and  a  copious  flow  of  urine  takes  place.  If  the 
peripheral  ends  of  these  nerves  be  stimulated  with  the  induced  electric 


4S4  TEXT-BOOK  OF  PHYSIOLOGY. 

current,  the  artery  contracts,  the  kidney  diminishes  in  size,  and  the 
flow  of  urine  ceases.  In  addition  to  these  vaso-constrictor  nerves, 
there  is  evidence  that  the  kidney  also  receives  vaso-dilator  nerves 
which  emerge  from  the  spinal  cord  and  are  found  in  the  anterior  roots 
of  the  eleventh,  twelfth,  and  thirteenth  dorsal  nerves,  in  the  dog. 
Direct  and  reflex  stimulation  of  these  nerves  gives  rise  to  a  dilatation 
of  the  artery,  a  swelling  of  the  kidney,  and  an  increase  in  secretion, 
independent  of  any  variation  in  general  blood  pressure. 

The  route  of  the  vaso-constrictor  nerves  is,  in  the  dog  at  least, 
through  the  splanchnics.  Section  of  these  nerves  is  followed  by  a 
dilatation  of  the  renal  vessels  and  an  increase  in  the  flow  of  urine. 
Stimulation  of  the  peripheral  ends  is  followed  by  a  constriction  of  the 
vessels  and  a  cessation  of  the  flow  of  urine. 

The  vaso-motor  center  for  the  blood-vessels  of  the  kidney  is  in  all 
probability  situated  in  the  medulla  oblongata  in  close  proximity  to 
the  general  vaso-motor  centers,  though  subordinate  centers  are  doubt- 
less present  in  the  spinal  cord.  It  was  found  by  Bernard  that  puncture 
of  the  medulla  was  occasionally  followed  by  a  profuse  secretion  of 
urine  without  the  presence  of  sugar.  The  route  of  the  vaso-motor 
impulses  which  influence  the  renal  blood-supply  is  down  the  cord 
through  the  splanchnics  and  through  the  renal  plexus. 

Influence  of  Variations  in  the  Composition  of  the  Blood. — 
As  it  is  the  function  of  the  kidneys  to  excrete  water,  inorganic  salts, 
and  various  end-products  from  the  blood  and  thus  maintain  a  gen- 
eral average  composition,  it  is  highly  probable  that  as  soon  as  they 
accumulate  beyond  a  certain  percentage  they  themselves  act  as  stimu- 
lants to  renal  activity,  either  by  acting  directly  on  the  renal  epithelium 
or  by  increasing  the  glomerular  pressure.  There  is  evidence  at  least 
that  urea  acts  in  the  former  manner.  An  excess  of  water  in  the  blood, 
that  from  copious  drinking  or  from  a  sudden  checking  of  the  skin  from 
a  fall  of  temperature,  will  act  in  the  latter  way.  The  introduction  into 
the  blood  of  inorganic  salts,  such  as  potassium  nitrate,  sodium  acetate, 
etc.,  will  in  a  short  time  lead  to  increased  activity  of  the  .kidneys,  as 
shown  by  an  increase  in  the  quantity  of  urine  excreted.  The  manner 
in  which  these  agents  and  other  members  of  their  class,  the  so-called 
saline  diuretics,  increase  renal  activity  is  yet  a  subject  of  discussion. 
On  the  one  hand,  it  is  stated  that  they  promote  an  absorption  of  water 
from  the  tissues  to  such  an  extent  that  a  condition  of  hydremic  plethora 
is  produced,  which  in  itself  increases  not  only  the  general  blood- 
pressure  but  the  local  renal  pressure  as  well,  and  that  it  is  this  factor 
which  is  the  cause  of  the  increased  flow  of  urine.  On  the  other  hand, 
it  is  asserted  that  though  the  salts  increase  the  local  pressure  and  the 
volume  of  the  kidney,  they  nevertheless  act  specifically  on  the  renal 
epithelium,  and  therefore  may  be  regarded  as  secreto-motor  agents. 
An  increase  in  the  percentage  of  sugar  or  urea  in  the  blood  has  a  similar 
influence  on  the  kidney. 

The  Storage  and  Discharge  of  Urine.— Urination. — The  urin- 


EXCRETION.  485 

ary  constituents,  as  soon  as  they  are  eliminated  from  the  blood, 
pass  into  and  through  the  uriniferous  tubules  and  by  them  are  dis- 
charged into  the  pelvis  of  the  kidney.  They  then  enter  the  ureter  by 
which  they  are  conducted  to  the  bladder.  The  immediate  cause  of 
this  movement  is  undoubtedly  a  difference  of  pressure  between  the 
terminal  portions  of  the  tubules  and  the  terminal  portion  of  the  ureter, 
aided  by  the  peristaltic  contraction  of  the  muscle  wall  of  the  ureter. 

The  bladder  is  a  reservoir  for  the  temporary  reception  of  the  urine 
prior  to  its  expulsion  from  the  body.  When  distended  it  is  ovoid  in 
shape  and  is  capable  of  holding  from  600  to  800  cu.  cm.  The  bladder 
is  composed  of  four  coats:  viz.,  serous,  muscle,  areolar,  and  mucous. 
The  muscle  coat  consists  of  external  longitudinal  and  internal  circular 
and  oblique  layers  of  fibers  of  the  non-striated  variety  which  collec- 
tively encircle  the  entire  organ.  As  these  fibers  by  their  contraction 
expel  the  urine  from  the  bladder,  they  are  known  collectively  as  the 
detrusor  urincz  muscle.  At  the  exit  of  the  bladder  the  circular  fibers 
are  somewhat  increased  in  number,  giving  rise  to  the  appearance  of  a 
distinct  muscle  which  has  been  termed  the  sphincter  vesica,  muscle. 
The  presence  of  this  muscle  has,  however,  been  denied  and  the  reten- 
tion of  the  urine  has  been  attributed  to  mechanic  conditions  at  the 
neck  of  the  bladder.  The  urethra  just  beyond  the  bladder  is  provided 
with  a  distinct  circular  muscle  composed  of  striated  fibers,  the  sphincter 
urethra  muscle.  When  the  urine  passes  into  the  bladder  it  is  retained 
there  and  prevented  from  escaping  by  the  contraction  of  this  latter 
muscle.  Under  normal  conditions  the  urine  accumulates  to  a  con- 
siderable extent  before  the  intra-vesic  pressure  gives  rise  to  a  charac- 
teristic sensation  and  the  desire  for  urination. 

The  Nerve  Mechanism  of  Urination. — The  muscle  mechan- 
isms which  retain  as  well  as  expel  the  urine  are  under  the  control  of 
the  nerve  system.  The  sphincter  urethrse  muscle,  which  by  the  orifice 
of  the  bladder  is  closed,  is  kept  in  a  state  of  tonic  contraction  by  nerve 
impulses  coming  from  the  spinal  cord  through  the  anterior  roots  of 
the  third  and  fourth  sacral  nerves.  The  detrusor  urinas  muscle  is 
excited  to  contraction  by  impulses  coming  likewise  through  the  sacral 
nerves  and  through  the  upper  lumbar  nerves  from  the  cord.  The 
centers  of  origin  for  these  two  sets  of  motor  nerves  are  located  in 
the  cord  in  the  neighborhood  of  the  fifth  lumbar  vertebra.  The 
expulsion  of  the  urine  is  largely  a  reflex  act,  though  under  the  con- 
trol of  the  will.  When  the  desire  to  urinate  is  experienced,  nerve 
impulses  are  coming  through  sensory  nerves  from  the  mucous  mem- 
brane of  the  bladder  which  are  reflected  to  the  centers  governing 
the  sphincter  urethras  and  detrusor  urinas  muscles  and  to  the  brain. 
The  effect  of  the  reflected  impulses  is  to  inhibit  the  sphincter  center 
and  to  stimulate  the  detrusor  center.  If  the  act  of  urination  is  to  be 
permitted,  volitional  impulses  descend  through  the  spinal  cord  which 
have  the  effect  of  still  further  inhibiting  the  sphincter  center  and 
stimulating  the  detrusor  center,  the  result  being  a  relaxation  of  the 


4S6  TEXT-BOOK  OF  PHYSIOLOGY. 

sphincter  muscle  and  a  contraction  of  the  detrusor  muscle  and  the 
expulsion  of  the  urine.  If  the  act  of  urination  is  to  be  suppressed, 
volitional  impulses  inhibit  the  detrusor  center  and  stimulate  the 
sphincter. 

PERSPIRATION;  SEBUM. 

The  perspiration  or  sweat,  the  chief  secretion  of  the  skin,  is  a 
clear  colorless  fluid,  slightly  acid  in  reaction  and  saline  to  the  taste. 
Its  specific  gravity  varies  from  1.003  t0  i-°o6.  Unless  collected  from 
the  soles  of  the  feet  and  the  palms  of  the  hand,  it  is  apt  to  be  mixed 
with  epithelial  cells  and  sebum.  The  total  quantity  of  perspiration 
secreted  daily  has  been  variously  estimated  at  from  700  to  1000  grams; 
the  exact  amount,  however,  is  difficult  of  determination,  for  the  reason 
that  the  rate  of  secretion  varies  readily  with  variations  in  tempera- 
ture, food,  drink,  season  of  the  year,  etc. 

Chemic  analysis  of  the  sweat  shows  that  it  contains  but  from  0.5 
to  2.5  per  cent,  of  solid  constituents,  the  variation  in  the  percentage 
depending  on  the  quantity  of  water  secreted.  The  solids  consist 
of  traces  of  urea,  neutral  fats,  lactic  and  sudoric  acids  in  combination 
with  alkaline  bases,  and  inorganic  salts  (Fovel).  Other  observers, 
however,  have  not  been  able  to  detect  the  presence  of  either  lactic 
or  sudoric  acid.  Urea  is  a  constant  ingredient,  though  its  percentage 
is  extremely  small,  possibly  not  more  than  0.1  per  cent.  The  amount, 
however,  may  be  very  much  increased  in  uremic  conditions,  the 
result  of  acute  or  chronic  disease  of  the  kidneys.  The  inorganic 
constituents  consist  mainly  of  sodium  chlorid  and  alkaline  and  earthy 
phosphates.  Carbonic  acid  is  also  present  in  the  free  state  as  well  as 
in  combination  with  alkaline  bases. 

The  very  small  quantity  of  the  solid  constituents  in  the  sweat, 
taken  in  connection  with  the  fact  that  it  is  excreted  most  abundantly 
when  the  external  temperature  is  high,  indicates  that  it  is  not  so  im- 
portant as  an  excrementitious  fluid  as  it  is  as  a  means  for  the  regulation 
of  the  temperature  of  the  body. 

The  sweat  is  a  product  of  the  secretory  activity  of  specialized 
glands,  the  sweat-glands,  embedded  in  the  skin,  to  the  histologic 
structures  of  which  they  bear  a  special  relation. 

THE  SKIN. 

The  skin  is  a  complexly  organized  structure  investing  the  entire 
external  surface  of  the  body.  Its  total  area  varies  from  16  to  20  feet 
in  man  and  from  12  to  16  feet  in  woman.  It  varies  in  thickness  in 
different  localities  of  the  body  from  -J-  to  yl(r  of  an  inch.  The  skin 
consists  of  two  principal  layers:  viz.,  a  deep  layer,  the  derma  or  corium, 
and  a  superficial  layer,  the  epidermis. 

The  derma  or  corium  may  be  subdivided  into  a  reticulated  and  a 
papillary  layer.     The  reticulated  layer  consists  of  white  fibrous  and 


EXCRETION. 


'4S7 


yellow  elastic  tissue,  non-striated  muscle-fibers,  woven  together  in 
every  direction  and  forming  an  areolar  network,  in  the  meshes  of 
which  are  deposited  masses  of  fat  and  a  structureless  amorphous 
matter;  the  papillary  layer  consists  mainly  of  club-shaped  elevations 
or  projections  of  the  amorphous  matter  constituting  the  papillae. 
The  reticulated  layer  serves  to  connect  the  skin  with  the  underlying 
structures  and  to  afford  support  for  the  blood-vessels,  nerves,  and 
lymphatics  which  are  distributed  to  the  papillae  (Fig.  224). 

The  epidermis 
is  an  extra-vascular 
structure  consist- 
ing entirely  of  epi- 
thelial  cells.  It 
may  also  be  sub- 
divided into  two 
layers  —  the  Mal- 
pighian  or  pig- 
mentary layer,  and 
the  corneous  or 
horny  layer.  The 
former  is  closely 
applied  to  the  pa- 
pillary layer  of  the 
true  skin  and  is 
composed  of  large 
nucleated  cells,  the 
lowest  layer  of 
which, the  "prickle 
cells,"  contains  the 
pigment  granules 
which  give  to  the 
skin  its  varying 
hues  in  different 
individuals  and  in 
different  races  of 
men;  the  corneous 
layer  is  composed 
of  flattened  cells  which  from  their  exposure  to  the  atmosphere,  etc., 
are  hard  and  horny  in  texture. 

The  Sweat-glands. — These  glands  are  tubular  in  shape,  the  inner 
extremity  of  each  being  coiled  upon  itself  a  number  of  times,  forming 
a  little  ball  situated  in  the  derma  or  the  subcutaneous  connective 
tissue.  From  this  coil  the  duct  passes  up  in  a  straight  direction  to  the 
epidermis,  where  it  makes  a  few  spiral  turns,  after  which  it  opens 
obliquely  on  the  surface.  The  gland  consists  of  a  basement  membrane 
lined  with  epithelial  cells.  It  is  supplied  abundantly  with  blood- 
vessels and  nerves.     The  sweat-glands  are  extremely  numerous  all 


j^W     ^ 


Fig.  224. — Section  Perpendicularly  Through  the 
Healthy  Skin.  a.  Epidermis  or  scarfskin.  b.  Rcte  mu- 
cosum,  or  rete  malpighii.  c.  Papillary  layer,  d.  Derma, 
corium,  or  true  skin.  e.  Panniculus  adiposus,  or  fatty  tis- 
sue. /,  g,  h.  Sweat-gland  and  duct,  i,  k.  Hair,  with  its 
follicle  and  papilla.     /.   Sebaceous  gland. 


488  TEXT-BOOK  OF  PHYSIOLOGY. 

over  the  cutaneous  surface,  though  they  are  more  thickly  disposed 
in  some  situations  than  others.  They  probably  average  2500  to  the 
square  inch;  the  total  number  has  been  estimated  at  from  2,000,000 
to  2,500,000. 

The  Influence  of  the  Nerve  System  on  the  Production  of 
Sweat. — The  secretion  of  sweat,  though  a  product  of  the  activity 
of  epithelial  cells  and  dependent  on  a  variety  of  conditions,  is  reg- 
ulated to  a  large  extent  by  the  nerve  system.  Here  as  in  other  secreting 
glands  the  fluid  is  derived  from  materials  in  the  lymph-spaces,  fur- 
nished by  the  blood.  Generally  the  two  conditions,  increased  blood- 
flow  and  increased  glandular  action,  coexist.  At  times,  however, 
a  profuse  clammy  perspiration  is  secreted  with  diminished  blood-flow. 
Two  sets  of  nerves  are  evidently  concerned  in  this  process:  viz., 
vaso-motor  nerves,  which  regulate  the  blood-supply,  and  secret  or 
nerves,  which  stimulate  the  gland  cells  to  activity. 

The  nerve-centers  which  control  the  sweat-glands  are  situated  in 
the  spinal  cord,  though  the  number  of  such  centers  and  their  exact 
location  for  the  different  regions  of  the  body  have  not  yet  been  satis- 
factorily determined.  In  a  general  way  it  may  be  stated  that  the 
centers  for  the  head  and  face  lie  in  the  upper  cervical  portion  of  the 
cord;  for  the  upper  extremities,  in  the  lower  cervical  portion;  for  the 
lower  extremities,  in  the  lower  dorsal  and  upper  lumbar  portion. 
The  secretor  nerves  which  emerge  from  these  centers  reach  the  glands 
of  the  face  and  head  through  the  cervical  sympathetic;  of  the  arms 
and  legs,  through  the  brachial  plexus  and  the  sciatic  nerves.  It  is 
probable  that  there  is  also  a  general  dominating  sweat  center  located 
in  the  medulla  oblongata. 

That  the  sweat-glands  are  stimulated  to  activity  by  nerve  impulses 
is  shown  by  the  fact  that  stimulation  of  the  peripheral  end  of  the 
divided  cervical  sympathetic,  of  the  brachial  plexus,  or  of  the  sciatic 
nerve  is  followed  in  a  few  seconds  by  a  profuse  secretion.  Though 
under  physiologic  conditions  there  is  a  simultaneous  dilatation  of  the 
blood-vessels  and  an  increased  supply  of  blood,  this  is  merely  a  con- 
dition and  not  a  cause  of  the  secretion;  for  the  secretion  can  be  excited 
and  the  flow  maintained  for  a  period  of  from  ten  to  fifteen  minutes 
after  ligation  of  the  blood-vessels  of  the  limb  or  even  after  its  ampu- 
tation, when  the  corresponding  nerve  is  stimulated. 

The  sweat-glands  may  be  excited  to  activity  by  their  related  nerve- 
centers,  either  by  central,  reflex,  or  peripheral  influences.  Among 
the  first  may  be  mentioned  mental  emotions,  venosity  of  the  blood, 
increased  temperature  of  the  blood,  hot  drinks,  violent  muscular 
exercise,  etc.  Among  the  second  may  be  mentioned  powerful  stim- 
ulation of  various  afferent  or  sensor  nerves,  heightened  external 
temperature,  etc.  Among  the  last  may  be  mentioned  various  drugs. 
Pilocarpin  injected  into  the  blood  causes  a  profuse  secretion  even 
when  the  nerves  have  been  divided.  Its  action  is  supposed  to  be 
exerted  on  the  terminal  branches  of  the  nerves  and  possibly  on  the 


EXCRETION. 


489 


cells  themselves.     As  in  the  case  of  the  salivary  glands  atropin  sus- 
pends the  activity  of  the  terminal  branches  of  the  secretor  nerves. 

Hairs. — Hairs  are  found  in  almost  all  portions  of  the  body,  and 
can  be  divided  into — • 

1.  Long,  soft  hairs,  on  the  head. 

2.  Short,  stiff  hairs,  along  the  edges  of  the  eyelids  and  nostrils. 

3.  Soft,  downy  hairs  on  the  general  cutaneous  surface. 

They  consist  of  a  root  and  a  shaft.  The  shaft  is  oval  in  shape 
and  about  -^j^  of  an  inch  in  diameter;  it  consists  of  fibrous  tissue, 
covered  externally  by  a  layer  of  imbricated  cells,  and  internally  by 
cells  containing  granular  and  pigment  material. 

The  root  of  the  hair  is  embedded  in  the  hair-follicle,  formed  by 
a  tubular  depression  of  the  skin,  ex- 
tending nearly  through  to  the  sub- 
cutaneous tissue;  its  walls  are  formed 
by  the  layers  of  the  corium,  covered 
by  epidermic  cells.  At  the  bottom  of 
the  follicle  there  is  a  papillary  projec- 
tion of  amorphous  matter,  corres- 
ponding to  a  papilla  of  the  true  skin, 
containing  blood-vessels  and  nerves, 
upon  which  the  hair-root  rests.  The 
investments  of  the  hair-roots  are 
formed  of  epithelial  cells,  constituting 
the  internal  and  external  root-sheaths. 

The  lower  portion  of  the  hair- 
follicle  is  connected  with  the  upper 
surface  of  the  derma  by  bundles  of 
non-striated  muscle-fibers  which  are 
termed  arrectores  pilorum  muscles. 
Their  inclination  and  insertion  are 
such  that  their  contraction  is  followed 
by  erection  of  the  hair-follicle  and  hair-shaft.  These  muscles  are  ex- 
cited to  action  by  nerves  termed  pilo-motor  nerves. 

THE  SEBUM. 

The  sebum  or  sebaceous  matter  is  a  peculiar  oily  material 
produced  by  specialized  glands  in  the  skin.  It  consists  of  water, 
epithelium,  proteids,  fat,  cholesterin,  and  inorganic  salts. 

The  sebaceous  glands  are  simple  and  compound  racemose 
glands  opening  by  a  common  excretory  duct  on  the  surface  of  the 
epidermis  or  into  the  shaft  of  a  hair-follicle  (Fig.  225),.  These  glands 
are  extremely  numerous  and  found  in  all  portions  of  the  body,  with 
the  exception  of  the  palms  of  the  hands  and  soles  of  the  feet,  and  most 
abundantly  in  the  face.  They  are  formed  by  a  delicate  structureless 
membrane  lined  by  polyhedral  epithelium. 

The  sebum  is  not  produced  by  an  act  of  true  secretion,  but  is 


Fig.  225. — Large  Sebaceous 
Gland,  i.  Hair  in  its  follicle.  2,  3, 
4,  5.  Lobules  of  the  gland.  6.  Ex- 
cretory duct  traversed  by  the  hair. 
— (Sa'ppey.) 


4QO  TEXT-BOOK  OF  PHYSIOLOGY. 

formed  by  a  proliferation  and  degeneration  of  the  gland  epithelium. 
"When  first  poured  on  the  surface,  the  sebum  is  oily  and  semiliquid 
in  character,  but  soon  hardens  and  acquires  a  cheese-like  consistence. 
It  serves  to  lubricate  the  hair  and  skin  and  prevent  them  from  be- 
coming dry  and  harsh. 

The  surface  of  the  fetus  is  generally  covered  with  a  thick  layer  of 
sebaceous  matter,  the  vernix  caseosa,  which  possibly  keeps  the  skin 
in  a  normal  condition  by  protecting  it  from  the  effects  of  the  long- 
continued  action  of  the  amniotic  fluid  in  which  the  fetus  is  suspended. 


CHAPTER  XIX. 

THE  CENTRAL  ORGANS  OF  THE  NERVE  SYSTEM  AND 
THEIR  NERVES. 

The  central  organs  of  the  nerve  system  are  the  encephalon 
and  the  spinal  cord  lodged  within  the  cavity  of  the  cranium  and  the 
cavity  of  the  spinal  or  vertebral  column  respectively.  The  general 
shape  of  these  two  portions  of  the  nerve  system  corresponds  with 
that  of  the  cavities  in  which  they  are  contained.  The  encephalon  is 
broad  and  ovoid,  the  spinal  cord  is  narrow  and  elongated. 

The  encephalon  is  subdivided  by  deep  fissures  into  four  distinct, 
though  closely  related  portions:  viz.,  (i)  the  cerebrum,  the  large  ovoid 
mass,  occupying  the  entire  upper  part  of  the  cranial  cavity;  (2)  the 
cerebellum,  the  wedge-shaped  portion  placed  beneath  the  posterior 
part  of  the  cerebrum  and  lodged  within  the  cerebellar  fossae  of  the 
cranium;  (3)  the  isthmus  of  the  encephalon,  the  more  or  less  pyramidal- 
shaped  portion  connecting  the  cerebrum  and  cerebellum  with  each 
other  and  both  with  (4)  the  medulla  oblongata.     (Fig.  226.) 

The  spinal  cord  is  narrow  and  cylindric  in  shape.  It  occupies 
the  spinal  canal  as  far  as  the  second  or  third  lumbar  vertebra.  The 
central  nerve  system  is  bilaterally  symmetric,  consisting  of  distinct 
halves  united  in  the  median  line.  The  cerebrum  is  subdivided  by  a 
deep  fissure,  running  antero-posteriorly,  into  two  ovoid  masses  termed 
cerebral  hemispheres;  the  cerebellum  is  also  partially  subdivided  into 
hemispheres;  the  isthmus  likewise  presents  in  the  median  line  a  partial 
division  into  halves;  the  medulla  oblongata  and  spinal  cord  are  sub- 
divided by  an  anterior  or  ventral  and  a  posterior  or  dorsal  fissure  into 
halves,  a  right  and  a  left. 

The  peripheral  organs  of  the  nerve  system  in  anatomic  and 
physiologic  relation  with  the  central  organs  are  the  encephalic  and  the 
spinal  nerves.  The  encephalic  nerves,  twelve  in  number  on  each 
side  of  the  median  line,  are  in  relation  with  the  base  of  the  encephalon, 
and  because  of  the  fact  that  they  pass  through  foramina  in  the  walls  of 
the  cranium  they  are  usually  termed  cranial  nerves. 

The  spinal  nerves,  thirty-one  in  number  on  each  side,  are  in  re- 
lation with  the  spinal  cord,  and  because  of  the  fact  that  they  pass  through 
foramina  in  the  walls  of  the  spinal  column  they  are  termed  spinal  nerves. 
As  both  cranial  and  spinal  nerves  are  ultimately  distributed  to  the 
structures  of  the  body — i.  e.,  the  general  periphery — they  collectively 
constitute  the  peripheral  organs  0}  the  nerve  system. 

The  central  organs  of  the  nerve  system  are  supported  and  protected 
by  three  membranes  named,  in  their  order  from  without  inward,  the 
dura  mater,  the  arachnoid,  and  the  pia  mater. 

4Qi 


49: 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  dura  mater  is  a  tough  membrane  composed  of  fibrous  tissue. 
It  consists  of  two  layers,  the  outer  of  which  lines  the  cranial  cavity  and 
forms  an  internal  periosteum;  the  inner  layer  is  closely  attached  to 
the  outer  except  at  certain  regions  where  it 
separates  and  forms  supporting  structures, 
such  as  the  falx  cerebri,  falx  cerebelli,  ten- 
torium cerebelli,  etc.;  at  the  margin  of  the 
foramen  magnum  the  outer  layer  becomes  con- 
tinuous with  the  periosteal  tissue,  while  the 
inner  layer  invests  the  cord  down  to  its  ulti- 
mate termination.     (Fig.  227.) 

The  arachnoid  is  a  delicate  serous  mem- 
brane. The  external  surface  is  smooth  and 
well  defined  and  separated  from  the  dura  by 
a  narrow  space,  the  subdural  space.  The 
inner  surface  sends  inward  fine  connective- 
tissue  processes  which  interlace  in  every  direc- 
tion, constituting  the  subarachnoid  tissue. 
This  tissue  is  abundant  in  the  cranium,  much 
less  so  in  the  spinal  canal.  The  spaces  be- 
tween the  connective  tissue,  taken  collectively, 
constitute  the  general  subarachnoid  space. 
Around  the  spinal  cord  this  space  is  well  de- 
fined, and  at  the  base  of  the  encephalon  ex- 
pands to  form  large  cavities  known  as  the 
cisterna  magna,  cisterna  pontis,  etc. 

The  pia  mater  is  a  delicate  membrane 
composed  of  areolar  tissue.  It  closely  invests 
the  encephalon  and  spinal  cord,  dipping  into 
the  various  fissures.  It  is  exceedingly  vascular 
and  sends  small  blood-vessels  for  some  distance 
into  the  brain  and  spinal  cord. 

The  Encephalo-spinal  Fluid. — The  gen- 
eral subarachnoid  space,  as  well  as  certain 
cavities  within  the  encephalon,  contain  a  clear 
transparent  fluid,  termed  the  encephalo-spinal. 
This  fluid  has  an  alkaline  reaction  and  a 
specific  gravity  of  1.007  or  1-008.  It  is  com- 
posed of  water,  proteids  (proteoses  and  serum- 
globulin),  and  a  compound  pyrocatechin, 
capable  of  reducing  copper  salts,  though  not 
exhibiting  any  other  of  the  properties  of  sugar. 
In  many  respects  this  fluid  resembles  lymph. 
The  subarachnoid  space  and  the  general  encephalic  cavities,  termed 
ventricles,  communicate  with  one  another  by  an  opening  in  the  pia 
mater  (the  foramen  of  Magendie)  as  it  passes  over  the  lower  part  of 
the  fourth  ventricle. 


Fig.  226. — The  Central 
Organs  of  the  Nerve 
System,  f.  t.  o.  Frontal, 
temporal,  and  occipital 
lobes  of  the  cerebrum,  c. 
Cerebellum,  p.  Pons.  mo. 
Medulla  oblongata.  ms., 
ms.  The  upper  and  lower 
limits  of  the  spinal  cord. 
The  remaining  letters  in- 
dicate the  region  and  num- 
ber (if  the  spinal  nerves. — 
(Quain,  after  Bonrgery.) 


THE  ENCEPHALO-SPINAL  MEMBRANES. 


493 


the 


It  was  stated  in  Chapter  VIII  that  the  entire  nerve  or  neuron  system 
can  be  resolved  into  a  single  morphologic  unit,  the  neuron:  the  histo- 
logic features  and  the  physiologic  properties  of  the  neuron  were  there 
also  described;  the  anatomic  relation  of  the  neurons  constituting  the 
peripheral  organs  of  the  nerve  system,  to  the  neurons  constituting  the 
central  organs  of  the  nerve  system  were  also  stated  and  illustrated  in 
part  diagrammatically,  page  125.  From  the  statements  made  regarding 
the  functions  of  the  different  neurons  in  their  individual  and  collective 
capacity  the  functions  of  the  nerve  system  will  become  apparent. 

The  Functions  of  the  Nerve  System. — The  functions  of 
nerve  system  are  twofold:  (1)  It  unites 
and  coordinates  the  organs  and  tissues 
of  the  body  in  such  a  manner  that  they 
are  enabled  to  cooperate  for  the  accom- 
plishment of  a  definite  object.  (2)  It 
serves  to  arouse  in  the  individual  a  con- 
sciousness of  the  existence  of  an  external 
world,  by  virtue  of  the  impressions  which 
it  makes  on  his  sense  organs,  and  conse- 
quently to  enable  him  to  adjust  himself 
to  his  environment. 

By  virtue  of  the  coordination,  a  stim- 
ulus, if  of  sufficient  intensity,  applied  to 
one  organ  or  tissue  will  call  forth  activity 
in  one  or  more  organs  near  or  remote 


from  the  part  stimulated.  This  coordi- 
nation is  accomplished  mainly  by  the 
spinal  cord  and  the  medulla  oblongata. 
All  actions  which  take  place  in  response 
to  a  peripheral  stimulus  and  independ- 
ently of  volition  are  termed  reflex  ac- 
tions. The  reflex  activities  connected 
with  digestion,  the  circulation  of  the 
blood,  with  respiration,  excretion,  etc., 
are  illustrations  of  the  coordinating  capa- 
bilities of  the  nerve-centers  located  in 
these  portions  of  the  central  nerve  system. 

Consciousness  of  the  existence  of  the  external  world  and  of  the  re- 
lation existing  between  it  and  the  individual  is  associated  with  the 
physiologic  activities  of  the  encephalon,  and  more  particularly  of  the 
cerebral  hemispheres.  This  portion  of  the  nerve  system  is  the  chief, 
though  perhaps  not  the  sole,  organ  of  the  mind,  and  its  main  functions 
are  for  the  most  part  mental. 

The  function  of  a  part  at  least  of  the  peripheral  nerve  system  is  to 
afford  a  means  of  communication  between  the  central  nerve  system 
and  the  remaining  structures  of  the  bodv.     The  nerve-trunks  consti- 


Fig.  227. — The  Membranes  of 
the  Spinal  Cord.  i.  Dura 
mater.  2.  Arachnoid.  3.  Poste- 
rior root  of  spinal  nerve.  4.  An- 
terior root  of  spinal  nerve.  5. 
Ligamentum  dentatum.  6.  Linea 
splendens. — (Morris,    ajter  Ellis.) 


tuting  this  part  may  be  divided  into  two  groups,  as  follows: 


494  TEXT-BOOK  OF  PHYSIOLOGY. 

i.  The  first  group  comprises  nerves  in  connection  with  the  special 
sense-organs,  e.  g.,  eye,  ear,  nose,  tongue,  skin,  as  well  as  nerves 
in  connection  with  the  general  or  organic  sense-organs,  e.  g.,  mu- 
cous membranes,  viscera,  etc.,  which  transmit  nerve  impulses 
to  certain  localized  areas  in  the  cerebral  cortex,  where  they  are 
translated  into  conscious  sensations.  These  sensations,  both 
special  and  general,  by  their  grouping  and  combinations  are  the 
primary  elements  of  intelligence. 
2.  The  second  group  comprises  those  nerves  which  terminate  in  the 
muscle  apparatus  and  which  transmit  nerve  impulses,  by  way  of 
the  medulla  and  spinal  cord,  from  localized  areas  in  the  cerebral 
cortex  to  the  muscles  of  the  face,  trunk  and  extremities,  which  are 
in  consequence  excited  to  activity.  The  muscle  movements  thus 
become  physical  expressions  of  mental  states,  and  if  directed  in  a 
definite  manner  to  the  overcoming  of  the  resistances  offered  by 
the  external  world  become  capable  of  modifying  it  in  accordance 
with  the  mental  states. 

The  first  group  of  nerves,  the  afferent,  especially  those  connected 
with  the  special  sense-organs,  are  excited  to  activity  by  impressions 
made  on  their  peripheral  terminations  by  agencies  in  the  external 
world,  and  thus  become  a  means  of  communication  between  the 
physical  and  the  mental  worlds. 

The  second  group  of  nerves,  the  efferent,  are  excited  to  activity 
bv  those  molecular  disturbances  in  their  related  nerve-cells  which 
accompany  volitional  efforts,  and  thus  they  become  a  means  of  com- 
munication between  the  mental  and  the  physical  worlds. 

The  central  nerve  system  is  thus  composed  of  a  number  of  separate 
though  closely  related  parts,  to  each  of  which  a  separate  function  has 
been  assigned.  In  the  study  of  the  structure  and  function  of  these 
separate  parts  it  will  be  found  convenient,  and  conducive  to  clearness, 
to  consider  them  in  the  order  of  their  complexity,  beginning  with  the 
spinal  cord  and  ending  with  the  cerebrum. 

THE  SPINAL  CORD. 

The  spinal  cord  is  the  narrow  elongated  portion  of  the  central 
nerve  system  contained  within  the  spinal  canal.  It  is  cylindric  in 
shape  though  presenting  an  enlargement  in  both  the  lower  cervical 
and  lower  lumbar  regions  corresponding  to  the  origins  of  the  nerves 
distributed  to  the  upper  and  lower  extremities.  The  cord  varies  in 
length  from  40  to  45  cm.,  measures  12  mm.  in  diameter,  weighs  42  gms., 
and  extends  from  the  atlas  to  the  second  lumbar  vertebra,  beyond 
which  it  is  continued  as  a  narrow  thread,  the  filum  terminate.  (Fig. 
228.)  It  is  divided  by  the  anterior  and  posterior  longitudinal  fissures 
into  halves,  and  is  therefore  bilaterally  symmetric.  A  transverse  sec- 
tion of  the  cord  shows  that  it  is  composed  of  both  white  and  gray  mat- 
ter, the  former  covering  the  surface,  the  latter  occupying  the  center. 


THE  SPIXAL  CORD. 


495 


Structure  of  the  Gray  Matter. — The  gray  matter  is  arranged 
in  the  form  of  two  crescents,  united  in  the  median  line  by  a  trans- 
verse band  or  commissure  forming  a  figure  resembling  the  letter  H. 
Though  varying  in  shape  in  different  regions  of  the  cord,  the  gray 
matter  in  all  situations  presents  on  either  side  an  anterior  or  ventral 
and  a  posterior  or  dorsal  horn.     Between  the  two  horns  there  is  a 


Superior  or  Cervical  Segment 
of  Spinal  Cord. 


Middle  or  Dorsal  Portion 
of  Cord. 


Inferior  Portion  of  Cord  and 
Cauda  Equina. 


Fig.  228.- — Superior,  Middle,  and  Inferior  Portions  of  Spinal  Cord.  i.  Floor 
of  ^fourth  ventricle.  2.  Superior  cerebellar  peduncle.  3.  Middle  cerebellar  peduncle. 
4-»Inferior  cerebellar  peduncle.  5.  Enlargement  at  upper  extremity  of  postero-median 
column.  6.  Glosso-pharyngeal  nerve.  7.  Vagus.  S.  Spinal  accessory.  9,  9,  9,  9. 
Ligamentum  denticulatum.  10,  10,  10,  10.  Posterior  roots  of  spinal  nerves.  11,  11,  n. 
11.  Postero-lateral  fissure.  12,  12,  12,  12.  Ganglia  of  posterior  roots.  13,  13.  Anterior 
roots.  14.  Division  of  united  roots  into  anterior  and  posterior  nerves.  15.  Terminal 
extremity  of  cord.  16,  16.  Filum  terminale.  17,  17.  Cauda  equina.  I,  VIII.  Cervical 
nerves.     I,  XII.  Dorsal  nerves.     I,  V.  Lumbar  nerves.     I,  V.  Sacral  nerves. — (Sappcy.) 


portion  termed  the  intermediate  gray  substance.  The  commissure 
presents  in  its  center  a  narrow  canal  which  extends  throughout  the 
entire  length  of  the  cord.  This  canal  is  lined  by  cylindric  epithelium 
and  surrounded  by  gelatinous  material.     (Fig.  229.) 

The  anterior  horn  is  short  and  broad  and  entirely  surrounded 
by  white  matter.  The  posterior  horn  is  narrow  and  elongated  and 
extends  quite  up  to  the  surface  of  the  cord,  where  it  is  capped  by  gelat- 


496 


TEXT-BOOK  OF  PHYSIOLOGY. 


inous  matter,  the  substantia  gelatinosa.  In  the  lower  cervical  and 
thoracic  regions  a  portion  of  the  intermediate  gray  substance  pro- 
jects outward  and  forms  the  so-called  lateral  horn.  The  gray  matter 
fundamentally  consists  of  a  framework  of  tine  neuroglia  supporting 

blood-vessels,  lymphatics,  medullated 
and  non-medullated  nerves,  and  groups 
of  nerve-cells. 

The  Nerve-cells. — The  nerve-cells 
of  the  cord  are  very  numerous  and  pre- 
sent a  variety  of  shapes  and  sizes  in 
different  regions.  They  are  usually 
arranged  in  groups  which  extend  for 
some  distance  up  and  down  the  cord, 
forming  columns  more  or  less  continuous. 
In  the  anterior  horn  two  well-marked 
groups  are  found,  one  situated  at  the 
anterior  and  inner  angle,  known  as  the 
antero-median  group,  the  other  situated 
at  the  posterior  and  lateral  angle  and 
known  as  the  postero-lateral  group.  In 
the  lower  cervical  and  upper  thoracic  re- 
gions, in  the  region  of  the  lateral  horn, 
another  group  of  cells  is  found,  known  as 
the  intermediate  group.  In  the  central 
portion  of  the  horn  there  is  also  a  central 
group. 

The  cells  of  the  anterior  horns  are  of 
large  size,  nucleated  and  multipolar. 
They  are  the  modified  descendants  of 
pear-shaped  cells,  the  neuroblasts,  which 
migrated  from  the  medullary  tube  (see 
page  105).  In  the  course  of  their  migra- 
tion they  developed  dendrites  which  form 
an  intricate  felt-work  throughout  the 
anterior  horn.  One  of  the  processes,  the 
axon,  approached  the  surface  of  the  c6rd, 
penetrated  it,  grew  outward,  became 
covered  with  myelin  and  neurilemma, 
and  developed  into  an  anterior  root- 
fiber.  These  nerve-cells,  with  their 
dendrites,  axons,  and  terminal  branches, 
form  efferent  neurons  of  the  first  order. 
The  intimate  histologic  and  physiologic 
relationship  existing  between  the  nerve-cell  and  the  axon  is  revealed  by 
the  degenerative  changes  which  arise  in  the  latter  when  separated  from 
the  former.  The  cell  apparently  determines  the  nutrition  of  the  axon 
and  may  be  regarded  as  trophic  in  function.     wSome  of  the  cells  of  the 


C 


D 


Fig.  229. — Sections 
Different     Regions 
Spinal  Cord.     A.     At 
of    the    sixth    cervical    nerve. 
At     the     mid-dorsal     region. 


THROUGH 
OF       THE 

the  level 
B. 
C. 
At  the  center  of  the  lumbar 
enlargement.  D.  At  the  upper 
pari  of  the  conus  medullaris.  1. 
Posterior  roots.  2.  Anterior  roots. 
3.  Posterior  fissure.  4.  Anterior 
6  in-.  5.  Central  canal. — (Mor- 
ris'   "Anatomy,"   ajter  Schwalbe.) 


THE  SPIXAL  CORD. 


497 


anterior  horn  send  their  axons  into  the  immediately  surrounding  white 
matter  of  the  same  side,  after  which  they  divide  into  two  branches, 
one  passing  up,  the  other  down,  the  cord,  to  re-enter  the  gray  matter  at 
different  levels.  They  are  probably  associative  in  function.  Other 
cells  send  their  axons  into  that  portion  of  the  white  matter  on  the  same 
and  opposite  sides  known  as  Gower's  antero-lateral  tract.  (Fig.  230.) 
In  the  posterior  horn  nerve-cells  are  also  present,  though  they 
are  not  so  numerous  as  in  the  anterior  horn.  At  the  base  of  the  horn 
and  on  its  inner  side  there  is  a  well-marked  group  of  cells  which  ex- 
tends from  the  seventh  or  eighth  cervical  nerves  downward  to  the 


Oorsal 


l/entral 


Fig.  230. — Scheme  of  the  Structure  of  the  Cord. — (Howell  after  Lenhossek.) 
On  the  right  the  nerve  cells;  on  the  left  the  entering  nerve  fibers.  Right  side:  i,  Motor 
cells,  anterior  horn,  giving  rise  to  the  fibers  of  the  anterior  root;  2,  tract  cells  whose  axons 
pass  into  the  white  matter  of  the  anterior  and  lateral  columns;  3,  commissural  cells  whose 
axons  pass  chief!}'  through  the  anterior  commissure  to  reach  the  anterior  columns  of  the 
other  side;  4,  Golgi  cells  (second  type),  whose  axons  do  not  leave  the  gray  matter;  5,  tract 
cells  whose  axons  pass  into  the  white  matter  of  the  posterior  column.  Left  side:  1,  Enter- 
ing fibers  of  the  posterior  root,  ending,  from  within  outward,  as  follows:  Clarke's  column, 
posterior  horn  of  opposite  side,  anterior  horn  same  side  (reflex  arc),  lateral  horn  of  same 
side,  posterior  horn  of  same  side;  2,  collaterals  from  fibers  in  the  anterior  and  lateral 
columns;  3,  collaterals  of  descending  pyramidal  fibers  ending  around  motor  cells  in  anterior 
horn. 


second  or  third  lumbar  nerves,  being  most  prominent  in  the  thoracic 
region.  This  column  is  known  as  Clarke's  vesicular  column.  From 
the  nerve-cells  constituting  this  column  axons  pass  obliquely  outward 
into  that  portion  of  the  white  matter  known  as  the  direct  cerebellar  tract. 
Other  nerve-cells  send  their  axons  into  the  white  matter  in  the 
posterior  portion  of  the  cord  bordering  the  posterior  median  fissure. 
Some  of  the  nerve-cells,  their  situation  and  the  distribution  of  their 
axons  are  shown  in  Fig.  230. 

Classification  of  Nerve-cells. — The  cells  of  the  gray  matter  may 
be  divided  into  three  main  groups:  viz.,  intrinsic,  efferent,  and  atferent. 


498  TEXT-BOOK  OF  PHYSIOLOGY. 

The  intrinsic  cells  are  associative  in  function.  The  axons  to  which 
these  cells  give  origin  pass  more  or  less  horizontally  into  the  white 
matter,  where  they  divide  into  two  branches,  one  of  which  passes 
upward,  the  other  downward.  At  various  levels  they  re-enter  the 
gray  matter  and  arborize  around  other  intrinsic  cells. 

The  efferent  cells,  independently  of  their  trophic  influence,  are 
also  motor  in  function,  inasmuch  as  the  excitation  arising  in  them 
is  transmitted  outwardly  through  their  axons  to  muscles,  blood-vessels, 
glands  and  viscera,  imparting  to  them  motion,  either  molar  or  molec- 
ular. As  the  efferent  fibers  in  the  ventral  roots  of  the  spinal  nerves 
are  classified  (see  page  107)  in  accordance  with  their  physiologic  action 
into  motor,  vaso-motor,  secretor,  viscero-motor  and  pilo-motor  nerves, 
so  the  nerve-cells  of  which  the  nerves  are  integral  parts  may  be  classified 
physiologically  as  motor,  vaso-motor,  secretor,  viscero-motor  and  pilo- 
motor.    Collections  or  groups  of  such  cells  are  termed  "centers." 

The  afferent  cells  are  largely  sentient  or  receptive  in  function, 
inasmuch  as  the  excitations  brought  to  the  spinal  cord  by  the  afferent 
nerves  in  the  dorsal  roots  from  the  general  periphery  are  received 
by  them  and  transmitted  by  and  through  their  axons  to  the  cortex  of  the 
cerebrum,  where  they  are  translated  into  conscious  sensations.  As 
the  nerve-fibers  in  the  dorsal  roots  of  the  spinal  nerves  are  classified, 
in  accordance  with  the  sensations  to  which  they  give  rise,  as  sensor, 
thermal,  tactile,  etc.,  so  these  nerve-cells  may  be  similarly  classified 
according  as  they  transmit  their  excitations  to  those  specialized  areas 
in  the  cerebral  cortex  in  which  these  different  sensations  arise. 

Structure  of  the  White  Matter. — A  transverse  section  of  the 
cord  shows  that  the  white  matter  completely  covers  the  gray  matter 
except  where  the  posterior  horns  reach  the  surface.  Anteriorly  the 
white  matter  of  each  lateral  half  is  connected  by  a  narrow  strip  or 
bridge  of  white  matter,  the  anterior  commissure.  Microscopic  ex- 
amination shows  that  the  white  matter  is  composed  of  vertically  dis- 
posed medullated  nerve-fibers  which  are  devoid  of  a  neurilemma. 
These  fibers  are  supported  partly  by  a  framework  of  connective 
tissue,  and  partly  by  neuroglia.  The  white  matter  of  each  side  of 
the  cord  is  anatomically  divided  into  an  anterior,  a  lateral,  and  a  pos- 
terior column  by  the  anterior  and  posterior  roots  of  the  spinal  nerves. 
Classification  of  the  Nerve-fibers. — From  a  study  of  the  em- 
bryologic  development  of  the  white  matter  and  of  the  degenerative 
changes  which  follow  its  pathologic  and  experimental  destruction,  it 
has  been  differentiated  into  a  number  of  specialized  tracts  which  have 
different  origins,  destinations,  and  functions.  Some  of  the  more  im- 
portant tracts  are  shown  in  Fig.  231.  They  may  be  divided,  however, 
into  efferent,  afferent,  and  associative  fibers. 

1.  The  anterior  column,  comprising  that  portion  between  the 
anterior  longitudinal  fissure  and  the  anterior  roots,  has  been  sub- 
divided into: 

(a)   The  direct  pyramidal  tract,  or  column  of  Tiirck.     This  tract 


THE  SPINAL  CORD. 


499 


borders  the  longitudinal  fissure  and  extends  from  the  upper  extremity 
of  the  cord  as  far  down  as  the  mid-thoracic  region.  From  above 
downward  this  tract  diminishes  in  size,  for  the  reason  that  its  fibers 
or  their  collaterals  cross  at  successive  levels  to  the  opposite  side  of  the 
cord  by  way  of  the  anterior  commissure  to  enter  the  gray  matter  of 
the  anterior  horn.  These  fibers  are  the  continuations  of  fibers  which 
take  their  origin  in  cells  wdiich  are  located  in  the  cortex  of  the  cerebral 
hemisphere  of  the  same  side.  The  terminal  filaments  of  these  fibers 
or  axons  are  in  physiologic  relation  either  directly  or  indirectly  through 
intercalated  neuron  cells  with  the  dendrites  of  the  cornual  cells. 
When  divided  in  any  part  of  their  course,  these  fibers  undergo  de- 
scending degeneration.  They  are  therefore  efferent  neurons  and  of  the 
second  order. 


Fig.  231. — Transection  of  the  Cervical  Spinal  Cord  Showing  Its  Chief  Sub- 
divisions.— (After  Mills.) 


(b)  The  ant ero -lateral  ground  bundle  or  root  zone.  This  tract  lies 
external  to  the  pyramidal  tract,  surrounds  the  anterior  horn  of  the 
gray  matter  and  extends  throughout  the  length  of  the  cord.  It  is 
composed  of  short  commissural  or  associative  fibers  which  come  from 
nerve-cells  in  the  gray  matter  from  the  same  and  opposite  sides  of  the 
cord.  After  entering  the  white  matter  the}7  divide  into  two  branches, 
pursue  opposite  directions,  then  re-enter  the  gray  matter  at  higher 
and  lower  levels  and  come  into  relation  with  other  nerve-cells. 

2.  The  lateral  column,  comprising  that  portion  between  the 
ventral  and  dorsal  roots,  has  been  divided  into : 


500  TEXT-BOOK  OF  PHYSIOLOGY. 

(a)  The  antero-lateral  tract  of  Gowers.  This  tract  is  somewhat 
crescentic  in  shape  and  situated  on  the  lateral  aspect  of  the  cord  ex- 
ternal to  the  antero-lateral  root  zone.  It  extends  throughout  the  entire 
length  of  the  cord.  When  divided  it  undergoes  ascending  degenera- 
tion, which  would  indicate  that  the  axons  originate  in  nerve-cells 
in  the  gray  matter.    This  tract  is  therefore  afferent  in  function. 

(b)  The  lateral  limiting  tract.  This  tract,  which  is  quite  narrow,  lies 
close  to  the  external  border  of  the  gray  matter.  It  is  composed  of 
fibers  which  do  not  degenerate  to  any  considerable  extent  and  are  in 
all  probability  associative  fibers  which  come  from  nerve-cells  in  the 
gray  matter  to  re-enter  at  lower  and  higher  levels.  It  is  also  believed 
by  some  investigators  that  the  anterior  portion  contains  efferent  and 
the  posterior  portion  afferent  fibers;  for  this  reason  it  is  frequently 
termed  the  mixed  lateral  tract. 

(c)  The  crossed  pyramidal  tract.  This  tract  occupies  the  posterior 
portion  of  the  lateral  column,  though  its  exact  position  varies  some- 
what in  different  regions  of  the  cord.  In  the  cervical  and  thoracic 
regions  it  is  covered  by  a  layer  of  fibers.  In  the  lumbar  region,  how- 
ever, it  comes  to  the  surface.  From  above  downward  this  tract  grad- 
ually diminishes  in  size,  for  the  reason  that  its  fibers  and  their  col- 
laterals enter  the  gray  matter  at  successive  levels.  The  terminal 
branches  of  these  fibers  are  in  close  physiologic  relation  either  directly 
or  indirectly  through  intercalated  neuron  cells  with  the  dendrites  of 
the  cornual  cells.  These  fibers  are  the  continuations  of  fibers  which 
take  their  origin  in  cells  which  are  located  in  the  cortex  of  the  cerebral 
hemispheres  of  the  opposite  side.  When  divided  in  any  part  of  their 
course,  they  undergo  descending  degeneration.  They  are  therefore 
efferent  neurons  and  of  the  second  order. 

(d)  The  direct  cerebellar  tract,  or  column  of  Flechsig.  This  tract 
is  situated  on  the  surface  of  the  lateral  column  external  to  the  crossed 
pyramidal  tract.  It  slightly  increases  in  size  from  belowr  upward. 
It  is  composed  of  fibers  the  cells  of  which  are  found  on  the  inner  side 
and  base  of  the  posterior  horn  (Clark's  vesicular  column).  From 
this  origin  the  fibers  pass  obliquely  outward  to  the  surface  and  then 
directly  upward  to  terminate,  as  its  name  implies,  in  the  cerebellum. 
Decussation  of  these  fibers  takes  place  in  the  superior  vermiform  lobe 
of  the  cerebellum.  When  divided  this  tract  degenerates  upward.  It 
is  therefore  in  all  probability  an  afferent  tract  and  of  the  second  order. 

3.  The  posterior  column,  comprising  that  portion  between  the 
dorsal  roots  and  the  posterior  longitudinal  fissure,  has  been  sub- 
divided into: 

(a)  The  poster  o-extemal  tract  of  Burdach.  This  tract  lies  just 
within  the  posterior  horns.  A  portion  of  this  tract  is  composed 
of  ground  fibers  which,  though  vertically  disposed,  have  but  a  short 
course.  They  take  their  origin  in  cells  in  the  gray  matter,  and  after 
entering  this  tract  divide  into  ascending  and  descending  branches, 
which    with    their   collaterals   re-enter   the   gray    matter   at   different 


THE  SPINAL  CORD.  501 

levels.  Another  portion  of  this  tract  is  made  up  of  nerve-fibers  de- 
rived from  the  dorsal  roots  of  the  spinal  nerves,  which  cross  this  col- 
umn toward  the  median  line  in  an  oblique  or  horizontal  direction. 
The  fibers  of  the  upper  portion  of  this  tract  terminate  around  the 
nucleus  cuneatus  at  the  medulla  oblongata.  When  divided,  these 
fibers  degenerate  for  but  a  short  distance.  The  ground  fibers  are  prob- 
ably associative  in  function. 

(b)  The  postero-internal  tract,  or  column  of  Goll.  This  tract  is 
separated  from  the  former  by  a  septum  of  connective  tissue  which  is 
most  marked  above  the  eleventh  thoracic  segment.  The  fibers  which 
compose  this  tract  are  long  and  derived  for  the  most  part  from  the 
dorsal  roots  of  the  spinal  nerves  of  the  same  side.  This  is  shown  by 
the  fact  that  division  of  these  roots  central  to  the  ganglion  is  followed 
by  ascending  degeneration  of  the  column  of  Goll  as  far  as  the  nucleus 
gracilis  in  the  medulla  oblongata.  Fibers  derived  from  cells  in  the 
gray  matter  are  also  contained  in  this  column.  This  tract  is  largely 
afferent  in  function. 

(c)  Lissauefs  tract.  This  tract  embraces  the  tip  of  the  posterior 
horn  and  is  composed  principally  of  fibers  from  the  dorsal  roots  of  the 
spinal  nerves.  After  entering  the  tract  the  fibers  divide,  into  ascend- 
ing and  descending  branches,  which  finally  terminate  around  cells 
in  the  posterior  horn. 

In  addition  to  the  tracts  described  in  foregoing  paragraphs  a  num- 
ber of  small  narrow  tracts  have  been  discovered  in  different  regions 
of  the  spinal  cord  the  functional  significance  of  which,  however,  has 
not  been  determined.     Of  these  may  be  mentioned: 

1.  The  antero-lateral  tract  of  Marchi  and  Lowenthal,  situated  at 
the  anterior  and  inner  angle  of  the  anterior  column,  which  degenerates 
downward  after  removal  of  one-half  of  the  cerebellum. 

2.  The  comma  tract  a  narrow  bundle  of  fibers  situated  in  the 
anterior  portion  of  the  column  of  Burdach.  When  it  is  divided  it 
degenerates  downward. 

3.  The  septo-marginal  tract,  an  oval  shaped  tract  situated  along 
the  margin  of  the  posterior  longitudinal  fissure. 

4.  The  cornu-commissural  tract  found  along  the  border  of  the 
anterior  portion  of  the  posterior  column  as  far  forward  as  the  posterior 
commissure.  Both  of  these  tracts  are  best  developed  in  the  lumbo- 
sacral region.  They  arise  from  nerve-cells  in  the  gray  matter.  They 
undergo  descending  degeneration  when  divided,  but  not  after  division 
of  the  dorsal  roots. 

The  Relation  of  the  Spinal  Nerves  to  the  Spinal  Cord. — The 

spinal  nerves  present  near  the  spinal  cord  two  divisions  which  from 
their  connection  with  the  anterior  or  ventral  and  the  posterior  or 
dorsal  surfaces  are  known  as  the  ventral  and  dorsal  roots. 

The  ventral  roots  are  the  axons  of  various  groups  of  nerve-cells 
situated  in  the  anterior  horns  of  the  grav  matter.     From  their  origin 


5o2  TEXT-BOOK  OF  PHYSIOLOGY. 

these  axons  pass  almost  horizontally  forward  through  the  anterior 
column  in  three  distinct  bundles.  After  emerging  from  the  cord 
they  curve  downward  and  backward  to  join  the  dorsal  roots.  The 
dorsal  roots  are  the  centrally  directed  axons  of  nerve-cells  in  the 
spinal  ganglia.  After  entering  the  cord  they  divide  into  two  main 
groups,  a  lateral  and  a  mesial.  A  portion  of  the  lateral  group  enters 
the  posterior  horn  directly  through  the  caput  cornu;  the  other  portion 
turns  upward  and  runs  through  Lissauer's  tract  and  ultimately  enters 
the  posterior  horn.  The  mesial  group  passes  into  the  postero-external 
column  (Burdach),  where  the  fibers  divide  into  descending  and  as- 
cending branches.  The  former  constitute  the  comma  tract,  the 
terminal  branches  of  which  surround  cells  in  the  gray  matter;  the 
latter  (ascending)  cross  the  column  obliquely  and  enter  the  postero- 
internal column  (Goll),  in  which  they  pass  upward  to  terminate  around 
the  cells  of  the  nucleus  gracilis  of  the  same  side.  As  these  root  fibers 
pass  up  and  down  the  cord,  collateral  branches  are  given  off  which 
enter  the  gray  matter  at  successive  levels  and  come  into  physiologic 
relation  with  the  cells  of  Clark's  vesicular  column  on  the  same  and 
opposite  sides  and  with  the  cells  of  the  anterior  horn. 

The  peripherally  directed  axons  of  the  nerve-cells  in  the  spinal 
nerve  ganglia  become  associated  with  the  axons  of  the  ventral  roots 
and  together  they  pass  as  a  spinal  nerve  to  peripheral  organs. 

The  ventral  root  axons  are  distributed  to  skeletal  muscles,  blood- 
vessels, glands  and  viscera.  The  dorsal  root  axons  are  distributed  to 
skin,  mucous  membranes  and  muscles.  The  classification  of  the 
nerve-fibers  in  the  ventral  and  dorsal  roots  in  accordance  with  the 
functions  they  subserve  will  be  found  on  pages  107,  108. 

Though  both  the  efferent  and  afferent  fibers  of  the  spinal  nerves 
are  directly  connected  with  nerve-cells  in  the  spinal  cord,  they  are 
also  indirectly  connected  by  efferent  and  afferent  nerve-tracts  with 
the  cerebral  cortex. 

Experimentally,  it  has  been  determined  that  the  anterior  or  ventral 
roots  contain  all  the  efferent  fibers,  the  posterior  or  dorsal  roots  all  the 
afferent  fibers.     The  proofs  in  support  of  this  view  are  as  follows: 
Stimulation  of  the  ventral  roots  produces: 

1.  Tetanic  contraction  of  skeletal  muscles. 

2.  Variations  in  the  degree  of  the  contraction,  the  tonus,  of  the 
muscle  walls  of  the  peripheral  arteries  either  in  the  way  of  aug- 
mentation or  inhibition. 

3.  Discharge  of  secretions  from  glands. 

4.  Variations  in  the  degree  of  the  contraction,  the  tonus,  of  the 
muscle  walls  of  certain  viscera  either  in  the  way  of  augmentation 
or  inhibition. 

Division  of  the  ventral  roots  is  followed  by: 

1.  Relaxation  of  skeletal   muscles  and  loss  of  movement. 

2.  Temporary  dilatation  and  loss  of  the  tonus  of  blood-vessels. 

3.  Cessation  in  the  discharge  of  secretions  from  glands. 


Diagram   Indicating  the  Course  or  the  Motor  and  Sensory   Fibers  of  the 
Spinal  Cord  and  Medulla. — (Gordinier.) 

a,  a.  Motor  cells  of  the  cerebral  cotrex.  b,  b.  Arborizations  of  the  fibers  of  the  sensory 
tract  in  the  cerebral  cortex,  c.  Nucleus  of  the  column  of  Burdach,  showing  terminal 
arborizations  of  the  long  sensory  fibers  of  the  cord.  d.  Nucleus  of  the  column  of  Goll, 
showing  terminal  arborizations  of  the  long  sensory  fibers  of  the  cord.  e.  Section  of  the 
medulla,  showing  sensory  decussation.  /.  Section  of  medulla,  showing  motor  or  pyramidal 
decussation,  g,  g.  Motorial  end  plates,  h.  Section  through  the  cervical  region  of  the  cord, 
showing  termination  in  the  anterior  horn  of  the  motor  fibers  of  the  direct  pyramidal  tract 
after  they  have  crossed  in  the  anterior  commissure;  also  fiber  of  crossed  pyramidal  tract 
ending  about  anterior  horn  cell  of  same  side,  i,  i.  Posterior  spinal  ganglia,  j,  k.  Sensory 
fibers  of  short  course.  /.  Sensory  fibers  of  long  course,  terminating  in  medulla,  in,  m, 
m.   Sensory  end  organs,     n.   Section  through  lumbar  cord. 


PLATE  II. 


THE  SPINAL  CORD.  503 

4.  Temporary  impairment  of  the  normal  activities  of  the  visceral 
muscles  from  loss  of  central  nerve  control;  the  degree  of  impair- 
ment depending  on  the  nature  of  the  viscus  involved. 
Peripheral  stimulation  of  the  dorsal  roots  produces: 

1.  Sensations  of  touch,  temperature,  pressure  and  pain. 

2.  Reflex  excitation  of  spinal  nerve  centers  in  consequence  of  which 
there  is  an  increased  activity  of  skeletal  muscles,  blood-vessels, 
glands  and  visceral  walls. 

3.  Reflex  inhibition  of  spinal  nerve  centers  in  consequence  of  which 
there  may  be  a  decrease  in  the  activities  of  skeletal  muscles, 
blood-vessels,  glands  and  viscera. 

4.  Sensations  of  the  duration  and  direction  of  muscle  movements, 
of  the  resistance  offered  and  of  the  position  of  the  body  or  of  its 
individual  parts^  (muscle  sensations) . 

Division  of  the  dorsal  roots  is  followed  by: 

1.  Loss  of  sensation  in  all  parts  to  which  they  are  distributed. 

2.  Loss  of  the  power  of  exciting  or  inhibiting  reflexly  the  activities 
of  spinal  nerve  centers  and  in  consequence  a  loss  of  the  power  of 
exciting  or  inhibiting  the  activities  of  peripheral  organs. 

The  ventral  roots  are  therefore  efferent  in  function  transmitting 
nerve  impulses  from  the  spinal  cord  to  the  peripheral  organs  which 
excite  them  to  activity. 

The  dorsal  roots  are  afferent  in  function  transmitting  nerve  impulses 
from  the  general  periphery  to  (a)  the  spinal  cord  where  they  excite 
its  contained  nerve-centers  to  activity,  and  (b)  to  the  cerebrum  where 
they  excite  its  centers  to  activity  with  the  development  of  sensations. 

Segmentation  of  the  Spinal  Cord. — For  the  elucidation  of  many 
problems  connected  with  the  physiologic  actions  of  the  spinal  cord, 
as  well  as  of  the  symptoms  which  follow  its  pathologic  impairment, 
it  will  be  found  helpful  to  consider  the  cord  as  consisting  physiologicly 
of  a  series  of-  segments  placed  one  above  the  other,  the  number  of 
segments  corresponding  to  the  number  of  spinal  nerves.  Each  spinal 
segment  would  therefore  comprise  that  portion  of  the  cord  to  which 
is  attached  a  pair  of  spinal  nerves.  The  nerve-cells  in  each  segment 
are  in  histologic  and  physiologic  relation  with  definite  areas  of  the 
body,  embracing  muscles,  blood-vessels,  glands,  skin,  etc. 

If  the  exact  distribution  of  the  nerves  of  any  segment  were 
then  known,  its  function  could  be  readily  stated.  By  virtue  of  this 
segmentation  it  becomes  possible  for  each  segment  to  act  inde- 
pendently, or  in  cooperation  with  other  segments  near  or  remote, 
with  which  they  are  associated  by  the  intrinsic  or  associative  cells 
and  their  axons;  and  the  spinal  cord  itself  is  enabled  to  act  as  a  unit. 

FUNCTIONS  OF  THE  SPINAL  CORD. 

Physiologic  investigation  has  demonstrated  that  the  spinal  cord, 
by  virtue  of  the  presence  of  nerve-cells  and  nerve-fibers,  may  be  re- 
garded as  composed  of: 


5o4  TEXT-BOOK  OF  PHYSIOLOGY. 

i.  Independent  nerve-centers,  each  of  which  has  a  special  function; 

and — 
2.   Conducting  paths  by  which  these  centers  are  brought  into  relation 

with  one  another  and  with  the  cerebrum  and  its  subordinate  or 

underlying  parts,  e.  g.,  medulla  and  pons  Varolii. 
The   Spinal   Cord  Segments   as   Independent    Centers. — The 
efferent  cells  of  the  spinal  segments  are  the  immediate  sources  of 
the  nerve  energy  which  excites  activity  in  muscles,,  blood-vessels,  glands. 
The  discharge  of  their  energy  may  be  caused: 
i.  By  variations  in  the  composition  of  the  blood  or  lymph  by  which 

they  are  surrounded.     The  activity  of  the  cell  thus  occasioned 

is  termed  automatic  or  autochthonic  (Gad). 

2.  By  the  arrival  of  nerve  impulses  coming  through  afferent  nerves 

from  the  general  periphery,  skin,  mucous  membrane,  etc. 

3.  By  the  arrival  of  nerve  impulses  descending  the  spinal  cord  from 

the  cerebrum  or  subordinate  structures.  The  peripheral  ac- 
tivity in  the  former  instance  is  said  to  be  reflex  or  peripheral  in 
origin;  in  the  latter  instance,  direct  or  cerebral  in  origin.  In  this 
latter  instance,  also,  the  muscle  movements  are  due  to  volitional, 
the  vascular  variations  and  glandular  discharges  to  emotional, 
forms  of  cerebral  activity. 
Each  segment  of  the  spinal  cord  may  be  regarded,  therefore, 
because  of  its  contained  nerve-cells: 

1.  As  a  center  for  automatic  activity. 

2.  As  a  center  for  the  reception  of  nerve  impulses  arising  either  at  the 

periphery  or  in  the  cerebrum,  and  for  their  subsequent  trans- 
mission through  efferent  nerves  to  various  peripheral  organs. 
Automatism. — The  growth,  the  nutrition  and  multiplication  of 
the  cells  of  various  tissues,  and  their  continuous  and  rhythmic  activity, 
have  been  attributed  to  an  automatic  action  of  the  spinal  nerve-cells. 
By  this  expression  is  meant  a  discharge  of  energy  from  the  cells  oc- 
casioned by  a  change  in  their  environment,  i.  e.,  in  the  chemic  com- 
position of  the  blood  or  lymph  by  which  they  are  surrounded,  and  in- 
dependent of  any  excitation  coming  through  afferent  nerves.  If  the 
cell  activity  is  continuous,  though  variable  in  degree  from  time  to 
time,  it  gives  rise  to  what  is  termed  tonus,  e.  g.,  trophic  tonus,  vas- 
cular, muscle  tonus,  etc.  If  the  cell  activity  is  intermittent,  it  imparts 
to  muscles  a  certain  rhythmic  activity,  e.  g.,  the  respiratory  movements. 
As  no  effect  arises  without  a  sufficient  cause,  the  term  automatic 
has  been  objected  to  and  the  term  autochthonic  has  been  suggested 
(Gad),  expressive  of  the  idea  that  the  energy  originates  in  the  nerve- 
cell  as  a  result  of  a  reaction  between  the  cell  and  its  ever-changing 
environment.  A  center  so  acting  could  not  be  regarded  as  primarily 
a  center  for  reflex  action,  however  much  it  might  be  influenced  or 
conditioned  secondarily  by  afferent  impulses.  Though  automatic 
activity  of  the  spinal  cord  centers  is  advocated  by  some  physiologists, 
the  fact  must  be  recognized  that  with  increasing  knowledge  of  reflex 


THE  SPINAL  CORD.  505 

activities  some  of  the  phenomena  hitherto  regarded  as  automatic 
have  been  found  to  be  reflex  in  origin.  Whether  this  will  eventually 
be  found  true  for  all  forms  of  so-called  automatic  or  autochthonic 
activity  remains  to  be  seen. 

Trophic  Tonus. — The  normal  metabolism  of  muscle,  gland,  and 
connective  tissue  which  underlies  the  assimilation  of  food,  the  storing 
of  energy,  and  the  production  of  new  compounds,  is  dependent,  in  the 
higher  animals  at  least,  on  the  connection  of  these  tissues  with  the  cen- 
tral nerve  system;  for  if  the  efferent  nerves  be  divided,  not  only  will 
they  undergo  degeneration  in  their  peripheral  portions,  but  the  muscles, 
glands,  and  connective  tissues  to  which  they  are  distributed  will  also 
undergo  similar  changes.  This  is  to  be  attributed  not  merely  to  in- 
activity, but  rather  to  a  loss  of  nerve  influence,  inasmuch  as  inactivity 
leads  merely  to  atrophy  and  not  to  degeneration.  It  would  appear 
from  facts  of  this  character  that  the  normal  metabolism  is  dependent 
for  its  continuance  on  nerve  influences.  There  is  no  evidence,  how- 
ever, as  to  the  existence  of  special  trophic  nerves,  separate  from  those 
which  impart  to  glands  and  muscles  their  customary  activities.  The 
trophic  centers  and  the  motor  centers  are  identical,  though  the  two 
modes  of  their  activity  are  separate  and  distinct. 

Vascular  Tonus. — The  state  of  moderate  contraction  of  the 
arterioles  throughout  the  body,  in  consequence  of  which  the  average 
arterial  pressure  is  maintained,  has  been  attributed  to  constant  activity 
of  the  vaso-motor  centers,  this  activity  being  conditioned  by  variations 
in  the  composition  of  blood,  either  an  increase  in  the  quantity  of 
carbon  dioxid  or  a  decrease  in  the  quantity  of  oxygen.  The  vaso- 
motor centers  are  regarded  as  primarily  automatic,  though  capable 
of  being  influenced  secondarily  by  reflected  excitations  from  the 
periphery  or  direct  excitations  from  the  cerebrum. 

Muscle  Tonus. — It  is  well  known  that  if  a  muscle  be  divided  in 
the  living  animal  the  two  portions  will  contract  and  separate  them- 
selves to  a  certain  distance.  This  indicates  that  the  muscle  when  in 
a  state  of  rest  is  in  a  slight  degree  cf  contraction.  This  condition  of 
the  muscle,  to  which  the  term  muscle  tonus  is  given,  was  formerly 
attributed  to  an  automatic  and  continuous  discharge  of  energy  from 
the  nerve-cells.  Brondgeest,  however,  showed  that  this  tonus  is 
entirely  reflex  in  origin  and  immediately  disappears  on  division  of 
the  posterior  roots  of  the  spinal  nerves,  which  would  not  be  the  case 
if  the  cells  in  the  cord  were  acting  automatically.  The  afferent  nerves 
in  this  reflex  arise  in  the  muscle  or  its  tendons,  and  the  stimulus  is 
the  slight  degree  of  extension  to  which  the  muscle  is  subjected  in  virtue 
of  its  attachments  and  the  ever-varying  position  of  the  limbs  and  trunk. 

The  tonic  contraction  of  the  visceral  muscles — e.  g.,  the  pyloric, 
the  vesical,  the  anal  sphincters — though  regarded  as  automatic  by 
some,  is  probably  reflex  in  origin,  dependent  on  the  arrival  of  afferent 
impulses  from  the  periphery.  It  is  probable  that  future  investiga- 
tion will  disclose  the  existence  and  pathway  of  these  afferent  fibers. 


506 


TEXT-BOOK  OF  PHYSIOLOGY. 


Reflex  Actions. — It  has  already  been  stated  that  the  nerve-cells 
in  the  spinal  cord  are  capable  of  receiving  and  transforming  afferent 
nerve  impulses,  the  result  of  peripheral  stimulation  into  efferent  nerve 
impulses,  which  are  transmitted  outward  to  muscles,  exciting  contrac- 
tion; to  glands,  provoking  secretion;  to  blood-vessels,  changing  their 
caliber;  and  to  organs,  inhibiting  or  accelerating  their  activity.  All 
such  actions  taking  place  through  the  spinal  cord  and  medulla  oblon- 
gata independently  of  sensation  or  volition  are  termed  reflex  actions. 
The  mechanism  involved  in  every  reflex  action  consists  of  at  least  the 
following  structures  (Fig.  232): 


sp.c 


Fig.  232. — Diagram  Showing  the  Structures  Involved  in  the  Production  of 
Reflex  Actions,  G.  Bachman.  r.s.  Receptive  surface;  af.n.  afferent  nerve;  [ex. 
emissive  or  motor  cells  in  the  anterior  horn  of  the  gray  matter  of  the  spinal  cord,  sp.c; 
ej.n.  efferent  nerves  distributed  to  responsive  organs,  e.  g.,  directly  to  skeletal  muscles, 
sk.m.,  and  indirectly  through  the  intermediation  of  sympathetic  ganglia,  sym.  g.,  to  blood- 
vessels, b.v.,  and  to  glands,  g.     The  nerves  distributed  to  viscera  are  not  represented. 


i.  A   receptive   surface;   e.   g.,   skin,    mucous   membrane,   sense 
organ,  etc. 

2.  An  efferent  fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises — 

4.  An   efferent   nerve,   distributed   to   a  responsive   organ,    as— 

5.  Muscle,  gland,  blood-vessel,  etc. 

In  this  connection  the  reflex  contractions  of  skeletal  muscles  only 
will  be  considered. 

If  a  stimulus  of  sufficient  intensity  be  applied  to  the  receptive  sur- 
face, there  will  be  developed  in  the  terminals  of  the  afferent  nerve  a 
series  of  nerve  impulses  which  will  be  transmitted  by  the  afferent  nerve 
to,  and  received  by,  the  dendrites  of  the  emissive  cell  in  the  anterior 
horn  of  the  gray  matter.     With  the  reception  of  these  impulses  there 


THE  SPINAL  CORD. 


5°7 


will  be  a  disturbance  in  the  equilibrium  of  the  molecules  of  the  cell, 
a  liberation  of  energy  and  a  transmission  of  nerve  impulses  outward 
through  the  efferent  nerve  to  the  muscle. 

A  reflex  mechanism  or  arc  of  this  simplicity  would  subserve  but  a 
simple  movement.  The  majority  of  the  reflexes,  however,  are  extremely 
complex  and  involve  the  cooperation  and  coordination  of  a  number 
of  centers  at  different  levels  of  the  spinal  cord  and  medulla,  on  the  same 
and  opposite  sides,  and  of  muscles  situated  at  distances  more  or  less 
remote  from  one  another.  The  transference  of  nerve  impulses  com- 
ing from  a  localized  area  of  a  receptive  surface,  to  emissive  cells  situated 
at  different  levels  is  accomplished  by  the  intermediation  of  a  third 
neuron  situated  in  the  gray  matter  which 
is  in  connection,  on  the  one  hand  with 
the  central  terminals  of  the  afferent 
nerve,  and,  on  the  other  hand  through 
collateral  branches  with  the  dendrites  of 
the  efferent  neurons  situated  at  different 
levels.  (Fig.  233.)  A  histologic  and 
physiologic  mechanism  of  this  character 
readily  explains  how  a  localized  stimula- 
tion can  give  rise  to  reflex  actions  ex- 
tremely complex  in  character. 

The  reflex  contractions  of  skeletal 
muscles  are  best  studied  after  division  of 
the  central  nerve  system  at  the  upper 
limit  of  the  spinal  cord.  xAiter  this  pro- 
cedure the  spinal  centers  can  act  inde- 
pendently of,  and  uninfluenced  by  either 
sensation  or  volitional  efforts  on  the  part 
of  the  animal.  Though.it  is  possible  to 
provoke  reflex  contractions  under  such 
circumstances  in  warm-blooded  animals, 
they  are,  as  a  rule,  incomplete  and  of 
short  duration,  owing  to  disturbances  of 
the  circulation  and  respiration  and  the 
consequent  loss  of  tissue  irritability.  In  frogs  and  in  cold-blooded 
animals  generally,  the  spinal  cord  retains  its  irritability  for  a  long 
period  of  time  after  removal  of  the  brain,  and  therefore  is  well  adapted 
for  the  study  of  reflex  actions. 

The  separation  of  the  spinal  cord  from  the  brain  is  readilv  effected 
by  destroying  the  medulla  oblongata.  This  can  be  done  by  inserting 
a  pin  through  the  skin,  the  occipito-atlantal  membrane  covering  the 
space  between  the  occipital  bone  and  the  atlas,  until  it  strikes  the  bodies 
of  the  vertebras  below.  If  the  pin  is  properly  directed  it  passes  through 
the  medulla.  Care  should  be  taken  to  avoid  injury  to  the  blood- 
vessels on  either  side.  The  brain  itself  should  then  be  destroved,  so 
as  to    remove  all  consciousness,  by  inserting  the  pin  into  the  brain 


Fig.  233. — Diagram  Showing 
the  Relation  of  the  Third 
Neuron  a,  to  the  Afferent 
Neuron  b,  and  to  the  Efferent 
Neurons  c,  c,  c. — (After  KdUiker.) 


5oS  TEXT-BOOK  OF  PHYSIOLOGY. 

cavity  through  the  foramen  magnum,  and  giving  it  a  few  rotatory 
movements. 

A  frog  so  prepared,  and  placed  on  the  table  and  allowed  to  remain 
at  rest  for  a  few  moments  until  the  shock  of  the  operation  passes  away, 
will  draw  the  limbs  close  to  the  body  and  assume  a  position  not  unlike 
that  of  a  normal  frog.  If  then  the  posterior  limbs  be  extended,  they 
will  immediately  be  drawn  close  to  the  side  of  the  trunk  in  the  usual 
flexed  position.  If  the  toes  are  pinched  with  forceps,  the  foot  will 
execute  a  series  of  movements  as  if  it  were  trying  to  free  itself  from 
the  source  of  irritation. 

If  the  frog  be  suspended,  the  limbs,  through  the  force  of  gravity, 
will  be  gradually  extended  and  hang  down  freely.  In  this,  as  in  the 
sitting  position,  the  animal  will  remain  perfectly  quiet  and  will  not 
exhibit  spontaneous  movements.  Any  stimulus  applied  to  the  skin, 
however,  provided  it  is  of  sufficient  intensity,  will  be  followed  by  a 
more  or  less  pronounced  movement.  Mechanic,  chemic  and  electric 
stimuli  applied  to  any  part  of  the  skin  will  call  forth  the  characteristic 
reflex  movements.  Chemic  stimuli  such  as  weak  solutions  of  sulphuric 
or  acetic  acid  placed  on  the  toes  will  be  followed  by  feeble  flexion  of 
the  corresponding  leg,  to  be  succeeded  in  a  short  time  by  extension. 
Stronger  solutions  will  produce  more  extensive  and  vigorous  move- 
ments, the  foot  at  the  same  time  being  rubbed  against  the  thigh, 
apparently  for  the  purpose  of  freeing  it  from  the  irritant.  Similar 
phenomena  follow  the  application  of  the  acid  to  the  fingers  or  the 
trunk.  As  a  rule,  the  extent  and  complexity  of  the  movement  is  within 
limits  proportional  to  the  strength  of  the  stimulus.  By  limiting  the 
sphere  of  action  of  the  stimulus  to  definite  but  different  areas  of  the 
skin  a  great  variety  of  movements,  more  or  less  complex  and  coordin- 
ated and  apparently  purposive  and  defensive  in  character,  can  be 
produced.  The  coordinated  and  purposive,  character  of  the  move- 
ments exhibited  by  a  brainless  frog  led  Pfluger  to  the  assumption  that 
the  spinal  cord  in  this  as  well  as  in  other  cold-blooded  animals  is  pos- 
sessed of  sensorial  functions,  and  endowed  with  rudimentary  con- 
sciousness. This  view,  however,  is  not  generally  accepted,  the 
movement  being  attributed  to  specialized  mechanisms  in  the  cord, 
partially  inherited,  which  permit  of  one  and  the  same  movement  with 
mechanic  regularity  and  precision,  so  long  as  the  conditions  of  the 
experiment  remain  the  same. 

In  warm-blooded  animals  similar  results  may  be  obtained  for  a 
short  time  after  division  of  the  cord,  especially  if  artificial  respiration 
is  maintained  and  the  circulation  of  the  blood  continued.  The  cord 
will  then  retain  its  irritability  for  some  time.  If  the  conditions  of 
experimentation  were  favorable,  it  is  highly  probable  that  the  human 
spinal  cord  would  execute  similar  movements.  Thus  it  was  observed 
by  Robin  in  a  man  who  had  been  decapitated  that  reflex  muscle  con- 
tractions could  be  elicited  by  stimulating  the  skin  after  the  lapse  of  an 
hour  after  execution.     "While  the  right  arm  was  lying  extended  by 


THE  SPINAL  CORD.  509 

the  side,  with  the  hand  about  25  centimeters  distant  from  the  upper 
part  of  the  thigh,  I  scratched  with  the  point  of  a  scalpel  the  skin  of  the 
chest  at  the  areola  of  the  nipple,  for  a  space  of  10  or  11  centimeters 
in  extent,  without  making  any  pressure  on  the  subjacent  muscles. 
We  immediately  saw  a  rapid  and  successive  contraction  of  the  great 
pectoral  muscle,  the  biceps,  probably  the  brachialis  anticus,  and 
lastly  the  muscles  covering  the  internal  condyle.  The  result  was  a 
movement  by  which  the  whole  arm  was  made  to  approach  the  trunk; 
with  rotation  inward  and  half-flexion  of  the  forearm  upon  the  arm; 
a  true  defensive  movement,  which  brought  the  hand  toward  the 
chest  as  far  as  the  pit  of  the  stomach.  Neither  the  thumb,  which 
was  partially  bent  toward  the  palm  of  the  hand,  nor  the  fingers,  which 
were  half  bent  over  the  thumb,  presented  any  movements.  The  arm 
being  replaced  in  its  former  position,  we  saw  it  again  execute  a  similar 
movement  on  scratching  the  skin,  in  the  same  manner  as  before,  a 
little  below  the  clavicle.  This  experiment  succeeded  four  times,  but 
each  time  the  movement  was  less  extensive;  and  at  last  scratching  the 
skin  over  the  chest  produced  only  contractions  in  the  great  pectoral 
muscle  which  hardly  stirred  the  limb"  (Dalton). 
Laws  of  Reflex  Action  (Pfliiger). 

1.  Law  of   Unilateral  it  y. — If  a  feeble  irritation  be  applied  to  one  or 

more  sensory  nerves,  movement  takes  place  usually  on  one  side 
only,  and  that  the  same  side  as  the  irritation. 

2.  Law  of  Symmetry. — If  the  irritation  becomes  sufficiently  intense, 

motor    reaction    is    manifested,    in    addition,    in    corresponding 
muscles  of  the  opposite  side  of  the  body. 

3.  Law   of   Intensity. — Reflex    movements   are  usually  more  intense 

on  the  side  of  irritation;  at  times  the  movements  of  the  opposite 
side  equal  them  in  intensity;  but  they  are  usually  less  pronounced. 

4.  Law  of  Radiation. — If  the  excitation  still  continues  to  increase,  it 

is  propagated  upward,  and  motor  reaction  takes  place  through 
centrifugal  nerves  coming  from  segments  of  the  cord  higher  up. 

5.  Law  of  Generalization. — When  the  irritation  becomes  very  intense, 

it  is  propagated  in  the  medulla  oblongata;  motor  reaction  then 
becomes  general,  and  it  is  propagated  up  and  down  the  cord, 
so  that  all  the  muscles  of  the  body  are  thrown  into  action,  the 
medulla  oblongata  acting  as  a  focus  whence  radiate  all  reflex 
movements. 
Special    Reflex    Movements. — Among    the    reflexes    connected 
with  the  more  superficial  portions  of  the  body  there  are  some  which 
are  so  frequently  either  increased  or  diminished  in  pathologic  con- 
ditions of  the  spinal  cord  that  their  study  affords  valuable  indications 
as  to  the  seat  and  character  of  the  lesions.     They  may  be  divided  into: 

1.  The  skin  or  superficial,  and 

2.  The  tendon  or  deep  reflexes. 

3.  The  organ  reflexes. 

The  skin  reflexes,  characterized  bv  contraction  of  underlyine;  mus- 


5io  TEXT-BOOK  OF  PHYSIOLOGY. 

cles,  are  induced  by  stimulation  of  the  skin — e.  g.,  pricking,  pinching, 
scratching,  etc.     The  following  are  the  principal  skin  reflexes: 
i.    Plantar  reflexe,  consisting  of  contraction  of  the  muscles  of  the  foot, 
induced  by  stimulation  of  the  sole  of  the  foot;  it  involves  the 
integrity  of  the  reflex  arc  through  the  lower  end  of  the  cord. 

2.  Gluteal   reflex,    consisting   of    contraction   of    the   glutei   muscles 

when  the  skin  over  the  buttock  is  stimulated;  it  takes  place 
through  the  segments  giving  origin  to  the  fourth  and  fifth  lumbar 
nerves. 

3.  Cremasteric  reflex,  consisting  of    a  contraction  of   the  cremaster 

muscle  and  a  retraction  of  the  testicle  toward  the  abdominal 
ring  when  the  skin  on  the  inner  side  of  the  thigh  is  stimulated; 
it  depends  upon  the  integrity  of  the  segments  giving  origin  to 
the  first  and  second  lumbar  nerves. 

4.  Abdominal  reflex,  consisting  of   a  contraction  of   the  abdominal 

muscles  when  the  skin  upon  the  side  of  the  abdomen  is  gently 
scratched;  its  production  requires  the  integrity  of  the  spinal 
segments  from  the  eighth  to  the  twelfth  dorsal  nerves. 

5.  Epigastric  reflex,  consisting  of   a   slight  muscular  contraction  in 

the  neighborhood  of  the  epigastrium  when  the  skin  between  the 
fourth  and  sixth  ribs  is  stimulated;  it  requires  the  integrity  of 
the  cord  between  the  fourth  and  seventh  dorsal  nerves. 

6.  Scapular  reflex  consisting  of  a  contraction  of  the  scapular  muscles 

when  the  skin  between  the  scapulae  is  stimulated;  it  depends 
upon  the  integrity  of  the  cord  between  the  fifth  cervical  and  third 
dorsal  nerves. 

The  skin  or  superficial  reflexes,  though  variable,  are  generally 
present  in  health.  They  are  increased  or  exaggerated  when  the  gray 
matter  of,  the  cord  is  abnormally  excited,  as  in  tetanus,  strychnin- 
poisoning,  and  disease  of  the  lateral  columns. 

The  so-called  "tendon  reflexes,'1'1  characterized  by  the  contraction 
of  a  muscle,  are  also  of  much  value  in  the  diagnosis  of  lesions  of  the 
cord  and  are  elicited  by  a  sharp  tap  on  a  given  tendon.  The  term, 
tendon  reflex,  is,  however,  somewhat  inaccurate.  The  fundamental 
condition  for  the  production  of  the  tendon  reflex  is  the  normal  tone  of 
the  muscle,  which  is  a  true  reflex,  maintained  by  afferent  nerve  impulses 
developed  in  the  muscle  itself  in  consequence  of  its  extension  and  hence 
compression  of  the  end-organs  of  the  afferent  nerves,  the  muscle 
spindles.  When  the  muscle  is  passively  extended,  as  it  is  when  the 
reflex  is  to  be  elicited,  there  is  an  exaltation  of  the  tonus  and  an  increase 
in  the  irritability.  To  this  condition  of  the  muscle  due  to  passive 
tension,  the  term  myotatic  irritability  has  been  given.  If  the  muscle 
extension  be  now  suddenly  increased,  as  it  is  when  the  tendon  is  sharply 
tapped,  the  increased  compression  of  the  muscle  spindles  will  develop 
additional  afferent  impulses  which  after  transmission  to  the  spinal 
cord  will  give  rise  to  contraction  of  the  corresponding  muscle. 

The  following  are  the  principal  forms  of  the  tendon  reflexes: 


THE  SPINAL  CORD.  511 

1.  Patellar  reflex  or  knee-jerk,  consisting  of  a  contraction  of  the  ex- 

tensor muscles  of  the  thigh  when  the  ligamentum  patellae  is 
struck  between  the  patella  and  tibia.  This  reflex  is  best  ob- 
served when  the  legs  are  freely  hanging  over  the  edge  of  a  table. 
The  patella  reflex  is  generally  present  in  health,  being  absent 
in  only  2  per  cent.;  it  is  greatly  exaggerated  in  lateral  sclerosis, 
in  descending  degeneration  of  the  cord;  it  is  absent  in  locomotor 
ataxia  and  in  atrophic  lesions  of  the  anterior  gray  corn.ua. 

2.  Ankle-jerk  or  Ankle  Reflex.—  If  the  extensor  muscles  of  the  leg  be 

placed  on  the  stretch  and  the  tendo  Achillis  be  sharply  struck,  a 
quick  extension  of  the  foot  will  take  place. 

3.  Ankle  Clonus.—  This  consists  of  a  series  of  rhythmic  reflex  con- 

tractions of  the  gastrocnemius  muscle,  varying  in  frequency  from 
six  to  ten  per  second.     To  elicit  this  reflex,  pressure  is  made  upon 
the  sole  of  the  foot  so  as  to  suddenly  and  energetically  flex  the 
foot  at  the  ankle,  thus  putting  the  tendo  Achillis  and  the  gas- 
trocnemius muscle  upon  the  stretch.     The  rhythmic  movements 
thus   produced   continue   so   long   as   the   tension   within   limits 
is  maintained.     Ankle  clonus  is  never  present  in  health,  but  is 
very  marked  in  lateral  sclerosis  of  the  cord. 
The  toe  reflex,  peroneal  reflex,  and  wrist  reflex  are  also  present  in 
sclerosis  of  the  lateral  columns  and  in  the  late  rigidity  of  hemiplegia. 
The  organ  reflexes,  e.  g.,  the  activities  of  the  genito-urinary  organs, 
the  stomach,  intestines,  gall-bladder,  etc.,  and  which  are  induced  by 
peripheral  stimulation  have  been  considered  in  connection  with  the 
physiologic    action   of   these    organs.     The    genito-urinary  center    is 
located  in  the  lumbar  region  of  the  spinal  cord.     In  diseased  conditions 
of  this  region  the  genito-urinary  reflexes  are  sometimes  increased,  at 
other  times  decreased  or  even  abolished. 

Reflex  Irritability. — The  general  irritability  or  quickness  of 
response  of  the  mechanism  involved  in  reflex  action  can  be  approxi- 
mately determined  by  observation  of  the  length  of  time  that  elapses 
between  the  application  of  a  minimal  stimulus  and  the  appearance  of 
the  muscle  response.  The  method  of  Tiirck  is  sufficiently  accurate 
for  general  purposes.  This  consists  in  suspending  a  frog,  after 
removal  of  the  brain,  and  immersing  the  foot  in  a  0.2  per  cent,  solu- 
tion of  sulphuric  acid.  The  time  is  determined  by  means  of  a  metro- 
nome beating  one  hundred  times  a  minute.  Stimulation  of  the  skin 
can  also  be  effected  by  the  induced  electric  current,  as  suggested  by 
Gaskell.  A  single  shock  is,  however,  ineffective.  When  the  shocks 
follow  each  other  with  sufficient  rapidity,  they  give  rise  to  a  summa- 
tion of  effects  in  the  nerve-centers  which  will  soon  be  followed  by  a 
muscle  response.  It  is  highly  probable  that  the  chemic  stimulation 
gives  rise  to  a  similar  summation  of  effects. 

The  period  of  time  thus  obtained  is  distributed  over  the  entire 
mechanism.  The  true  reflex  time,  however — i.  e.,  the  time  occupied 
in  the  passage  of  the  nerve  impulses  across  the  spinal  mechanism — 


5i2  TEXT-BOOK  OF  PHYSIOLOGY. 

is  shorter  and  is  obtained  by  subtracting  from  the  whole  period 
the  time  occupied  by  the  passage  of  the  impulses  through  the  affer- 
ent and  efferent  nerves  as  well  as  the  latent  period  of  muscle  con- 
traction. This  corrected  period,  the  true  reflex  time,  has  been  found 
to  be  twelve  times  longer  than  the  time  occupied  by  the  passage  of  the 
nerve  impulse  through  the  nerves,  including  the  latent  period  of  the 
muscle. 

The  reflex  irritability  is  increased  by: 
i.  Separation  of  the  Brain  from  the  Cord. — This  is  at  once  followed 
by  an  increase  in  reflex  irritability,  and  is  taken  as  evidence  that 
the  brain  normally  exerts  an  inhibitor  influence  over  the  reflex 
centers  of  the  cord.  The  same  increase  is  observed  upon  hemi- 
section  of  the  cord,  though  the  increase  is  limited  to  the  same 
side. 

2.  The  Toxic  Action  of  Drugs. — Strychnin   even  in  small  doses  in- 

creases the  irritability  to  such  an  extent  that  a  minimal  stimulus 
is  sufficient  to  call  forth  spasmodic  contractions  of  all  the 
skeletal  muscles.  Under  its  influence  the  usual  coordinated 
reflexes  disappear  and  are  succeeded  by  incoordinated  reflexes. 
The  explanation  of  this  fact  is  believed  to  be  a  diminution  in  the 
resistance  offered  by  the  cord  to  the  passage  of  the  afferent  im- 
pulses rather  than  to  a  direct  stimulation  of  the  efferent  cells. 
So  much  is  this  resistance  decreased  that  the  nerve  impulses, 
instead  of  being  confined  to  their  accustomed  paths,  are  radiated 
in  all  directions.  Absolute  repose  of  the  animal  and  the  exclu- 
sion of  all  external  stimuli  greatly  diminish  the  tendency  to 
the  occurrence  of  spasms. 

3.  Degeneration   of   the   Pyramidal  Tracts. — In  primary  lateral  scle- 

rosis, a  pathologic  condition  characterized  primarily  by  a  degen- 
eration of  the  terminal  filaments  of  the  pyramidal  tract  fibers, 
the  reflex  activity  of  the  cord  becomes  exalted.  As  the  disease 
progresses  the  irritability  increases  to  such  an  extent  that  violent 
spasmodic  contractions  of  the  arms  and  legs  arise  when  the  skin 
or  tendons  are  mechanically  stimulated.  The  explanation 
offered  is  practically  the  same  as  in  division  of  the  cord:  viz., 
withdrawal  of  the  inhibitor  and  controlling  influence  of  the 
brain. 

The  reflex  excitability  may  be  decreased  by: 
1.  Stimulation  of  Certain  Regions  of  the  Brain. — It  was  discovered 
by  Setchenow  that  when  the  frog  brain  is  divided  just  anterior 
to  the  optic  lobes  and  the  reflex  time  subsequently  determined 
according  to  the  method  of  Tiirck,  that  the  time  can  be  consider- 
ably lengthened  by  stimulation  of  the  optic  lobes.  This  is  read- 
ily accomplished  by  placing  small  crystals  of  sodium  chlorid  on 
the  optic  lobes.  It  was  concluded  from  this  fact  that  these  lobes 
contain  centers  which  exert  an  inhibitor  influence  over  centers 
in     the    spinal     cord     through     descending     nerve-libers.     This 


THE  SPINAL  CORD. 


5i3 


conclusion  is  strengthened  by  the  fact  that  division  of  the 
brain  just  behind  the  optic  lobes  causes  a  temporary  inhibition 
of  the  reflexes  in  consequence  of  a  mechanical  irritation  of  these 
fibers.  It  is  quite  probable  that  the  volitional  inhibition  of 
certain  reflexes  is  accomplished  through  the  intermediation  of 
this  center  localized  by  Setchenow.     (Fig.  234.) 

2.  Stimulation    of    Sensor    Nerves. — If    during  the  application  of  a 

stimulus  sufficient  to  call  forth  a  characteristic  reaction  in  a 
definite  period  of  time,  a  sensor 
nerve  in  a  distant  region  of  the 
body  be  simultaneously  stimu- 
lated, it  will  be  found  that  the 
reflex  time  will  be  lengthened 
or  the  reaction  completely  in- 
hibited. 

3.  Lesions  of  the  spinal  cord;  e.  g., 

atrophy  of  the  multipolar  cells 
of  the  anterior  horns  of  the 
gray  matter;  degeneration  of 
the  terminals  of  the  dorsal  root 
fibers. 

4.  The  toxic  action  of  various  drugs 

— e.  g.,  chloroform,  chloral — 
which  are  believed  to  exert  a 
depressing  action  on  the  nerve- 
cells  themselves. 
The   Spinal  Cord  as  a  Con- 
ductor.— The  white  matter  of  the 
spinal  cord  consists  of  nerve-fibers 
the  specific  function  of  which  is 

1.  To  conduct  nerve  impulses  from 

one  segment  of  the  cord  to 
another. 

2.  To  conduct  nerve  impulses  com- 

ing to  the  cord  through  afferent 
nerves,  directly  or  indirectly  to 
various  areas  of  the  encephalon . 

3.  To  conduct  nerve  impulses  from  the  encephalon  to  the  spinal  cord 

segments. 
Intersegmental  or  Associative  Conduction. — The  spinal  cord 
consists  of  a  series  of  physiologic  segments  each  of  which  has  specific 
functions  and  is  associated  through  its  related  spinal  nerve  with  a 
definite  segment  of  the  body.  For  the  harmonious  cooperation  and 
coordination  of  all  the  spinal  segments  it  is  essential  that  they  should 
be  united  by  commissural  or  associative  fibers.  This  is.  in  fact, 
accomplished  by  the  axons  of  the  intrinsic  cells  of  the  gray  ma  iter, 
which  constitute  such  a  large  part  of  the  antero-lateral  and  posterior 
3.3 


7/icd.  ob. 


Fig.  234. — Diagram  of  the  Brain  of 
the  Frog.  olj.  n.  olfactory  nerves;  olj.l. 
olfactory  lobes ;  c.h.  cerebral  hemispheres ; 
op.  thl.  optic  thalamus;  op.  I.  optic  lobes; 
c.  cerebellum;  mod.  of.  medulla  oblon- 
gata; IV.  v.  fourth  ventricle. 


5i4  TEXT-BOOK  OF  PHYSIOLOGY. 

root  zones.  In  consequence  of  this  association,  the  cord  becomes  ca- 
pable of  complex  coordinated  and  purposive  reflex  actions. 

Spino-encephalic  or  Sensor  Conduction. — The  nerve  impulses 
that  arise  in  consequence  of  impressions  made  on  the  terminals  of 
the  nerves  in  the  cutaneous  and  mucous  surfaces,  in  the  viscera  and 
in  the  muscles,  are  transmitted  through  the  dorsal  roots  of  the  spinal 
nerves  to  the  cord.  When  transmitted  through  the  cord  to  the  cere- 
bral hemispheres  directly  or  indirectly,  they  are  received  by  specialized 
nerve-cells  in  the  cortex  and  translated  into  conscious  sensations. 
The  sensations  thus  arising  may  be  divided  into  special  and  general 
sensations.  Of  the  former  may  be  mentioned  pain,  touch,  pressure, 
temperature;  of  the  latter  may  be  mentioned  hunger,  thirst,  fatigue, 
well-being,  etc. 

The  pathways  through  the  spinal  cord  that  conduct  these  afferent 
impulses  to  the  brain  are  ill  defined  and  imperfectly  known,  and  only 
for  a  few  sensations  can  it  be  said  that  their  pathways  have  been 
determined.  The  reason  for  this  obscurity  lies  partly  in  the  difficulties 
of  experimentation,  partly  in  the  difficulties  of  interpretation.  Clinical 
observations  are  for  special  reasons  more  or  less  untrustworthy. 

Section  of  one  lateral  half  of  the  cord,  or  a  lesion  involving  the 
one  lateral  half,  as  a  rule  abolishes  all  forms  of  cutaneous  sensibility 
on  the  opposite  side  below  the  injury.  This  would  seem  to  prove  that 
the  nerve  impulses  cross  the  median  line  of  the  cord  immediately  or 
very  shortly  after  entering.  At  the  same  time,  muscle  sensibility  is 
abolished  on  the  corresponding  side  below  the  injury.  This  would 
seem  to  prove  that  the  fibers  of  the  posterior  roots  which  enter  and 
cross  the  column  of  Burdach  and  ascend  in  the  column  of  Goll  are 
derived  mainly  from  the  muscles.  It  is,  however,  believed  by  some 
investigators  that  those  fibers  which  subserve  the  sense  of  touch  do 
not  decussate  at  once,  but  ascend  in  the  column  of  Goll  as  far  as  the 
medulla  oblongata,  where  they,  in  common  with  the  fibers  coming 
from  the  muscles,  arborize  around  the  nerve-cells  in  the  gracile  and 
cuneate  nuclei.  The  afferent  path  is  then  continued  by  new  nerve- 
fibers  which  emerge  from  these  cells,  and  which,  after  crossing  the 
median  plane  and  decussating  with  the  fibers  coming  from  the  oppo- 
site side,  join  the  afferent  path  from  the  spinal  cord.  These  fibers 
are  known  as  the  internal  arcuate  fibers  and  assist  in  the  formation 
of  the  lemniscus  or  fillet.  (Fig.  235.)  The  sensor  pathway  decus- 
sates in  part  at  different  levels  of  the  spinal  cord  and  in  part  at  the 
level  of  the  gracile  and  cuneate  nuclei.  The  former  is  often  termed 
the  lower,  the  latter  the  upper  sensor  decussation. 

The  pathways  for  the  impulses  that  give  rise  to  the  different  sen- 
sation have  been  variously  located  by  different  observers,  e.  g.,  in  the 
gray  matter,  in  the  limiting  layer,  and  in  the  antcro-lateral  tract  of 
Govvers;  the  pathway  for  the  impulses  that  give  rise  to  temperature 
sensations  has  been  located  in  the  gray  matter;  the  pathway  for 
tactile  impressions  has  been  located  in  the  posterior  columns,  though 


THE  SPINAL  CORD. 


5i5 


this  is  not  beyond  dispute.     The  pathway  for  pain  sensations  has 
been  located  in  Gowers'  tract. 

Encephalo-spinal  or  Motor  Conduction.  —At  birth  the  child  is 
capable  of  performing  all  the  functions  of  organic  life,  such  as  sucking, 
swallowing,  breathing,  etc.  It  is,  however,  deficient  in  psychic  activity 
and  in  volitional  control  of  its  muscles.  Its  movements  are  therefore 
largely,  if  not  entirely,  reflex  in  character. 


Fig.  235. — Diagram  of  the  Sensor  Pathways  in  the  Spinal  Cord  Augmented 
above  by  Fibers  of  the  Sensor  Cranial  Nerves  and  Nerves  of  Special  Sense. 
V.  The  trifacial  nerve.  VIII.  The  vestibular  branch  of  the  acoustic  nerve.  IX.  The 
glosso-pharyngeal  nerve.     X.  The  pneumogastric  nerve. — (Van  Gehuchten.) 


Embryologic  and  histologic  examination  of  the  spinal  cord  and 
medulla  show  that  so  far  as  their  mechanisms  for  independent  phys- 
iologic activities  are  concerned  both  are  fully  developed.  Similar 
investigations  of  the  cerebral  hemispheres  and  of  the  nerve-fibers 
which  bring  their  nerve-cells  into  relation  with  the  spinal  segments 
show  that  the  cells  of  the  cortex  are  not  only  immature,  but  that  their 
descending  axons  are  incompletely  invested  with  myelin.     With  the 


5i6  TEXT-BOOK  OF  PHYSIOLOGY. 

growth  of  the  child,  psychic  life  unfolds  and  volitional  control  of  mus- 
cles is  acquired.  Coincidently  the  cells  of  the  cerebral  cortex  grow 
and  develop  and  the  fibers  become  covered  with  myelin. 

The  nerve-fibers  which  have  their  origin  in  the  cells  of  the  cerebral 
cortex,  and  which  terminate  in  tufts  around  the  cells  in  the  anterior 
horns  of  the  gray  matter  of  the  spinal  segments,  are  to  be  regarded  as 
long  commissural  tracts  uniting  and  associating  these  two  portions 
of  the  central  nerve  system. 

Experimental  investigations  and  observations  of  pathologic  lesions 
accord  with  the  view  that  physiologically  these  fibers  are  efferent 
pathways  for  the  transmission  of  motor  or  volitional  impulses  from 
the  cortex  to  the  spinal  segments.  The  nerve-cells  in  which  the 
motor  impulses  originate  are  located  for  the  most  part,  as  will  be  fully 
stated  later,  in  the  central  portion  of  the  cortex  of  the  cerebral  hemi- 
spheres in  the  neighborhood  of  the  central  or  Rolandic  fissure.  The 
axons  of  these  cells  from  each  hemisphere  descend  through  the  corona 
radiata  to  and  through  the  internal  capsule,  along  the  inferior  sur- 
face of  the  crura  cerebri,  behind  the  pons  to  the  medulla,  of  which 
they  constitute  the  anterior  pyramids.  (Fig.  236.)  At  this  point  the 
pyramidal  tract*  of  each  side  divides  into  two  portions,  viz. : 

1.  A  large  portion,  containing  from  80  to  90  per  cent,  of  the  fibers, 

which  decussates  at  the  lower  border  of  the  medulla  and  passes 
downward  in  the  posterior  part  of  the  lateral  column  of  the 
opposite  side,  constituting  the  crossed  pyramidal  tract;  as  it  descends 
it  gradually  diminishes  in  size  as  its  fibers  or  their  collaterals 
enter  the  gray  matter  of  each  successive  segment. 

2.  A  small  portion,  containing  from  20  to  10  per  cent,  of  the  fibers, 

which  does  not  decussate  at  the  medulla  but  passes  downward  on 
the  inner  side  of  the  anterior  column  of  the  same  side,  consti- 
tuting  the   direct   pyramidal  tract   or   column   of   Tiirck.     This 
tract  can  be  traced  down,  as  a  rule,  only  as  far  as  the  mid-dorsal 
region.     As  it  descends  it  becomes  smaller  as  its  fibers  cross  the 
anterior  commissure  to  enter  the  gray  matter  of  the  opposite 
side.     Thus  all  the  fibers  of  the  pyramidal  tract  from  each  cere- 
bral hemisphere  eventually  are  brought  into  relation  with  the 
cells  of  the  gray  matter  of  the  opposite  side  of  the  cord. 
That  the  pyramidal  tracts  are  the  conductors  of  volitional  impulses 
throughout  the  length  of  the  cord  to  its  various  segments  has  been 
made  evident  by  the  results  of  section,  electric  stimulation,  and  disease. 
iJivision  of  the  anterior  and  lateral  columns  of  one  side  of  the  cord  in 
any  part  of  its  extent  is  invariably  followed  by  a  loss  of  motion  or  paral- 

*From  the  fact  that  the  region  included  between  the  origin  of  these  fibers  and 
the  internal  capsule  presents  somewhat  the  form  of  a  pyramid  with  four  sides, 
Charcot  designated  it  the  pyramidal  region  and  the  fibers  composing  it  the  pyram- 
idal tract.  The  base  of  the  pyramid  includes  the  cortex  of  the  convolutions 
around  the  Rolandic  fissure.  The  summit  of  the  pyramid  is  truncated  and  covers 
the  pyramidal  region  of  the  internal  capsule. 


THE  SPINAL  CORD. 


5i7 


ysis  of  the  muscles  below  the  section,  while  electric  stimulation  of  the 
peripheral  end  of  the  isolated  crossed  pyramidal  tract  is  followed  by 
marked  "characteristic  movements  of  the  muscles.  Similar  results 
follow  division  of  the  pyramidal  tract  in  any  part  of  its  course  from 


x 


'< 


% 


m 

Fig.  236. — Diagram  of  the  Pyramidal  Tract  or  Motor  Path.  III.  Common 
oculo-motor  nerve.  IV.  Pathetic  nerve.  V.  Motor  division  of  the  trigeminal  nerve. 
VI.  The  abducens  nerve.  VII.  Facial  nerve.  IX.  and  X.  Motor  divisions  of  the  glosso- 
pharyngeal and  pneumogastric  nerves.  XL  Spinal  accessory  nerve.  XII.  Hypoglossal 
nerve. — ( Van  Geh iichten.) 


the  cerebral  cortex  downward.  Electric  stimulation  of  the  cortical 
cells  which  give  origin  to  the  pyramidal  tract  is  also  followed  by  con- 
traction of  the  muscles  of  the  opposite  side,  while  their  destruction  is 
attended  by  paralysis  of  the  same  muscles.     As  the  nutrition  of  the 


5i8  TEXT-BOOK  OF  PHYSIOLOGY. 

fibers  is  governed  by  the  cells,  it  follows  that  when  the  axon  is  separated 
from  its  cell  it  degenerates.  It  has  been  found  that  a  lesion  of  the  py- 
ramidal tract  in  any  part  of  its  course  is  followed  by  descending  degen- 
eration, which  is  taken  in  evidence  that  it  conducts  nerve  impulses 
from  above  downward.  Thus  experimental  investigation  and  path- 
ologic observation  are  in  accord  in  the  view  that  physiologically  these 
nerve-fibers  are  the  pathways  for  the  transmission  of  motor  or  volitional 
impulses  from  the  encephalon  to  the  spinal  cord. 

The  relation  of  the  motor  and  sensor  pathways  to  each  other  in 
the  spinal  cord  and  brain  are  shown  in  Plate  II.  The  afferent  fibers 
which  decussate  at  various  levels  through  the  spinal  cord  are  not  repre- 
sented. 


CHAPTER  XX. 

THE  MEDULLA  OBLONGATA;   THE  ISTHMUS  OF  THE  EN- 
CEPHALON ;  THE  BASAL  GANGLIA. 

THE  MEDULLA  OBLONGATA. 

The  medulla  oblongata  is  that  portion  of  the  central  nerve  system 
immediately  superior  to  and  continuous  with  the  spinal  cord.  It  has 
the  shape  of  a  truncated  cone,  the  base  of  which  is  directed  upward, 
the  truncated  apex  downward.  It  is  38  mm.  in  length,  18  mm.  in 
breadth,  and  12  mm.  in  thickness.  By  the  continuation  upward  of 
the  anterior  and  posterior  median  fissures,  the  medulla  is  divided  into 
symmetric  halves  (Figs.  237  and  238).  Like  the  cord,  of  which  it  is  a 
continuation,  it  is  composed  of  white  matter  externally  and  gray  matter 
internally. 

Structure  of  the  Gray  Matter. — The  gray  matter  of  the  medulla 
is  continuous  with  that  of  the  cord,  though  owing  to  the  shifting  of 
position  of  the  different  tracts  of  the  white  matter  it  is  arranged  with 
much  less  regularity.  The  appearance  which  the  gray  matter  presents 
on  transverse  section  varies  also  at  different  levels. 

At  the  level  of  the  first  cervical  nerve  the  posterior  horns  are  narrow, 
elongated,  and  directed  outward.  The  lateral  horns  are  well  devel- 
oped and  present  a  collection  of  cells  near  their  bases  which  can  be 
traced  forward  and  backward  for  some  distance.  At  the  level  of  the 
decussation  of  the  pyramidal  tracts  the  head  of  the  anterior  horn  be- 
comes detached  from  the  rest  of  the  gray  matter  and  is  pushed  backward 
toward  the  posterior  horn ;  the  bases  of  the  anterior  horns  become  spread 
out  to  form  a  layer  of  gray  matter  near  the  dorsal  aspect  of  the  medulla. 
Transverse  sections  of  the  medulla  at  all  levels  show  a  more  or  less  ex- 
tensive network  of  nerve-fibers  known  as  the  reticular  formation.  In 
its  meshes  are  found  collections  of  nerve-cells  of  varying  size.  To- 
ward the  dorsal  aspect  of  the  medulla  special  groups  of  cells  are  found 
from  which  axons  arise  to  become  the  fibers  of  various  efferent  cranial 
nerves,  e.  g.,  the  hypoglossal,  the  efferent  fibers  of  the  vagus,  and  glosso- 
pharyngeal. 

Structure  of  the  White  Matter. — The  white  matter  is  composed 
of  nerve-fibers  supported  by  connective  tissue  and  neuroglia.  It  is 
subdivided  on  either  side  by  grooves  into  three  main  columns:  viz., 
an  anterior  column  or  pyramid,  a  lateral  column,  and  a  posterior  column. 

The  anterior  column  or  pyramid  is  composed  partly  of  fibers  con- 
tinuous with  those  of  the  anterior  column  of  the  spinal  cord  (the  direct 
pyramidal  tract),  and  partly  of  fibers  continuous  with  those  of  the 
lateral  column  of  the  cord  of  the  opposite  side  (the  crossed  pyram- 

5i9 


520 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  237. — Anterior  or  Ventral 
View  of  the  Medulla  Oblongata 
and  Isthmus,  i.  Infundibulum.  2. 
Tuber  cinereum.  3.  Corpora  albi- 
cantia.  4.  Cerebral  peduncle.  5. 
Tuber  annulare.  6.  Origin  of  the 
middle  peduncle  of  the  cerebellum. 
7.  Anterior  pyramids  of  the  medulla 
oblongata.  8.  Decussation  of  the  an- 
terior pyramids.  9.  Olivary  bodies. 
10.  Restiform  bodies,  n.  Arciform 
fibers.  12.  Upper  extremity  of  the 
spinal  cord.  13.  Ligamentum  dentic- 
ulatum.  14,  14.  Dura  mater  of 
the  cord.  15.  Optic  tracts.  16. 
Chiasm  of  the  optic  nerves.  17. 
Motor  oculi  communis.  18.  Patheti- 
cus.  jo.  Fifth  nerve.  20.  Motor 
oculi  externus.  21.  Facial  nerve.  22. 
Auditory  nerve.  23.  Nerve  of  Wris- 
berg.  24.  Glosso-pharyngeal  nerve 
25.  Pneumogastric.  26,  26.  Spinal 
accessory.  27.  Sublingual  nerve.  28, 
29,  30.  Cervical  nerves. — (Sappey.) 


Fig.  238. — Posterior  or  Dorsal  View 
of  the  Medulla  Oblongata,  Isthmus, 
and  Basal  Ganglia,  i.  Corpora  quad- 
rigemina.  2.  Corpus  quadrigeminum  an- 
terior (pregeminum).  3.  Corpus  quadri- 
geminum posterior  (post-geminum).  4. 
Tract  of  fibers  (brachium)  passing  to  the 
corpus  geniculatum  externum.  5.  Tract  of 
fibers  (brachium)  passing  to  6,  the  corpus 
geniculatum  internum.  7.  Posterior  com- 
missure. 8.  Pineal  gland.  9.  Superior  cere- 
bellar peduncle.  10,  11,  12.  The  valve  of 
Vieussens.  13.  The  pathetic  nerve.  14. 
Lateral  groove  of  the  isthmus.  15.  Triangu- 
lar bundle  of  the  isthmus.  16.  Superior 
cerebellar  peduncle.  17.  Middle  cerebellar 
peduncle.  18.  Inferior  cerebellar  peduncle. 
19.  Anteroinferior  wall  of  the  fourth  ventri- 
cle. 20.  Acoustic  nerve.  21.  Spinal  cord. 
22.  The  postero-median  column.  23.  The 
posterior  pyramids. — {Sappey.) 


ISTHMUS  OF  THE  ENCEPHALON.  521 

idal  tract),  which  decussate  at  the  anterior  portion  of  the  medulla. 
The  united  fibers  can  be  traced  upward  to  the  pons,  where  they  dis- 
appear from  view. 

The  lateral  column  is  composed  of  fibers  continuous  with  those 
of  the  lateral  column  of  the  cord.  As  the  fibers  pass  upward,  how- 
ever, they  diverge  in  several  directions.  The  fibers  of  the  crossed 
pyramidal  tract  cross  the  median  line,  as  previously  stated,  to  enter 
into  the  formation  of  the  anterior  column;  the  fibers  of  the  direct 
cerebellar  tract  gradually  curve  backward,  and  in  so  doing  unite  with 
other  fibers  to  form  the  restiform  body,  after  which  they  enter  the 
cerebellum  by  way  of  the  inferior  peduncle.  Situated  between  the 
anterior  pyramid  and  the  restiform  body  is  a  small  oval  mass,  the 
olivary  body,  composed  of  both  white  and  gray  matter. 

The  posterior  column  is  composed  largely  of  fibers  continuous  with 
those  of  the  posterior  column  of  the  cord.  The  subdivision  of  this 
column  into  a  postero-external  (Burdach)  and  a  postero-internal 
(Goll)  is  more  marked  in  the  medulla  than  in  the  cord.  The  former 
is  here  known  as  the  funiculus  cuneatus,  the  latter  as  the  funiculus 
gracilis.  These  two  strands  of  fibers  are  apparently  continued  into 
the  restiform  body.  Owing  to  the  divergence  of  the  restiform  bodies 
a  V-shaped  space  is  formed,  the  floor  of  which  is  covered  with  epithe- 
lium resting  on  the  ependyma.  At  the  upper  extremity  of  the  funicu- 
us  cuneatus  and  funiculus  gracilis,  two  collections  of  gray  matter  are 
found,  known  respectively  as  the  nucleus  cuneatus  and  nucleus  gracilis. 
Around  the  cells  of  these  nuclei  many  of  the  fibers  of  the  posterior 
column  end  in  brush-like  expansions. 

The  Fillet  or  Lemniscus. — From  the  ventral  surface  of  the  cu- 
neate  and  gracile  nuclei  axons  emerge  which  pass  forward  and  upward 
through  the  gray  matter  and  decussate  with  corresponding  fibers  com- 
ing from  the  opposite  nuclei.  They  then  assume  a  position  just  pos- 
terior to  the  pyramids  and  between  the  olivary  bodies.  These  fibers 
thus  form  a  new  distinct  tract,  termed  the  fillet  or  lemniscus.  As  this 
tract  ascends  toward  the  brain  it  receives  additional  axons  from  the 
sensory  end-nuclei  of  all  the  afferent  cranial  nerves  of  the  opposite 
side  with  the  exception  of  the  auditory.  From  the  end-nuclei  of  the 
auditory  nerve  new  axons  ascend  as  a  distinct  tract  situated  near  the 
lateral  aspect  of  the  pons.  From  their  position  these  two  separate 
tracts  have  been  termed  the  mesial  and  lateral  fillets  respectively. 

Before  proceeding  to  a  consideration  of  the  functions  of  the  medulla 
oblongata  it  will  be  found  conducive  to  clearness  to  sketch  the  salient 
anatomic  features  of  the  parts  anterior  to  it  and  their  relations  one  to 
another. 

THE  ISTHMUS  OF  THE  ENCEPHALON. 

The  isthmus  of  the  encephalon  comprises  that  portion  of  the 
central  nerve  system  connecting  the  cerebrum  above,  the  cerebellum 
behind,  and   the  medulla  below.     Its  ventral  surface  presents  below 


52- 


TEXT-BOOK  OF  PHYSIOLOGY. 


an  enlargement,  convex  from  side  to  side,  the  pons  Varolii.  On 
each  side  the  fibers  of  which  the  pons  consists  converge  to  form  a  com- 
pact bundle,  the  middle  peduncle,  which  enters  the  corresponding  half 
of  the  cerebellum.  Above  the  pons,  this  surface  presents  two  large 
columns  of  white  matter  which,  diverging  somewhat  from  below  up- 
ward, enter  the  base  of  the  cerebrum  and  are  known  as  the  crura  cerebri. 
Embracing  the  crura  above  are  two  large  bands  of  white  matter,  the 
optic  tracts  (Fig.  237). 

The  dorsal  surface  presents  below  two  diverging  columns  of  white 
matter,  the  inferior  peduncles;  above,  two  converging  columns,  the 
superior  peduncles  of  the  cerebellum  (Fig.  238).  At  the  extreme 
upper  part  of  this  surface  there  are  four  small  grayish  eminences, 
the  corpora  quadrigemina.  From  the  disposition  of  the  white  matter 
on  the  dorsal  surface  of  the  isthmus  and  medulla,  there  is  formed  a 

lozenge-shaped  space,  the  fourth  ventricle. 
The  space  is  merely  an  expansion  of  the 
central  cavity  of  the  cord,  the  result  of 
the  changed  relations  of  the  white  and 
gray  matter  in  this  region  of  the  central 
nerve  system.  Above,  this  ventricle 
communicates  by  a  narrow  canal,  the 
aqueduct  of  Sylvius,  with  the  third  ven- 
tricle. The  floor  of  the  fourth  ventricle 
is  covered  with  a  layer  of  epithelium 
resting  on  the  ependyma  continuous  with 
that  lining  the  central  canal  of  the  cord. 
Beneath  this  is  a  layer  of  gray  matter. 

The  pons  Varolii  comprises  in  a 
general  way  that  portion  of  the  central 
nerve  system  situated  between  the  me- 
dulla oblongata  and  the  crura  cerebri. 
The  ventral  surface  is  convex  from  side  to  side;  the  lateral  surface, 
owing  to  the  convergence  of  the  fibers  of  which  it  is  composed,  is 
contracted  to  form  the  middle  peduncle  of  the  cerebellum;  the  posterior 
surface  is  flat  and  forms  the  upper  half  of  the  floor  of  the  fourth  ven- 
tricle. The  pons  consists  of  white  fibers  and  gray  matter  supported 
by  connective  tissue  and  neuroglia.  Transverse  sections  of  the  pons 
show  that  it  is  divided  into  an  anterior  or  ventral,  and  a  posterior  or 
dorsal  portion,  the  latter  being  usually  termed  the  tegmentum. 

The  ventral  portion  consists  for  the  most  part  of  white  fibers,  ar- 
ranged longitudinally  and  transversely  (Fig.  239).  The  longitudinal 
fibers  are  largely  continuations  of  the  pyramidal  tracts,  or  the  fibers 
composing  the  anterior  pyramid  of  the  medulla.  In  the  lower  part  of 
the  pons  these  fibers  are  compactly  arranged,  but  at  higher  levels  they 
are  separated  into  a  number  of  bundles  by  the  interlacing  of  the  trans- 
verse fibers.  The  transverse  fibers  are  divided  into  a  superficial  and 
a  deep  set.     Among  these  fibers  are  groups  of  nerve-cells  which  collec- 


Fig.  239. — Transection  of  the 
Pons  through  its  Middle  Por- 
tion, Showing  the  Relation 
of  the  Nerve  Tracts  of  Which 
it  is  Composed.  D.  1.  f.  Dorsal 
longitudinal  fasciculus.  L.c.  and 
c.  Locus  ceruleus.  L.f.  Lateral 
fillet. 


ISTHMUS  OF  THE  ENCEPHALON.  523 

tively  are  known  as  the  nucleus  ponds.  Some  of  the  transverse  fibers, 
especially  the  superficial  ones,  are  commissural  in  character — i.  e., 
they  connect  corresponding  parts  of  the  gray  matter  of  the  lateral 
halves  of  the  cerebellum;  others  coming  from  the  gray  matter  of  the 
cerebellum  cross  the  median  line  and  terminate  around  the  cells  of 
the  nucleus  pontis;  others  again  are  connected  with  the  gray  cells  of 
the  same  side.  Through  the  intermediation  of  the  nucleus  pontis 
and  certain  of  the  longitudinal  fibers  of  the  pons,  the  cerebellum  is 
brought   into  relation  with  the  cerebrum. 

The  dorsal  or  tegmental  portion  consists  of:  (1)  The  fillet;  (2)  the 
formatio  reticularis;  (3)  the  posterior  longitudinal  bundle;  (4)  the 
substantia  ferruginosa;  (5)  groups  of  nerve-cells  from  which  arise 
various  cranial  nerves—  e.  g.,  the  fifth,  sixth,  seventh,  and  eighth. 

The  fillet  or  lemniscus  in  this  region  is  divided  into  a  mesial  and  a 
lateral  portion.  The  fibers  of  the  mesial  portion  are  partly  the  axons 
of  the  nerve-cells  of  the  gracile  and  cuneate  nuclei  of  the  opposite  side 
of  the  medulla,  and  partly  of  the  axons  of  the  sensor  nerve-cells  of  the 
afferent  cranial  nerves  with  the  exception  of  the  auditory.  The  fibers 
of  the  lateral  portion  are  mainly  the  axons  of  the  cells  in  the  floor  of  the 
fourth  ventricle  around  which  the  auditory  nerve-fibers  end.  They 
are  therefore  a  continuation  of  the  auditory  tract. 

The  formatio  reticularis  is  a  continuation  of  that  of  the  medulla. 

The  posterior  longitudinal  bundle  is  triangular  in  shape  and  situated 
behind  the  formatio  reticularis  and  close  to  the  median  line.  The 
fibers  composing  it  are  largely  derived  from  the  ground  fibers  of  the 
antero-lateral  column  of  the  spinal  cord. 

The  superior  olive  is  a  cylindric  mass  of  gray  matter  situated  in 
the  pons  in  the  anterior  part  of  the  formatio  reticularis.  It  consists 
of  nerve-cells  the  axons  of  which  pass  dorso-laterally,  decussate  in 
the  median  line,  and  form  the  lateral  fillet  of  the  opposite  side.  Some 
few  axons  go  to  the  lateral  fillet  of  the  same  side. 

The  substantia  ferruginosa  is  composed  mainly  of  pigmented  cells. 

The  groups  of  nerve-cells  lying  just  beneath  the  floor  of  the  fourth 
ventricle  give  origin  to  axons  of  the  motor  portion  of  the  fifth,  the  sixth, 
the  seventh  cranial  nerves.  Some  of  the  groups  are  the  sensor  end- 
nuclei  of  the  fifth  and  eighth  cranial  nerves. 

The  crura  cerebri  comprise  that  portion  of  the  central  nerve  sys- 
tem situated  between  the  pons  below  and  the  cerebrum  above.  They 
are  composed  of  strands  of  nerve-fibers  which  are  divided,  as  shown 
on  cross-section,  into  a  ventral  and  a  dorsal  portion  by  a  crescentic 
shaped  layer  of  gray  matter,  the  substantia  nigra  (Fig.  240).  Of  the 
fibers  which  compose  the  ventral  portion  of  each  crus,  the  crusta  or  pes, 
the  larger  part  is  continuous  below,  through  the  longitudinal  fibers  of 
the  pons,  with  the  pyramid  of  the  medulla  and  the  pyramidal  tract; 
above  they  assist  in  the  formation  of  the  internal  capsule.  On  the 
inner  and  on  the  outer  side  of  each  crusta  there  is  a  bundle  of  fibers 
derived  from  the  frontal,  and  from  the  temporal  and  occipital  por- 


5^4 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  240.- — Scheme  of  Transverse  Sec- 
tion of  the  Cerebral  Peduncles.  CQ 
Corpora  quadrigemina.  Aq.  Aqueduct 
p. Lb.  Posterior  longitudinal  bundle.  F 
Fillet  or  lemniscus.  RX.  Red  nucleus.  SN 
Substantia  nigra.  III.  Third  nerve.  Py 
Pyramidal  tracts.  Fc.  Fronto-cerebellar; 
and  TOC,  temporo-occipital  fibers  of  the 
crusta.  CC.  Caudate-cerebellar  fibers  in 
upper  part  of  crusta. — {After  Wernicke  and 
Gowers.) 


tions  of  the  cerebrum  respectively.  These  fibers  are  connected  directly 
with  the  nuclei  pontis  and  indirectly  with  the  cerebellum  of  the  same 
and  opposite  sides.  The  fibers  which  compose  the  dorsal  portion,  the 
tegmentum,  are  continuous  with  those  which  pass  upward  from  the 

medulla  and  pons,  e.  g.,  the  fillet, 
both  mesial  and  lateral,  the 
formatio  reticularis,  the  posterior 
longitudinal  bundle,  and,  in  ad- 
dition, the  fibers  of  the  superior 
peduncles  of  the  cerebellum. 
Above,  the  fibers  terminate 
largely  in  collections  of  gray 
matter  at  the  base  of  the  cere- 
brum. 

The  aqueduct  of  Sylvius  is  a 
short  narrow  canal  which  con- 
nects the  cavity  of  the  fourth 
with  the  cavity  of  the  third 
ventricle.  It  is  lined  by  the 
ependyma  and  surrounded  by  a 
layer  of  gray  matter  continuous 
with  that  forming  the  floor  of 
the  fourth  ventricle..  In  that 
portion  of  the  gray  matter  lying 
beneath  or  ventral  to  the  aqueduct  there  are  groups  of  nerve-cells 
which  give  origin  to  axons  which  unite  to  form  the  third  and  fourth 
cranial  nerves. 

THE  CORPORA  QUADRIGEMINA. 

The  corpora  quadrigemina  are  four  small  grayish  eminences 
situated  beneath  the  posterior  border  of  the  corpus  callosum  and  behind 
the  third  ventricle.  They  rest  upon  the  lamina  quadrigemina,  which 
forms  the  roof  of  the  aqueduct  of  Sylvius.  The  anterior  pair  are  termed 
the  nates,  or  the  pregemina,  the  posterior  pair  the  testes,  or  the  post- 
gem  ina. 

From  the  external  surface  of  each  body  there  pass  outward  bundles 
of  fibers  termed  brachia.  The  fibers  which  compose  the  brachium 
of  the  pregeminum  pass  outward  and  enter  a  small  collection  of  gray 
matter,  the  corpus  geniculalum  externum,  and  the  optic  tract.  The 
fibers  which  compose  the  brachium  of  the  postgeminum  are  divided 
into  two  bundles,  one  of  which  enters  a  second  small  collection  of  gray 
matter,  the  corpus  geniculatum  internum,  while  the  other  passes  forward 
beneath  this  body  to  enter  the  internal  capsule,  beyond  which  it  passes 
to  the  cortex  of  the  temporal  region  of  the  cerebrum. 

Though  these  bodies  are  closely  associated  anatomically,  they 
differ  in  origin,  in  their  relations  and  in  their  functions. 

Microscopic  examination  of  sections  of  the  quadrigeminal  bodies 


BASAL  GANGLIA.  525 

shows  that  they  are  composed  of  nerve-ceils  and  nerve-fibers,  both  of 
which  are  so  intricately  arranged  that  it  is  difficult  to  trace  their  rela- 
tion one  to  another  and  to  adjoining  structures.  Some  of  the  cells  of 
the  pregeminum  give  off  axons  which  course  outward  and  forward, 
enter  the  internal  capsule,  and  pass  through  the  optic  radiation  to  the 
cortex  of  the  occipital  region  of  the  cerebrum.  Many  fibers  of  the 
optic  tract,  axons  of  the  cells  of  the  retina,  end  in  brush-like  expansions 
around  these  same  cells.  There  is  thus  formed  a  connected  pathway 
between  the  retina  and  the  occipital  cortex. 

The  cells  of  the  occipital  cortex,  however,  send  axon  fibers  in  the 
reverse  direction  through  the  optic  radiation  to  terminate  around  the 
cells  of  the  pregeminum,  while  axons  of  pregeminal  cells  pass  for- 
ward to  the  retina  and  to  the  cells  of  origin  of  the  third  nerve. 

The  cells  of  the  postgeminum  give  origin  to  axons  which  pass  up- 
ward, forward,  and  outward,  enter  the  internal  capsule,  and  pass  by 
way  of  the  auditory  tract  to  the  cortex  of  the  temporo-sphenoidal 
region  of  the  cerebrum.  Many  of  the  fibers  of  the  lateral  fillet,  a 
portion  of  the  auditory  tract,  terminate  in  brush-like  expansions 
around  these  same  cells.  There  is  thus  established  a  connected  path- 
way between  the  cochlea  and  the  temporo-sphenoidal  cortex.  The 
cells  of  the  temporal  cortex,  however,  send  axons  in  the  reverse  direc- 
tion by  way  of  the  auditory  tract  to  the  cells  of  the  postgeminum. 
There  is  thus  established  a  double  communication  between  the  occipital 
and  temporal  region  of  the  cerebral  cortex,  and  the  pregeminal  and 
postgeminal   bodies   respectively. 

THE  BASAL  GANGLIA;  THE  CORPORA  STRIATA  AND 
OPTIC  THALAMI. 

The  basal  ganglia  surmount  the  crura  cerebri,  but  are  only  made 
visible  by  removal  of  the  cerebrum  (Fig.  241). 

The  corpora  striata  are  two  large  ovoid  collections  of  gray  and 
white  matter  situated  at  the  base  of  the  cerebrum.  The  larger  portion 
of  each  body  is  embedded  in  the  cerebral  white  matter,  while  the 
smaller  portion  projects  into  the  anterior  part  of  the  lateral  ventricle. 
A  transection  of  the  corpus  striatum  shows  that  it  is  divided  by  a  band  of 
white  matter  into  two  portions,  viz. : 

1.  The  caudate  nucleus,  the  intra- ventricular  portion,  convex  in  shape 

with  its  base  directed  forward,  its  apex  or  tail  directed  backward 
and  downward. 

2.  The  lenticular  nude  us,  the  extra-ventricular  portion,  somewhat  bicon- 

vex in  shape  and  embedded  largely  in  the  white  matter.  Each  len- 
ticular nucleus  is  subdivided  by  two  lamina  of  white  matter  into 
three  portions.  The  two  inner,  from  their  pale  yellow  color, 
form  the  globus  pallidas,  the  outer,  somewhat  darker  in  color,  is 
the  put  a  men. 
The  Internal  Capsule. — The  band  of  white  matter  separating  the 
caudate  from  the  lenticular  nucleus  has  been  termed  the  internal  cap- 


526  TEXT-BOOK  OF  PHYSIOLOGY. 

sule  from  the  manner  in  which  it  embraces  the  inner  surface  of  the 
lenticular  nucleus.  It  consists  of  nerve-fibers  which  associate  histo- 
logically and  physiologically  all  portions  of  the  cerebral  cortex  with  the 
optic  thalamus,  pons,  medulla,  spinal  cord,  and  cerebellum.  The 
relation  of  the  capsule  to  the  nuclei  through  which  it  passes  is  readily 


Fig.  241. — Dissection  of  Brain,  from  above,  Exposing  the  Lateral  Fourth  and 
Fifth  Ventricles  with  the  Surrounding  Parts,  h. — a.  Anterior  part,  or  genu  of 
corpus  callosum.  b.  Corpus  striatum,  b'.  The  corpus  striatum  of  left  side,  dissected  so 
as  to  expose  its  gray  substance,  c.  Points  by  a  line  to  the  taenia  semicircularis.  d.  Optic 
thalamus,  e.  Anterior  pillars  of  fornix  divided;  below  they  are  seen  descending  in  front 
of  the  third  ventricle,  and  between  them  is  seen  part  of  the  anterior  commissure;  in  front 
of  the  letter  e  is  seen  the  slit-like  fifth  ventricle,  between  the  two  laminae  of  the  septum 
lucidum.  /.  Soft  or  middle  commissure;  g  is  placed  in  the  posterior  part  of  the  third 
ventricle;  immediately  behind  the  latter  are  the  posterior  commissure  (just  visible)  and  the 
pineal  gland,  the  two  crura  of  which  extend  forward  along  the  inner  and  upper  margins 
of  the  optic  thalami.  h.  and  i.  The  corpora  quadrigemina.  k.  Superior  crus  of  cere- 
bellum. Close  to  k  is  the  valve  of  Vieussens,  which  has  been  divided  so  as  to  expose  the 
fourth  ventricle.  /.  Hippocampus  major  and  corpus  fimbriatum,  or  taenia  hippocampi. 
m.  Hippocampus  minor,  n.  Eminentia  collateralis.  0.  Fourth  ventricle,  p.  Posterior 
surface  of  medulla  oblongata,  r.  Section  of  cerebellum.  5.  Upper  part  of  left  hemisphere 
of  cerebellum  exposed  by  the  removal  of  part  of  the  posterior  cerebral  lobe. — {Hirschjeld 
and  Leveille.) 

shown  on  cross-section  (Fig.  242).  The  appearance  which  it  presents, 
however,  varies  considerably  at  different  levels.  At  a  given  level  it 
may  be  said  to  consist  of  two  segments  or  limbs,  an  anterior,  situated 
between  the  caudate  nucleus  and  the  anterior  extremity  of  the  lenticu- 
lar nucleus,  and  a  posterior,  situated  between  the  optic  thalamus  and 
the  posterior  extremity  of  the  lenticular  nucleus.     The  two  segments 


BASAL   GANGLIA. 


527 


unite  at  an  obtuse  angle,  termed  the  knee,  which  is  directed  toward  the 
median  line. 

The  optic  thalami  are  two  oblong  masses  of  gray  matter  situated 
upon  the  crura  cerebri  and  behind  the  corpora  striata.  The  anterior  and 
posterior  extremities  of  each  thalamus  present  enlargements  known 
respectively  as  the  anterior  tubercle  and  the  posterior  tubercle  or  pul- 
vinar. The  mesial  surface  of  the  thalamus  forms  the  lateral  wall  of  t he- 
third  ventricle  and  is  covered  by  epithelium  resting  on  a  thin  layer  of 
ependyma. 

A  transection  of  the  thalamus  shows 
that  it  is  not  only  covered  externally  but 
penetrated  by  white  matter,  which  sub- 
divides its  contained  gray  cells  into  four 
more  or  less  distinct  masses  termed 
nuclei,  viz.,  an  anterior,  a  lateral,  occu- 
pying the  external  part  of  the  thalamus, 
a  ventral,  close  to  the  entire  ventral  sur- 
face, and  a  posterior,  situated  beneath 
the  pulvinar.  Beneath  and  somewhat 
internal  to  each  optic  thalamus  there  is 
a  region,  the  subthalamic,  consisting  of 
an  intricate  network  of  nerve-fibers  and 
several  nuclei  of  gray  matter,  e.  g.,  the 
red  or  tegmental  nucleus,  the  subthalamic 
nucleus,  or  Luys'  body,  and  the  sub- 
stantia nigra. 

Though  the  thalamus  has  extensive 
connections  with  many  portions  of  the 
central  nerve  system,  the  most  important 
are  with  the  cortex,  the  tegmentum,  and 
the  optic  tracts. 

From  the  cells  of  these  various  nuclei 
axons  emerge  which  pass  into  the  in- 
ternal capsule,  and  through  the  corona 
radiata  to  all  portions  of  the  cortex. 
Those  which  come  from  the  pulvinar  and  pass  to  the  occipital  lobe 
constitute  a  part  of  the  optic  radiation;  those  from  the  lateral  and 
ventral  nuclei  ultimately  reach  the  parietal  lobe;  those  from  the 
anterior  nucleus  pass  to  the  hippocampal  and  uncinate  convolutions. 
In  a  similar  manner  all  portions  of  the  cortex  are  brought  into  relation 
with  the  thalamus,  axons  from  the  cortical  cells  passing  downward  to 
terminate  in  tufts  around  the  thalamic  nuclei. 

The  tegmentum  is  intimately  related  to  the  thalamus,  though  the 
exact  distribution  of  various  strands  of  fibers  is  a  subject  of  much  dis- 
cussion. Most  of  the  libers  of  the  mesial  fillet  end  in  tufts  around  the 
cells  of  the  ventral  and  lateral  nuclei;  other  fibers  pass  directly  to  the 
cortex. 


Fig.  242. — Horizontal  Sec- 
tion of  the  Internal  Cap- 
sule showing  its  Relations 
to  the  Caudate  Nucleus, 
Optic  Thalamus,  and  the 
Lenticular  Nucleus,  i. 
Caudate  nucleus.  2.  Anterior 
segment  0/  the  internal  capsule. 
3.  External  capsule.  4.  Len- 
ticular nucleus.  5.  Claustrum. 
6.  Posterior  segment  of  internal 
capsule.  7.  Optic  thalamus. — 
(Modified   from    Landois.) 


528  TEXT-BOOK  OF  PHYSIOLOGY. 

The  optic  tract  sends  fibers  directly  into  the  pulvinar,  around  the 
cells  of  which  they  terminate  in  brush-like  expansions. 


SUMMARY  OF  THE  STRUCTURE  OF  THE  MEDULLA,  ISTHMUS, 
AND  BASAL  GANGLIA. 

Structure  of  the  Central  Gray  Matter. — Though  the  general 
arrangement  of  the  central  gray  matter  has  been  incidentally  alluded 
to  in  the  foregoing  presentation  of  the  anatomic  features  of  the  medulla 
and  isthmus,  it  will  be  convenient  to  summarize  its  arrangement  and 
structure  at  this  point. 

The  gray  matter  of  the  cord,  of  the  dorsal  aspect  of  the  medulla 
and  pons,  of  the  region  surrounding  the  aqueduct  of  Sylvius,  and  of  the 
lining  of  the  third  ventricle,  constitute  practically  a  continuous  system, 
though  presenting  modifications  in  various  parts  of  its  extent.  In 
the  transition  region  of  the  spinal  cord  and  medulla  the  gray  matter 
of  the  former  becomes  much  changed  in  shape  owing  to  the  shifting 
of  position  of  the  various  tracts  of  white  matter,  until  in  the  medulla 
and  pons  it  is  spread  out  in  the  form  of  a  thin  layer  near  their  dorsal 
surfaces,  where,  together  with  the  ependyma,  it  forms  the  floor  of 
the  fourth  ventricle. 

In  the  region  of  the  aqueduct  of  Sylvius  the  gray  matter  again 
converges  and  ultimately  surrounds  the  canal,  to  again  expand  at  its 
anterior  extremity,  to  form  the  lining  of  the  third  ventricle. 

The  Nerve-cells. — The  nerve-cells  in  these  different  regions  do 
not  differ  morphologically  from  those  in  the  gray  matter  of  the  spinal 
cord.  The  corpus,  or  body  of  the  cell,  presents  a  number  of  dendrites 
as  well  as  the  sharply  defined  axon.  As  a  rule,  the  cells  are  arranged 
in  groups,  or  clusters,  or  nests,  partially  surrounded  and  enclosed  by 
supporting  tissue,  and  situated  beneath  the  floor  of  the  fourth  ventricle 
and  the  floor  of  the  aqueduct  of  Sylvius.  From  some  of  the  cell  groups 
axons  pass  ventrally  through  the  white  matter  to  merge  on  the  ventral 
and  lateral  surfaces  of  the  medulla,  pons,  and  crura,  where  they  are 
known  as  efferent  or  motor  cranial  nerves.  From  other  groups  of  cells, 
axons  cross  the.  median  line,  and  after  joining  the  mesial  fillet  ascend 
toward  the  cerebrum.  Around  these  latter  cells  the  terminal  filaments 
of  the  afferent  or  sensor  cranial  nerves  arborize.  The  collection  of 
cells  found  in  the  central  gray  matter  may  be  divided  into  two  groups — 
efferent  and  afferent. 

The  efferent  cells,  like  those  of  the  cord  independent  of  a  trophic 
influence,  are  motor  in  function,  inasmuch  as  the  excitation  arising 
in  them  is  transmitted  outward  through  their  related  axons  to  mus- 
i  les,  glands,  viscera  or  blood-vessels,  imparting  to  them  motion,  either 
molar  or  molecular. 

The  afferent  cells  are  largely  sentient  or  receptive  in  function, 
inasmuch  as  the  excitations  brought  to  them  by  the  afferent  cranial 
nerves  from  skin  and   mucous   membranes  and   from  sense-organs, 


MEDULLA  AND  BASAL  GANGLIA. 


529 


such  as  the  tongue  and  ear,  are  received  by  them  and  transmitted 
through  their  ascending  axons  to  the  cortex  of  the  cerebrum,  where 
thev  are  translated  into  conscious  sensations. 

Structure  of  the  White  Matter. — The  white  matter  is  com- 
posed of  medullated  nerve-fibers,  and  though  arranged  in  a  very 
complex  manner  may  be  divided  into  longitudinal  and  transverse 
fibers. 

The  longitudinal  -fibers  which  compose  the  main  portion  of  the 
isthmus  may  be  subdivided  into  (1)  a  ventral  or  pedal  portion  and  (2) 


Fig.  243. — Diagrammatic  Arrangement  of  the  Projection  Tracts  Connecting 
the  Cerebral  Cortex  with  the  Lower  Nerve-centers.  A.  Fronto-cerebellar 
tract.  B.  The  pyramidal  or  motor  tract.  C.  Sensory  tract.  D.  Visual  tract  from 
optic  thalamus  (6.T.)  to  the  occipital  lobe.  E.  Central  auditory  tract.  F.  Superior 
cerebellar  peduncle.  G.  Middle  cerebellar  peduncle.  H.  Inferior  cerebellar  peduncle. 
C.N.  Caudate  nucleus.  C.Q.  Corpora  quadrigemina.  Vt.  Fourth  ventricle.  The 
numerals  refer  to  cranial  nerves.     J.  Eighth  nerve  nucleus. — {After  Starr.) 


a  dorsal  or  tegmental  portion.  The  fibers  constituting  the  ventral 
or  pedal  portion  may  for  convenience  be  said  to  extend  from  the  cere- 
bral cortex  to  the  pons,  medulla,  and  spinal  cord.  They  may  be  di- 
vided into  three  distinct  tracts:  e.  g.,  the  pyramidal  tract,  the  fronto- 
cerebellar  tract,  and  the  occipito-temporo-cerebellar  tract  (Fig.  243). 
The  pyramidal  tract  descends  from  the  cortex  of  the  cerebrum 
bordering  the  fissure  of  Rolando,  passes  through  the  posterior  one- 
third  of  the  anterior  segment  and  the  anterior  two-thirds  of  the  pos- 
terior segment  of  the  internal  capsule,  the  middle  two-fifths  of  the 
crusta,  behind  the  transverse  fibers  of  the  pons,  to  become  the  anterior 
34 


530  TEXT-BOOK  OF  PHYSIOLOGY. 

pyramids  of  the  medulla,  beyond  which  it  divides  into  the  direct  and 
crossed  pyramidal  tracts  of  the  cord.  In  its  course  some  of  the  fibers 
and  their  collaterals  arborize  around  efferent  cells  from  the  anterior 
extremity  of  the  aqueduct  of  Sylvius  to  the  termination  of  the  spinal 
cord. 

The  fronto-cerebellar  tract  descends  from  the  cortex  of  the  frontal 
portion  of  the  anterior  lobe,  passes  through  the  anterior  portion  of  the 
anterior  segment  of  the  internal  capsule,  the  inner  fifth  of  the  crusta 
to  the  pons,  where  its  fibers  terminate  or  arborize  around'the  nucleus 
pontis  of  the  same  and  opposite  sides. 

The  occipito-temporo-cerebellar  tract  descends  from  the  occipital 
and  temporal  lobes,  passes  to  the  inner  side  of  the  lenticular  nucleus, 
and  continues  downward  on  the  outer  side  of  the  crusta,  occupying 
about  one-fifth  of  its  bulk,  to  the  pons,  where  its  fibers  also  arborize 
around  the  nucleus  pontis  of  the  same  and  opposite  sides.  By  means 
of  fibers  in  the  middle  peduncle  these  descending  fibers  are  brought 
into  relation  with  the  cerebellum. 

The  fibers  constituting  the  dorsal  or  tegmental  portion  of  the  longit- 
udinal system  may  be  said  for  convenience  to  extend  from  the  pos- 
terior portion  of  the  medulla  and  pons  to  the  optic  thalamus  and  cere- 
brum. They  may  be  subdivided  into  several  tracts:  viz.,  the  fillet, 
the  posterior  longitudinal  bundle,  Gowers'  tract,  etc. 

The  fillet  or  lemniscus,  consisting  of  fibers  having  their  origin 
partly  from  the  cells  of  the  cuneate  and  gracile  nuclei  and  partly  from 
the  cells  of  the  sensor  end-nuclei  of  various  sensor  cranial  nerves, 
occupies  a  region  in  the  ventral  and  mesial  portion  of  the  tegmentum 
throughout  its  entire  extent.  Superiorly  this  mesial  fillet  divides 
into  two  portions,  one  of  which  passes  to  the  thalamus  and  pregem- 
inum  (anterior  corpus  quadrigeminum),  the  other  to  the  cortex  of  the 
parietal  and  limbic  lobes.  The  fibers  coming  from  the  sensor  end- 
nucleus  of  the  auditory  nerve  (the  lateral  fillet)  lie  on  the  lateral  aspect 
of  the  pons  and  crus.  Superiorly  they  terminate  in  the  postgeminum 
(the  posterior  corpus  quadrigeminum). 

The  posterior  longitudinal  bundle,  an  upward  extension  of  the  fibers 
composing  a  portion  of  the  ground  bundle  of  the  spinal  cord,  is  located 
on  cither  side  of  the  median  line  just  beneath  the  floor  of  the  fourth 
ventricle  and  the  aqueduct  of  Sylvius.  As  it  passes  upward  collateral 
branches  are  given  off,  some  of  which  arborize  around  the  cell  nuclei 
of  the  third,  fourth,  and  sixth  cranial  nerves  of  the  same  side,  while 
others  cross  the  median  line  and  arborize  around  the  corresponding 
cell  nuclei  of  the  opposite  side.  Superiorly  some  of  the  fibers  become 
related  to  cells  in  the  thalamus  and  subthalamic  region.  This  bundle 
of  fibers  appears  to  be  mainly  commissural  in  character. 

Gowers'  tract,  the  antero-lateral  tract  of  the  spinal  cord,  occupies 
a  position  in  the  lateral  region  of  the  formatio  reticularis  both  in  the 
medulla  and  pons.  Continuing  upward,  it  enters  the  mesial  fillet, 
and  in  company  with  it  passes  through  the  posterior  division  of  the 


FUNCTIONS  OF  THE  MEDULLA  OBLONGATA.  531 

internal  capsule  and  finally  terminates  around  cells  in  the  cortex  of 
the  parietal  lobe. 

The  transverse  -fibers  of  the  isthmus  are  found  in  the  pons.  The 
fibers  of  the  ventral  as  well  as  those  of  the  more  dorsal  regions  have 
their  origin  in  nerve-cells  in  the  cprtex  of  the  cerebellum.  From  their 
origin  they  pass  through  the  cerebellar  white  matter,  and  through 
the  middle  peduncle  as  far  as  the  median  line,  where  they  decussate 
with  fibers  coming  from  the  opposite  side.  Beyond  this  point  they 
pass  to  the  cerebellar  cortex.  From  their  anatomic  relations  it  is  prob- 
able that  these  transverse  fibers  are  commissural  in  character,  bring- 
ing into  relation  opposite  but  corresponding  regions  of  the  cerebellar 
cortex.  In  addition  to  the  commissural  fibers  other  transverse  fibers 
associate  the  cerebellar  cortex  with  the  gray  matter  in  the  pons  on 
both  the  same  and  opposite  sides.  In  this  way  the  cerebellum  is 
brought  into  relation  with  longitudinal  fibers  coming  from  and  going 
to  the  cerebrum. 

FUNCTIONS  OF  THE  MEDULLA  OBLONGATA,  ISTHMUS,  AND 
BASAL  GANGLIA. 

Microscopic  examination  of  the  white  and  gray  matter  of  these 
various  parts  of  the  central  nerve  system  shows  that  they  are  com- 
posed of  nerve-cells  and  nerve-fibers  which  morphologically  do  not 
differ  in  essential  respects  from  those  found  in  the  spinal  cord,  though 
their  arrangement  is  far  more  complicated  and  involved.  The  func- 
tions of  these  closely  related  structures  are  in  consequence  equally 
complex  and  involved  and  but  imperfectly  known. 

In  a  general  way  it  may  be  said  that  by  virtue  of  the  presence 
of  nerve-cells  and  definite  tracts  of  nerve-fibers  these  structures  col- 
lectively may  be  regarded  as  consisting: 

1.  Of  centers  for  reflex  actions;  and — 

2.  Of  conducting  paths  by  which  the  various  parts  are  brought  into 

relation  one  with  another  and  with  the  spinal  cord,  the  cerebel- 
lum, and  the  cerebrum. 

The  Medulla  Oblongata  and  Pons. — The  gray  matter  situated 
in  these  structures — i.  e.,  just  beneath  the  floor  of  the  fourth  ventricle 
— contains  nerve-cells  arranged  in  more  or  less  well-defined  groups 
which  may  be  divided  into  efferent  and  afferent. 

The  efferent  cells  are  the  immediate  sources  of  nerve  impulses  which 
are  transmitted  through  efferent  axons  to  various  peripheral  organs- 
muscles,  glands,  viscera  and  blood-vessels.  Their  activity  may  be  ex- 
cited by  the  same  influences  which  excite  the  efferent  cells  of  the  spinal 
cord:  e.  g.,  variations  in  the  composition  of  the  blood  or  lymph;  the  ar- 
rival of  nerve  impulses  coming  through  afferent  pathways  in  the  spinal 
cord  and  through  afferent  cranial  nerves;  the  arrival  of  nerve  impulses 
coming  through  efferent  pathways  from  the  cerebrum.  The  peripheral 
activity  resulting  from  their  excitation  may  therefore  be  automatic  or 
autochthonic,  peripheral  (reflex)  or  cerebral  (volitional)  in  origin. 


532  TEXT-BOOK  OF  PHYSIOLOGY. 

The  afferent  cells  are  sentient  or  receptive  in  function,  inasmuch 
as  they  receive  nerve  energies  coming  through  lower  afferent  pathways 
and  transmit  them  through  their  related  axons  to  the  cortex  of  the 
cerebrum,  where  they  are  translated  into  conscious  sensations. 

The  efferent  cells  give  origin  tq  nerve-fibers  which  pass  ventrally 
and  become  the  efferent  or  motor  cranial  nerves. 

The  afferent  cells  give  origin  to  fibers  which  pass  to  the  cerebral 
cortex.  Around  both  groups  of  cells,  the  afferent  or  sensor  cranial 
nerves  terminate  in  tuft-like  expansions.  In  a  subsequent  section  the 
origin,  course,  and  distribution  of  the  various  cranial  nerves  will  be 
considered.  But  as  the  function  of  the  nerve  is  but  to  transmit  energy 
from  the  cell  of  which  it  constitutes  a  part,  the  function  ascribed  to  it 
may  without  impropriety  be  transferred  to  the  cell  itself. 

Since  it  is  by  means  of  nerve-cells  and  their  associated  fibers  that 
many  important  functions  of  organic  life  are  initiated  and  maintained, 
it  would  naturally  be  expected  from  its  extensive  nerve  connections 
that  this  region  of  the  nerve  system  plays  an  extensive  role  in  this 
respect.  As  the  accomplishment  of  these  functions  requires  the 
cooperation  and  coordination  of  a  number  of  separate  but  related 
structures,  it  is  evident  that  there  must  exist  in  the  medulla  and  pons 
a  number  of  coordinating  mechanisms  consisting  of  nerve-cells  and 
nerve-fibers  which  are  associated  in  various  ways  for  the  accomplish- 
ment of  definite  functions.  To  such  a  coordinating  mechanism  the 
term  "center"  has  been  given:  e.  g.,  respiratory,  cardiac,  deglutitory, 
etc.* 

The  Medulla  Oblongata  and  Pons  as  Centers  for  Reflex  Activ- 
ities.— Experimentation  has  shown  that  the  medulla  and  pons  contain 
a  number  of  such  centers,  the  more  important  of  which  are  as  follows: 
i.   Cardiac   centers,   which   exert    (i)    an   accelerator  action   over  the 
heart's  pulsations  through  nerve-fibers  emerging  from  the  spinal 
cord  in  the  roots  of  the  first  and  second  dorsal  nerves  and  reach- 
ing the  heart  through  the  sympathetic  nerve;   (2)   an  inhibitor 
or  retarding  action  on  the  rate  of  the  heart  beat  through  efferent 
fibers  in  the  trunk  of  the  pneumogastric  or  vagus  nerve.    (See  pages 

3*2>  3I3-) 

2.  A  vaso-motor    center,   which    regulates  the  caliber  of    the  blood- 

vessels throughout  the  body  in  accordance  with  the  needs  of  the 
organs  and  tissues  for  blood,  through  nerve-fibers  passing  by 
way  of  the  spinal  nerves  to  the  walls  of  the  blood-vessels.  (See 
page  376.) 

3.  A  respiratory  center,  which  coordinates  the  muscles  concerned  in 

the  production  of  the  respiratory  movements.     (See  page  428.) 

*  By  the  term  center  as  here  employed  is  meant  a  collection  of  nerve-cells  and  nerve- 
fibers  occupying  an  area  of  greater  or  less  extent,  though  its  exact  anatomic  limits  may 
not  Ik-  ac(  urately  defined.  That  an  area  may  merit  the  term  center,  it  is  necessary  that 
its  stimulation  should  increase,  its  destruction  should  abolish  or  impair,  functional 
activity. 


FUNCTIONS  OF  THE  CRURA  CEREBRI.  533 

4.  A  mastication  center,  which  excites  to  activity  and  coordinates  the 

muscles  of  mastication.     (See  page  150.) 

5.  A  deglutition    center,  which  excites   and   coordinates  the  muscles 

concerned  in  the  transference  of  the  food  from  the  mouth  to 
the  stomach.     (See  page  170.) 

6.  An  articulation  center,  which  coordinates  the  muscles  necessary  to 

the  production  of  articulate  speech. 

In  addition,  the  gray  matter  contains  centers  which  influence  the 
secretion  of  saliva,  provoke  vomiting,  coordinate  the  muscles  of  the 
face  concerned  in  expression,  and  control  the  secretion  of  the  perspira- 
tion. 

As  Conducting  Pathways. — The  anterior  pyramids  of  the  medulla 
and  their  continuations  through  the  more  ventral  portions  of  the  pons, 
being  portions  of  the  general  pyramidal  tract,  serve  to  conduct  voli- 
tional efferent  nerve  impulses  from  higher  portions  of  the  brain  to  the 
spinal  cord.  Division  of  either  of  these  pathways  is  at  once  followed 
by  a  loss  of  volitional  control  of  the  muscles  below  the  section. 

The  dorsal  or  tegmental  portion,  containing  the  fillet  and  Gowers' 
tract,  serves  to  transmit  afferent  nerve  impulses  from  the  spinal  cord 
to  higher  portions  of  the  brain.  Transverse  division  of  one-half 
of  the  dorsal  portion  of  the  pons  is  followed  by  complete  anesthesia 
of  the  opposite  half  of  the  body  without  any  impairment  of  motion. 

The  restiform  bodies  constitute  a  pathway  between  the  spinal  cord 
and  the  cerebellum.  The  transverse  fibers  of  the  pons  associate 
opposite  but  corresponding  portions  of  the  cerebellar  hemispheres. 

The  Crura  Cerebri. — The  crura  cerebri  consist  ventrally  of  fibers 
which  are  largely  derived  from  the  pyramidal  tracts  and  are  con- 
tinuous with  the  longitudinal  fibers  of  the  ventral  portion  of  the  pons 
and  medulla;  and  dorsally  of  fibers  continuous  with  those  coming 
through  the  lower  portions  of  the  tegmentum.  Hence  they  are  con- 
ductors of  motor  impulses  in  the  former  and  of  sensor  impulses  in  the 
latter  region.  It  is  not  definitely  known  as  to  whether  reflex  actions 
take  place  through  the  gray  matter,  the  locus  niger,  or  not. 

The  gray  matter  beneath  the  aqueduct  of  Sylvius  contains  nerve- 
cell  groups  which  are  centers  for  reflex  actions  in  connection  with 
ocular  movements:  e.  g.,  closure  of  the  lids,  contraction  of  the  sphinc- 
ter pupillae,  convergence  of  the  eyes,  etc. 

The  Corpora  Quadrigemina.— From  the  anatomic  relation  of 
the  anterior  quadrigeminal  body  (the  pregeminum)  to  the  optic  tract, 
on  the  one  hand,  and  to  the  optic  radiation,  on  the  other,  the  inference 
can  be  drawn  that  it  is  in  some  way  essential  to  the  performance  of 
the  visual  process.  Experimental  investigations  and  pathologic 
changes  support  the  inference. 

Irritation  of  the  pregeminum  in  monkeys  on  one  side  is  followed 
by  dilatation  of  the  pupils  first  on  the  opposite  side  and  then  almost 
immediately  on  the  same  side.  The  eyes  at  the  same  time  are  also 
widely  opened  and  the  eyeballs  turned  upward  and  to  the  opposite 


TEXT-BOOK  OF  PHYSIOLOGY. 


side.  If  the  irritation  be  continued,  motor  reactions  are  exhibited 
in  various  parts  of  the  body.  Destruction  of  the  pregeminum  in  both 
monkeys  and  rabbits  is  followed  by  blindness,  dilatation  and  immo- 
bility of  the  pupils,  with  marked  disturbance  of  equilibrium  and 
locomotion  (Ferrier). 

From  the  anatomic  relation  of  the  posterior  quadrigeminal  body 
(the  postgeminum)  to  the  lateral  fillet,  the  basal  tract  for  hearing, 
the  inference  may  be  drawn  that  it  is  in  some  way  connected  with  the 
auditory  process. 

Stimulation  of  the  postgeminum  gives  rise  to  cries  and  various  forms 

of  vocalization.  Pathologic 
states  of  this  body  are  also 
attended  by  impairment  of 
hearing  and  disorders  of 
the  equilibrium. 

From  the  foregoing  facts 
it  is  probable  that  the  cor- 
pora quadrigemina  are  as- 
sociated with  station  and 
locomotion.  Ferrier  as- 
sumes that  in  these  bodies 
"sensory  impressions,  ret- 
inal and  others,  are  co- 
ordinated with  adaptive 
motor  reactions  such  as  are 
involved  in  equilibration 
and  locomotion." 

The  Corpora  Striata. 
— The  relation  of  these 
bodies  to  the  pyramidal 
motor  tract  would  indicate 
that  they  are  in  some  way 
connected  with  motor  ac- 
tivities. Their  function, 
however,  is  obscure.  While 
stimulation  of  one  corpus 
produces  convulsion  of  the  muscles  of  the  opposite  side  of  the  body, 
and  destruction  gives  rise  to  paralysis  of  the  corresponding  muscles,  it 
is  difficult,  owing  to  the  intimate  association  of  the  white  and  the  gray 
matter,  to  state  to  which  the  phenomena  are  to  be  attributed.  The 
evidence  at  hand  points  to  the  conclusion  that  if  a  lesion  is  limited  to 
the  gray  matter  the  paralysis  which  might  result  would  be  but 
temporary  and  of  short  duration.  The  pathologic  evidence  is  of  a 
similar  character.  Gowers  is  of  the  opinion,  that  if  the  lesion  is  small 
and  at  a  sufficient  distance  from  the  white  fibers  of  the  capsule,  there 
may  even  be  no  initial  hemiplegia;  neither  motor  nor  sensory  paralysis 
will  arise  if  the  lesion  is  confined  to  the  gray  matter. 


Fig.  244. — Horizontal  Section  of  the  Inter- 
nal Capsule  Showing  the  Position  and  Rela- 
tion oe  the  Motor  Tracts  for  the  Eye,  Head 
(Hd.),  Tongue  (Tg.),  Mouth  (Mth.),  Shoulder 
(Shi.),  Elbow  (Elb.),  Digits  of  Hand  (Dig.), 
Abdomen  (Abd.),  Hip,  Knee  (Kn.),  Digits  of 
Foot  (Dig.).  S.  Sensor  tract.  O.  T.  Optic  tract. 
A.  T.  Auditory  tract.  1.  Caudate  nucleus.  2. 
Anterior  segment  of  internal  capsule.  3.  Ex- 
ternal capsule.  4.  Island  of  Reil.  5.  Lenticular 
nucleus.  6.  Claustrum.  7.  Posterior  segment  of 
internal  capsule. — (Modified  from  Landois.) 


FUNCTION'S  OF  THE  INTERNAL  CAPSULE. 


It  is  stated  by  some  experimenters  that  localized  injuries,  both 
experimental  and  pathologic,  are  followed  by  a  persistent  rise  of 
temperature,  varying  from  i°  to  2.6°  C. 

The  Optic  Thalami. — From  the  anatomic  relation  of  the  optic 
thalami  to  the  general  and  special  sense  nerve-tracts,  on  the  one  hand, 
and  to  the  cerebral  cortex,  on  the  other  hand,  it  is  assumed  that  they 
are  connected  with  the  production  of  sensations  both  general  and 
special,  and  act  as  intermediates  between  the  peripheral  sense-organs 
and  the  cortex. 

The  results  of  experimental  stimulation  and  destruction  of  the 
thalami  are  extremely  con- 
tradictory and  fail  to  throw 
much  light  on  their  func- 
tions. Ferrier  states  that 
destruction  of  the  posterior 
part  of  one  thalamus  pro- 
duced blindness  in  the  op- 
posite eye  and  impairment 
of  the  sense  of  touch  and 
pain  in  the  opposite  side  of 
the  body.  In  a  patient 
under  the  care  of  Hugh- 
lings- Jackson  there  was 
blindness  in  the  right  half 
of  each  eye,  loss  of  hearing 
in  the  left  ear,  impairment 
of  taste  on  the  left  side  of 
the  tongue,  and  a  diminu- 
tion of  the  sense  of  touch 
on  the  left  side  of  the  body. 
Postmortem  examination 
showed  a  patch  of  soften- 
ing in  the  posterior  part  of 
the  right  thalamus,  the  re- 
mainder of  the  organ  being 
normal. 


Fig.  245. — Vertical  Sectiox  Through  the- 
Right  Cerebral  Hemisphere  in  Front  of  the 
Gray  Commissure.  1.  Caudate  nucleus.  2.  Cor- 
pus callosum.  3.  Pillars  of  the  fornix.  4.  Internal 
capsule.  5.  Optic  thalamus.  6.  Gray  commissure. 
7.  External  capsule.  S.  Claustrum  (1,  2,  3,  the 
three  divisions  of  the  lenticular  nucleus). — (Landois.) 


It  is  probable  that  in  the  thalamus  visual,  tactile,  and  labyrinthine 
impressions  are  received,  coordinated,  and  reflected  outward,  with 
the  result  of  producing  various  adaptive  motor  reactions  connected 
with  station  and  ecpuilibrium.  It  is  also  believed  by  some  investigators 
to  act  as  an  intermediate  between  emotional  states  and  their  expres- 
sion in  the  muscles  of  the  face,  this  power  being  lost  in  certain  patho- 
logic conditions.  The  power  of  regulating  the  temperature  of  the 
body  has  also  been  assigned  to  the  thalamus,  as  destruction  of  its 
anterior  extremity  is  usually  followed  by  a  rise  in  temperature. 

The  Internal  Capsule. — The  internal  capsule  has  been  shown  by 
the  results  both  of  experiment  and  of  pathologic  processes  to  be,  first, 


536  TEXT-BOOK  OF  PHYSIOLOGY. 

a  pathway  for  the  transmission  of  nerve  impulses  from  the  cerebral 
cortex  to  the  pons,  medulla,  and  spinal  cord,  which  give  rise  to  con- 
traction of  the  muscles  of  the  opposite  side  of  the  body;  and,  second, 
a  pathway  for  the  transmission  of  nerve  impulses  coming  from  skin, 
mucous  membrane,  muscles,  and  special  sense-organs  to  the  cortex, 
where  they  give  rise  to  sensations  general  and  special.  It  is  therefore 
the  common  motor  and  sensor  pathway.  For  the  reason  that  it  trans- 
mits both  motor  and  sensor  impulses,  and  for  the  further  reason  that 
it  is  frequently  the  seat  of  pathologic  lesions  which  are  followed  by 
either  a  loss  of  motion  or  sensation  or  both,  the  internal  capsule  is  one 
of  the  most  important  parts  of  the  central  nerve  system.  As  shown 
in  Fig.  244,  it  consists  of  two  segments  or  limbs  united  at  an  obtuse 
angle,  the  knee  or  elbow,  which  is  directed  toward  the  median  line. 
The  motor  tract  is  confined  to  the  posterior  one-third  of  the  anterior 
segment  and  the  anterior  two-thirds  of  the  posterior  segment.  The 
sensor  tract  is  confined  to  the  posterior  one-third  of  the  posterior  seg- 
ment, the  extreme  end  of  which  also  contains  the  optic  and  auditory 
tracts. 

The  region  of  the  anterior  segment  in  front  of  the  motor  tract 
contains  the  fibers  of  the  fronto-cerebellar  tract,  the  function  of  which 
is  unknown. 

The  motor  region  contains  fibers  which  descend  from  the  cerebral 
cortex  to  nerve-centers  situated  in  the  gray  matter  beneath  the  aque- 
duct of  Sylvius,  in  the  gray  matter  beneath  the  floor  of  the  fourth  ven- 
tricle, and  in  the  anterior  horns  of  the  gray  matter  of  the  spinal  cord, 
and  which  in  turn  are  connected  by  the  cranial  and  spinal  nerves  with 
the  muscles  of  the  eye,  head,  face,  trunk,  and  limbs.  The  positions 
occupied  by  these  different  tracts  are  shown  in  Fig.  244. 

The  relation  of  the  internal  capsule  to  the  caudate  nucleus  and 
the  optic  thalamus  internally,  and  to  the  lenticular  nucleus  externally, 
is  also  shown  in  a  vertical  section  of  the  cerebrum  made  in  front  of  the 
gray  commissure  (Fig.  245).  From  the  fact  that  the  internal  capsule 
contains  efferent  or  motor  tracts,  and  afferent  or  sensor  tracts,  it  is 
evident  that  a  destructive  lesion  of  the  motor  tract  would  be  followed 
by  a  loss  of  motion;  and  of  the  sensor  tract,  by  a  loss  of  sensation  on 
the  opposite  side  of  the  body. 


CHAPTER  XXI. 
THE  CEREBRUM. 

The  cerebrum  is  the  largest  portion  of  the  encephalon,  constitut- 
ing about  85  per  cent,  of  its  total  weight.  In  shape  it  is  ovate,  convex 
on  its  outer  surface,  narrow  in  front  and  broad  behind.  It  is  divided 
bv  a  deep  longitudinal  cleft  or  fissure  into  halves,  known  as  the  cerebral 
hemispheres.  The  hemispheres  are  completely  separated  anteriorly 
and  posteriorly  by  this  fissure,  but  in  their  middle  portions  are  united 
bv  a  broad  white  band,  the  corpus  callosum.  Each  hemisphere  or 
hemi-cerebrum  is  convex  on  its  outer  aspect,  and  corresponds  in  a 
general  way  with  the  cavity  of  the  skull;  the  inner  or  mesal  surface  is 
flat  and  forms  the  lateral  boundary  of  the  longitudinal  fissure. 

The  surface  of  each  hemi-cerebrum  presents  a  series  of  alternate 
indentations  and  elevations,  known  respectively  as  fissures  or  sulci, 
and  convolutions  or  gyres.  A  knowledge  of  the  situation  and  extent 
of  the  principal  fissures  and  convolutions,  as  well  as  of  their  relation 
one  to  another,  is  essential  to  a  clear  understanding  of  many  phys- 
iologic processes,  clinical  phenomena,  and  surgical  procedures,  The 
general  arrangement  of  the  primary  fissures  and  convolutions  is  rep- 
resented in  Figs.  246  and  247. 

Fissures. — 

1.  The  fissure  of  Sylvius,  one  of  the  most  important  of  the  primary 

fissures,  is  found  on  the  side  of  the  cerebrum.  It  begins  at  the 
base  and  extends  upward,  outward,  and  backward  to  a  point 
corresponding  to  the  eminence  of  the  parietal  bone,  where  it 
usually  terminates  in  a  more  or  less  vertically  directed  branch, 
the  epi- sylvian  branch.  Anteriorly  a  short  branch  is  given  off 
which  passes  upward  and  forward  into  the  frontal  lobe  and 
known  as  the  pre-sylvian;  a  horizontal  branch  is  known  as  the 
sub-sylvian.  The  Sylvian  fissure  is  the  first  to  appear  in  the  de- 
velopment of  the  fetal  brain,  becoming  visible  at  the  third  month. 
In  the  adult  it  is  deep  and  well  marked  and  divides  the  hemi- 
cerebrum  into  a  frontal  and  a  temporo-sphenoidal  lobe. 

2.  The  fissure   of  Rolando,  or  central  fissure,  equally  important,  is 

found  on  the  superior  and  lateral  aspects  of  the  cerebrum.  It 
runs  from  a  point  on  the  convexity  of  the  hemisphere  near  the 
median  line  transversely  outward  and  downward  toward  the 
fissure  of  Sylvius,  but  as  a  rule  does  not  pass  into  it.  It  divides 
the  frontal  from  the  parietal  lobe.  The  inclination  of  the  central 
fissure  h  such  as  to  form  with  the  longitudinal  fissure  an  angle 
of  about  67  degrees. 

537 


53* 


TEXT-BOOK  OF  PHYSIOLOGY. 

The  infra-parietal  fissure  arises  a  short  distance  behind  the  central 
fissure.  It  then  runs  upward,  backward,  and  downward  to 
terminate  near  the  posterior  extremity  of  the  parietal  lobe.  It 
divides  the  parietal  lobe  into  a  superior  and  an  inferior  portion. 

The  parieto-occipital  fissure,  situated  on  the  mesal  surface  of  the 
hemisphere,  divides  the  latter  into  a  parietal  and  an  occipital 
lobe.  It  begins  as  a  deep  notch  on  the  surface  of  the  hemisphere, 
and  is  then  continued  downward  and  forward  until  it  enters  the 
calcarine  fissure.      (Fig.  247.) 


Fig.  246.  —  Diagram  Showing  Fissures  and  Convolutions  on  the  Lateral 
Aspect  of  the  Left  Hemi-Cerebrum.  F.  Frontal.  P.  Parietal.  T.  Temporal  and 
O.  Occipital  lobes.  S.  Fissure  of  Sylvius.  EPS.  Epi-sylvian,  P  R  S.  Pre-sylvian,  S  B  S. 
Sub-sylvian  fissures.  C.  Central  fissure  or  Fissure  of  Rolando.  P  R  C.  Pre-central  fis- 
sure. SPFR.  Super-frontal  fissure.  MEFR.  Medi-frontal  fissure.  SBFR.  Sub- 
frontal  fissure.  PC.  PC.  Post-central  fissure.  PTL.  Parietal  fissure.  PAROC. 
Par-occipital,  EXOCC.  Ex-occipital  fissures.  SPTMP.  Super-temporal  fissure.  MTMP 
Medi-temporal  fissure. 

5.  The  calcarine  fissure  begins  on  the  posterior  extremity  of  the  mesal 

surface  of  the  occipital  lobe.  From  this  point  it  passes  downward 
and  forward  to  unite  with  the  parieto-occipital  fissure. 

6.  The  para-central  fissure  begins  at  the  supero-mesal  border  of  the 

hemisphere.  It  then  passes  downward  and  forward  for  a  varia- 
ble distance  and  then  turns  upward  enveloping  a  lobule  known 
as  the  para-central  lobule. 

7.  The  su  per -c  alio  sal  fissure  extends  from  a  point  just  anterior  to  the 

para-central  lobule  downward  and  forward  below  the  rostrum  of 

the  corpus  callosum. 
Secondary  fissures  of  more  or  less  importance  are  present  in  the 
different  lobes,  subdividing  the  surface  into  convolutions:  e.  g.,  in  the 
frontal  lobe  arc  found  the  pre-central,  the  super- frontal,  medi-jrontal 
and  sub-jrontal  fissures;  in  the  temporal  lobe  the  super-temporal  and 
medi- temporal  fissures. 


THE  CEREBRUM. 


5^9 


Convolutions. — The  convolutions  or  gyres  are  the  portions  of 
the  cerebral  surface  comprised  between  the  fissures.  The  arrange- 
ment of  the  surface  is  such  that  only  the  more  superficial  portions  are 
visible.  The  depth  of  the  convolution,  the  portion  bordering  the 
fissure,  is  concealed  from  view.  Each  lobe  presents  a  series  of  such 
convolutions  which  differ  considerably  in  their  relative  physiologic 
importance. 

The   Frontal  Lobe. — The  frontal   lobe  presents  on  its   convex 

surface  four  convolutions :     viz.,  the  anterior  or  pre-central  convolution, 

and  the  super-,  medi,  and  sub-frontal  convolutions. 

i.  The  anterior  or  pre-central  convolution  is  situated  just  in  front  of 

the   Rolandic  or  central  fissure,   with  which   it   corresponds  in 


Fig.  247. — Diagram  Showing  Fissures  and  Convolutions  on  the  Mesal  As- 
pect of  the  Left  Hemi-cerebrum.  C.  Upper  extremity  of  the  central  fissure.  PARC. 
Para-central  fissure.  S  P  C  L.  Super-callosal  fissure.  C  L.  Callosal  fissure.  O  C.  Oc- 
cipital fissure.     CLC.  Calcarine  fissure.     CLT.  Collateral  fissure. 


direction.  It  is  continuous  above  with  the  super-frontal  and 
below  with  the  sub-frontal  convolution. 

2.  The     super-frontal    convolution    is    bounded     internally    by    the 

longitudinal  fissure  and  externally  by  the  super-frontal  fissure. 
From  the  upper  end  of  the  pre-central  convolution,  with  which 
it  is  continuous,  it  runs  forward  and  downward  to  the  anterior 
extremity  of  the  frontal  lobe,  where  it  turns  backward  and  rests 
on  the  orbital  plate  of  the  frontal  bone. 

3.  The   medA- frontal  convolution  is  situated  on  the  side  of  the  lobe, 

between  the  super-frontal  fissure  above  and  the  medi-frontal 
fissure  below.     Its  general  direction  is  downward  and  forward. 

4.  The  sub-frontal  convolution  winds  around  the  pre-sylvian  branch 

of  the  fissure  of  Sylvius  in  the  anterior  and  inferior  portion  of 


54o  TEXT-BOOK  OF  PHYSIOLOGY. 

the  frontal  lobe.     It  is  continuous  posteriorly  with  the  lower  end 

of  the  pre-central  convolution. 
The    Parietal    Lobe. — The    parietal    lobe    presents    three    well- 
marked  convolutions:  viz.,  the  posterior  or  post-central  convolution, 
and   the   super-   and    sub-parietal.     The   latter   is  again  subdivided 
into  the  marginal  and  the  angular  convolution. 

i.  The  posterior  or  post-central  convolution  is  situated  just  behind 
the  Rolandic  or  central  fissure,  with  which  it  corresponds  in 
direction.  Above,  it  is  continuous  with  the  super  parietal 
convolution;  below,  with  the  marginal  and  the  pre-central  con- 
volutions. 

2.  The  super-parietal  convolution  is  bounded  internally  by  the  longit- 

udinal fissure  and  externally  by  the  intra-parietal  fissure.  From 
the  upper  end  of  the  post-central  convolution,  with  which  it  is 
connected,  it  runs  downward  and  backward  as  far  as  the  parieto- 
occipital fissure. 

3.  The    sub-parietal    convolution    is   connected    anteriorly   with   the 

post-central  convolution.  Passing  backward,  it  winds  around 
the  superior  extremity  of  the  fissure  of  Sylvius,  in  which  situa- 
tion it  is  known  as  the  supra-marginal  convolution.  Beyond 
this  point  it  divides  into  two  portions,  one  of  which  runs  forward 
into  the  temporal  lobe  above  the  super-temporal  fissure,  while 
the  other  runs  downward  and  backward,  following  the  intra- 
parietal  fissure  to  its  termination.  At  this  point  it  makes  a  sharp 
bend  and  runs  forward  into  the  temporal  lobe  just  beneath  the 
super-temporal  fissure.  In  the  neighborhood  of  the  bend  it  is 
generally  known  as  the  angular  convolution  or  gyrus. 

The  Temporo-sphenoidal  Lobe. — The  temporo-sphenoidal  lobe 
presents  on  its  external  surface  three  well-marked  convolutions:  viz., 
the  super-,  the  medi-,  and  the  sub-temporal,  separated  by  the  super-  and 
medi-  temporal  fissures.  These  three  convolutions  are  in  a  general 
way  parallel  with  each  other,  and  pursue  a  direction  from  before 
backward  and  upward.  Anteriorly,  they  are  fused  together,  but 
posteriorly  their  connections  are  somewhat  different.  The  super- 
temporal  is  continuous  behind  and  above  with  the  supra-marginal 
convolution,  and  behind  and  below  with  the  angular  convolution  or 
gyre.  The  medi-temporal  blends  with  the  preceding  and  with  the 
middle  occipital.  The  sub-temporal  is  continuous  with  the  inferior 
occipital. 

The  Occipital  Lobe. — The  occipital  lobe  is  triangular  in  shape 
and  forms  the  posterior  apex  of  the  hemisphere.  Its  base  on  the 
external  surface  is  formed  by  an  imaginary  line  drawn  from  the 
parieto-occipital  fissure  to  the  pre-occipital  notch  on  the  inferior  and 
lateral  border.  The  external  surface  presents  three  convolutions — 
the  superior,  middle,  and  inferior  occipital. 

The  inner  or  mesal  surface  of  the  hemisphere,  formed  in  part 


THE  CEREBRUM.  541 

by  the  frontal,  the  parietal,  the  occipital,  and  the  temporal  lobes,  pre- 
sents several  convolutions  of  much  physiologic  interest,  viz. : 

1.  The     callosal    convolution,    situated    between    the    super-callosal 

fissure  and  the  corpus  callosum.  From  its  origin  anteriorly 
at  the  base  of  the  brain  this  convolution  passes  backward,  grad- 
ually increasing  in  width  as  it  approaches  the  posterior  extremity 
of  the  corpus  callosum.  At  this  point  it  again  narrows  and 
descends  between  the  calcarine  and  hippocampal  fissures  to  blend 
with  the  hippocampal  convolution. 

2.  The   gyrus  hippocampus,  formed  by  the  union  of   the  posterior 

extremity  of  the  callosal  convolution  and  the  sub-calcar- 
ine  convolution  is  situated  just  below  the  dentate  or  hippo- 
campal fissure.  Anteriorly  it  becomes  enlarged,  and  just 
behind  the  apex  of  the  temporal  lobe  turns  backward  and 
inward  to  form  a  hook-shaped  eminence,  the  uncinate  gyrus 
or  uncus. 

The  limbic  lobe  is  the  name  given  to  an  area  of  the  brain  which 
includes,  among  other  structures,  the  callosal  convolution,  the  gyrus 
hippocampus,  and  the  uncus.  As  forming  a  part  of  this  general 
lobe  may  be  mentioned  the  dentate  fascia,  the  strise  and  peduncle 
of  the  corpus  callosum,  the  septum  lucidum,  the  fornix,  and  the 
infracallosal  gyrus. 

3.  The   collateral    convolution   is  bounded   by   the  collateral  fissure 

above,  and  its  inferior  border  extends  from  the  occipital  lobe  to 
the  anterior  pole  of  the  temporal  lobe. 

4.  The  quadrate  lobule,  or  precuneus,  a  square-shaped  convolution, 

is  situated  between  the  posterior  termination  of  the  para-central 
fissure  and  the  parieto-occipital  fissure.  It  blends  with  the  callosal 
convolution,  on  the  one  hand,  and  with  the  parietal  lobule  on  the 
other. 

5.  The  cuneus,  a  triangular  or  wedge-shaped  convolution  or  lobule, 

is  situated  on  the  mesal  surface  of  the  occipital  lobe  between  the 
parieto-occipital  and  calcarine  fissures. 

The  Insula  or  Island  of  Reil. — This  anatomic  structure  con- 
sists of  a  triangular  shaped  cluster  of  six  small  convolutions  situated 
at  the  bifurcation  of  the  Sylvian  fissure  and  concealed  from  view 
by  the  convolutions  bordering  it,  spoken  of  collectively  as  the  oper- 
culum. These  convolutions  are  connected  with  the  frontal,  the 
parietal,  and  the  temporal  lobes. 

Structure  of  the  Gray  Matter  of  the  Cortex. — The  gray  matter, 
the  cortex  of  the  cerebrum,  varies  from  two  to  four  millimeters  in 
thickness.  When  examined  with  a  lens  of  low  power,  it  presents  a 
laminated  appearance,  due  to  differences  in  color  and  arrangement 
of  its  constituent  elements.  With  higher  magnification  the  cortex 
is  seen  to  consist  of  neuroglia  cells,  nerve-cells  with  specialized  den- 
drites and  axons,  medullated  and  non-medullated  nerve-fibers,  blood- 
vessels, connective  tissue,  etc. — all  of  which  are  arranged  and  inter- 


54- 


TEXT-BOOK  OF  PHYSIOLOGY. 


blended  in  a  most  intricate  manner.  Notwithstanding  the  complexity 
of  its  structure,  modern  histologic  methods  have  enabled  Cajal  to 
divide  it  into  four  fairly  distinct  layers  or  zones,  from  without  inward, 
as  follows  (Fig.  248) : 

1.    The  Molecular  Layer. — The  most  superficial  portion  of  this  layer 

consists  mainly  of  neuroglia  or  glia  cells, 
the  processes  of  which  interlace  in  all 
directions,  forming  a  distinct  sheath 
just  beneath  the  pia.  The  deeper  por- 
tions of  this  layer  contain  a  specialized 
type  of  nerve-cell  (Cajal  cells),  of  which 
there  are  several  varieties.  These  cells 
give  off  nerve-fibers  which  pursue  a 
horizontal  direction  for  a  variable  dis- 
tance, but  in  their  course  give  off 
collateral  branches  which  ascend  to  the 
outer  surface  of  the  layer.  Among 
these  structures  are  to  be  found,  also, 
dendritic  processes  of  cells  situated  in 
the  subjacent  layer.  The  terminal 
filaments  of  medullated  nerve-fibers 
coming  from  nerve-cells  in  lower  re- 
gions of  the  encephalo-spinal  axis  are 
also  present,  but  for  the  most  part  they 
pursue  a  tangential  direction. 
The  Layer  of  Small  Pyramidal  Cells. — 
This  layer  consists  mainly  of  nerve- 
cells,  the  majority  of  which  are  pyra- 
midal in  shape  and  of  small  size. 
Other  cells,  however,  are  present,  which 
present  a  variety  of  shapes,  for  which 
reason  the  layer  was  at  one  time  termed 
the  ambiguous  layer.  The  apical  proc- 
ess of  the  pyramidal  cells  is  broad  at 
the  base,  but  narrows  rapidly  as  it 
passes  upward.  It  frequently  divides 
into  several  branches,  each  of  which 
develops  club-shaped  processes  or  gem- 
mules,  which  give  to  it  a  feathery  ap- 
pearance. Dendrites  are  also  given 
off  from  the  sides  and  base  of  the  cell- 
body.  From  the  base  a  single  axon 
descends  which  ultimately  becomes  the  axis-cylinder  of  a  medul- 
la u-d  nerve. 
The  Layer  of  Large  Pyramidal  Cells. — The  nerve-cells  of  this 
layer,  as  the  name  implies,  are  also  pyramidal  in  shape,  but  of 
large  size.     Each  cell  presents  the  same  features  as  the  cells  of  the 


Fig.  248. — Section  of  the 
Cerebral  Cortex  (Motor  Area) 
of  Child,  Stained  by  Golgi's 
Silver  Method.  A.  Layer  of 
neuroglia  cells.  B.  Layer  of  small 
pyramidal  ganglion  cells.  C. 
Layer  of  large  pyramidal  cells.  D. 
Layer  of  irregular  smaller  cells. — 
(Pier  sol.) 


THE  CEREBRUM.  543 

preceding  layer,  with  the  exception  that  the  apical  process  is 
larger,  better  developed,  and  branches  more  freely.     All  the  den- 
drites  are   extensively  provided   with  gemmules.     The   axon  is 
well  developed,  sharply  defined,  and  smooth.     After  giving  off 
collateral  branches,  the  axon  descends  into  the  cerebrum  and 
becomes  a  medullated  nerve-fiber. 
4.   The  Layer   of   Polymorphous  Cells. — In  this  layer  the  nerve-cells 
present  a  variety  of  forms:     e.  g.,  spindle,  polygonal,  pyramidal, 
etc.     The  spindle  form  is  the  most  common.     From  either  end 
of  the  spindle  a  large  dendrite  emerges  which  soon  branches  and 
becomes  gemmulated.     The  axon  is  well  defined  and  it  soon  de- 
scends into  the  white  matter. 
The  Number  of  Cortical  Cells. — Attempts  have  been  made  by 
various  histologists  to  estimate  the  total  number  of  functional  nerve- 
cells  in  the  cerebral  cortex  of  man.     Though  the  estimates  are  widely 
different,   the  lowest  presents  numbers  which  are  beyond   compre- 
hension.    Thus,    Meynert's    estimate    is    612    millions;    Donaldson's 
1200  millions;  while  Thompson's  is  9200  millions. 

Structure  of  the  White  Matter. — The  white  matter  of  the 
cerebrum  consists  of  medullated  nerve-fibers  which,  though  intri- 
cately arranged,  may  be  divided  into  three  systems:  viz.,  the  com- 
missural, the  association,  and  the  projection. 

1.  The  commissural  system.     The  fibers  which  compose  this  system 

unite  corresponding  areas  of  the  cortex  of  each  hemisphere. 
The  fibers  from  the  frontal,  parietal,  and  occipital  lobes  cross 
in  the  median  line  and  form  a  band  of  transversely  arranged 
fibers,  the  corpus  callosum.  The  fibers  which  unite  the  corre- 
sponding areas  of  the  temporo-sphenoidal  lobes  cross  in  the  an- 
terior commissure.  All  the  commissural  fibers  are  the  axons  of 
nerve-cells,  in  the  cortex,  the  terminals  of  which  are  to  be  found 
in  the  cortex  of  the  opposite  side. 

2.  The   association   system.      The  fibers  which  compose  this  system 

unite  neighboring  as  well  as  distant  parts  of  the  same  hemisphere, 
and  may  therefore  be  divided  into  long  and  short  fibers.  They 
associate  the  inexcitable  or  association  areas  with  the  excitable 
or  projection  areas. 

3.  The  projection  system.      The  fibers  composing  this   system   unite 

certain  areas  of  the  cortex  of  the  cerebrum  with  the  basal  gan- 
glia, the  pons,  medulla  oblongata,  and  spinal  cord.  They  may 
be  divided  into:  (1)  afferent  fibers  which  have  their  origin  in 
the  lower  nerve-centers  at  different  levels  and  thence  pass  to 
the  cortex;  and  (2)  efferent  fibers  which  have  their  origin  in  the 
cortex  and  thence  pass  to  the  lower  nerve-centers,  terminating 
at  different  levels.  The  former  are  also  termed  the  cortico- 
afferent  or  cortico- petal;  the  latter,  cortico-efjercnt  or  cortico-fugal. 
The  afferent  fibers,  the  so-called  sensor  tract,  which  transmit 
nerve  impulses  coming  from  the  general  periphery  and   the   sense- 


544  TEXT-BOOK  OF  PHYSIOLOGY. 

organs,  -pass  through  the  tegmentum  as  the  mesial  and  lateral  fillets, 
and  thence  to  the  cortex  directly  by  way  of  the  internal  capsule,  or 
indirectly  through  the  intermediation  of  the  thalamic  and  subthalamic 
nuclei.  The  distribution  of  these  fibers  to  the  various  areas  of  the 
cortex  will  be  found  in  following  paragraphs. 

The  efferent  fibers  of  the  so-called  motor  tract  which    transmit 
motor  or  volitional  nerve  impulses  from  the  cortex  to  the  pons,  medulla, 
and  spinal  cord,  emerge  from  the  layer  of  pyramidal  cells  of  the  gray 
matter  of  the  anterior  or  the  pre-central  convolution,  the  paracentral 
lobule  and  immediately  adjacent  areas.     From  this  origin  the  axons 
descend  through  the  white  matter  of  the  corona  radiata,  converging 
toward  the  internal  capsule,  into  and  through  which  they  pass,  occupy- 
ing the  anterior  two-thirds  of  the  posterior  limb  or  segment.     Be- 
yond the  capsule  they  continue  to  descend,  occupying  the  middle 
three-fifths  of  the  pes  or  crusta  of  the  crus  cerebri,  the  ventral  portion 
of  the  pons,  and  eventually  the  anterior  pyramid  of  the  medulla  oblon- 
gata.    At  this  point  the  tract  divides  into  two  portions,  viz.: 
i.  A  large  portion,   containing  from  ninety-one  to  ninety-seven  per 
cent,  of  the  fibers,  which  decussates  at  the  lower  border  of  the 
medulla  and  passes  down  the  lateral  column  of  the  cord,  con- 
stituting the  crossed  pyramidal  tract. 
2.  A   small  portion,   containing  from  three  to  nine  per  cent,  of  the 
fibers,    which   does   not   decussate   at   the   medulla,   but   passes 
down  the  inner  side  of  the  anterior  column  of  the  same  side, 
constituting  the  direct  pyramidal  tract  or  column  of  Tiirck. 
After  passing  through  the  internal  capsule,  and  as  it  descends 
through  the  crus,  pons,  and  medulla,  the  cortico-efferent  tract  gives 
off  a  number  of  fibers  which  cross  the  median  line  and  arborize  around 
the  nerve-cells  in  the  gray  matter  beneath  the  aqueduct  of  Sylvius 
(the  nuclei  of  origin  of  the  third  and  fourth  cranial  nerves),  and  around 
the  nerve-cells  in  the  gray  matter  beneath  the  floor  of  the  fourth  ven- 
tricle (the  nuclei  of  origin  of  the  remainder  of  the  motor  cranial  nerves). 
The  remaining  fibers  go  to  form  the  crossed  and  direct  pyramidal  tracts 
and  arborize  around  the  cells  in  the  anterior  horn  of  the  gray  matter 
of  the  opposite  side  of  the  cord  at  successive  levels.     By  this  means 
the  cortex  is  brought  into  anatomic  and  physiologic  relation  with  the 
general   musculature  of  the  body  through  the  various  cranial   and 
spinal  motor  nerves.     (See  Fig.  236,  page  517.) 

The  fronlo-cerebellar  and  the  occipito-lemporo-cerebellar  tracts 
are  also  efferent  tracts  and  parts  of  the  projection  system.  The 
fronto-cerebellar,  originating  in  the  nerve-cells  of  the  cortex  of  the 
frontal  lobe,  passes  down  to  and  through  the  internal  capsule,  occupy- 
ing the  anterior  one-third  of  the  anterior  segment.  It  then  descends 
;ilon,^  the  inner  side  of  the  crus  cerebri  to  the  pons,  where  its  fibers 
arborize  around  the  cells  of  the  nucleus  pontis.  Through  the  inter- 
im flialion  of  these  cells  this  tract  is  brought  into  relation  with  the 
cerebellum  of  the  same  but  chiefly  of  the  opposite  side.     The  occipito- 


THE  CEREBRUM.  545 

temporal  tract,  originating  in  the  cells  of  the  cortex  of  both  the  occip- 
ital and  temporal  lobes,  passes  downward  and  inward  toward  the 
lenticular  nucleus,  beneath  which  it  passes  to  enter  the  outer  one-fifth 
of  the  crusta.  It  then  enters  the  pons,  and  through  the  nucleus  pontis 
also  comes  into  relation  with  the  cerebellum  of  both  sides.     (See  Fig. 

243>  Page  529-) 

THE  FUNCTIONS  OF  THE  CEREBRUM. 

The  functions  of  the  cerebrum  comprehend,  in  man  at  least, 
all  that  pertains  to  sensation,  cognition,  feeling,  and  volition.  All 
subjective  experiences,  which  in  their  totality  constitute  mind,  are 
dependent  on  and  associated  with  the  anatomic  integrity  and  the 
physiologic  activity  of  the  cerebrum  and  its  related  sense-organs,  the 
eye,  ear,  nose,  tongue,  etc. 

From  an  examination  of  the  anatomic  development  of  the  brain 
in  different  classes  of  animals,  in  different  men  and  races  of  men, 
and  from  a  study  of  the  pathologic  lesions  and  the  results  of  experi- 
mental lesions  of  the  brain,  evidence  has  been  obtained  which  reveals 
in  a  striking  manner  the  intimate  connection  of  the  cerebrum  and  all 
phases  of  mental  activity. 

1.  Comparative  anatomic  investigations  show  that  there  is  a  general 

connection  between  the  size  of  the  brain,  its  texture,  the  depth 
and  number  of  convolutions,  and  the  exhibition  of  mental  power. 
Throughout  the  entire  animal  series  an  increase  in  intelligence 
goes  hand  in  hand  with  an  increase  in  the  development  of  the 
brain.  In  man  there  is  an  enormous  increase  in  size  over  that 
of  the  highest  animals,  the  anthropoid  apes.  The  most  culti- 
vated races  of  men  have  the  greatest  cranial  capacity,  that  of  the 
educated  European  or  American  being  approximately  92.1  cubic 
inches  (1835  c.c);  while  that  of  the  Australian  is  but  81.7  cubic 
inches  (1628  c.c).  '  Men  distinguished  for  great  mental  power 
usually  have  large  and  well-developed  brains;  e.  g.,  that  of  Cuvier 
weighed  64.4  ounces  (1830  grams);  that  of  Abercrombie,  63 
ounces  (1786  grams).  A  large  intelligence,  however,  is  not  incom- 
patible with  a  much  smaller  brain  weight;  thus,  the  brain  of 
Helmholtz  weighed  but  50.8  ounces  (1440  grams) ;  that  of  Leidy, 
49.9  ounces  (1415  grams);  that  of  Liebig,  47.7  ounces  (1352 
grams).  The  average  arithmetic  brain  weight  of  96  distinguished 
men  was  found  to  be  51.9  ounces  (1473  grams)  (Spitzka). 

2.  Pathologic    IcsioJis    and   mechanic  injuries  which  disorganize   the 

cerebrum  are  at  once  followed  by  a  disturbance  or  an  entire 
suspension  of  mental  activity.  -  Concussion  of  the  brain  or 
sudden  compression  from  a  hemorrhage  destroys  consciousness. 
Physical  and  chemic  alterations  of  the  gray  matter  of  the  cere- 
brum have  been  shown  to  coexist  with  insanity,  loss  of  memory, 
of  articulate  speech,  etc.  Congenital  defects  of  organization  are 
accompanied  by  a  deficiency  in  mental  capacity  and  the  higher 
35 


546  TEXT-BOOK  OF  PHYSIOLOGY. 

instincts.  Under  such  circumstances  no  great  advance  in  brain 
development  is  possible  and  the  intelligence  remains  at  a  low 
level.  In  congenital  idiocy  the  brain  is  small,  imperfectly  devel- 
oped, and  wanting  in  proper  chemic  composition. 

3.  Experimental  lesions  of  the  brain  in  lower  animals  are  attended 

by  results  similar  to  those  observed  in  disease  or  after  injury 
in  man.  Removal  of  the  cerebrum  in  the  pigeon  completely 
abolishes  intelligence  and  destroys  the  capability  of  performing 
volitional  movements.  The  pigeon  remains  in  a  state  of  pro- 
found stupor,  though  retaining  the  capability  of  executing  reflex 
or  instinctive  movements.  It  can  temporarily  be  aroused  by  loud 
noises,  light  placed  before  the  eyes,  pinching  of  the  toes,  etc., 
but  it  soon  relapses  into  a  condition  of  quietude.  Coincident 
with  the  destruction  of  the  cerebrum  there  occurs  a  loss  of  mem- 
ory, reason,  and  judgment,  and  the  animal  fails  to  associate  the 
impressions  with  any  previous  train  of  ideas.  The  higher  the 
animal  in  the  scale  of  development,  the  more  striking  is  the  loss 
of  mentality  after  removal  of  the  cerebrum. 

4.  Experimental  interference  with  the  blood-supply  to  the  cerebrum 

is  followed  by  a  diminished  or  complete  cessation  of  its  activities. 
There  is  perhaps  no  organ  of  the  body  that  is  so  directly  depend- 
ent upon  its  blood-supply  for  the  continuance  of  its  activities 
as  the  cerebrum.  The  supply  of  blood  is  furnished  by  four  large 
blood-vessels:  viz.,  the  two  carotid  and  the  two  vertebral  arteries. 
These  vessels,  after  entering  the  cavity  of  the  skull,  give  off 
branches  which  unite  to  form  the  "circle  of  Willis."  From  this 
circle,  large  branches  are  given  off  which  enter  the  cerebrum 
and  distribute  blood  to  all  its  parts.  After  passing  through  the 
capillaries  the  blood,  greatly  altered  in  chemic  composition,  is 
returned  by  large  veins.  The  large  volume  of  blood  that  is 
present  in  the  brain  and  the  marked  changes  in  composition 
that  it  undergoes  while  passing  through  the  brain  indicate  a 
very  active  and  complex  metabolism  in  this  organ.  By  means 
of  the  anatomic  arrangement  of  the  blood-vessels  at  the  base 
of  the  brain,  the  blood-supply  is  equalized.  It  also  explains 
why,  when  one,  or  even  two,  of  the  four  large  vessels  are  oc- 
cluded by  pathologic  deposits  or  surgical  procedures,  brain 
activity  continues,  though  perhaps  diminished  in  degree.  Occlu- 
sion of  all  four  vessels,  however,  is  at  once  followed  by  a  complete 
abolition  of  all  forms  of  cerebral  activity.  An  experiment  per- 
'  formed  by  Brown-Sequard  illustrates  the  dependence  of  cerebral 
activity  on  the  blood-supply.  A  dog  was  beheaded  at  the  junction 
of  the  neck  and  chest.  After  a  period  of  ten  minutes  all  evi- 
dences of  life  had  entirely  ceased.  Four  tubes  connected  with  a 
reservoir  of  warm  defibrinated  blood  were  then  connected  with 
the  four  arteries  of  the  head.  By  means  of  a  pumping  apparatus 
imitating  the  action  of  the  heart  the  blood  was  driven  into  and 


THE  CEREBRUM.  547 

through  the  brain.     After  a  few  minutes  cerebral  activity  returned, 
as  shown  by  contraction  of  the  muscles  of  the  face  and  eyes.     The 
character  of  the  contractions  were  such  as  to  convey  the  idea 
that  they  were  directed  by  the  will.     These  vital  manifestations 
continued  for  a  period  of  fifteen  minutes,  when  on  the  cessation  of 
the  artificial  circulation  they  disappeared,  and  the  head  exhibited 
once  again   the   usual  phenomena  observed  in  dying:  viz.,  con- 
traction and  then  dilatation  of  the  pupils  and  convulsive  move- 
ments of  the  muscles  of  the  face. 
Localization  of   Functions  in  the   Cerebrum. — By  the  term 
localization  of   functions  is  meant  the  assignment  of   definite  phys- 
iologic functions  to  definite  anatomic  areas  of  the  cerebral  cortex. 
From  experiments  made  on  the  brains  of  animals,  by  the  observation 
and  association  of  clinical  symptoms  with  pathologic  lesions  of  the 
central  nerve  system,  and  from  observation  of  the  developmental  stages 
of  the  embryonic  brain,  it  has  been  established  in  recent  years: 

1.  That  the  general  and  special  sense-organs  of  the  body  are  associ- 

ated through  afferent  nerve-tracts  with  definite  though  perhaps  not 
sharply  delimited  areas  of  the  cerebral  cortex;  and — 

2.  That   certain   areas  of  the  cortex  are  associated  through  efferent 

nerve-tracts  with  special  groups  of  skeletal  or  voluntary  muscles. 

Experimental  excitation  of  a  cortical  area  associated  with  a  sense- 
organ  is  undoubtedly  attended  by  the  production  of  a  sensation  at 
least  similar  to  that  produced  by  peripheral  excitation  of  the  sense- 
organ  itself;  destruction  of  the  area  is  followed  by  an  abolition  of  all 
the  sensations  associated  with  the  sense-organ.  For  these  reasons 
such  areas  are  termed  sensor. 

Excitation  of  a  cortical  area  associated  with  a  group  of  skeletal 
muscles  is  attended  by  their  contraction;  destruction  of  the  area  is 
followed  by  their  relaxation  or  paralysis.  For  these  reasons  such  areas 
are  termed  motor. 

Since  the  sense-organs  are  remote  from  the  brain  and  the  impres- 
sions made  upon  them  by  the  objective  world  can  be  utilized  by  the 
mind,  only  after  they  have  been  reproduced  in  the  cortical  areas,  it 
may  be  said  that  each  sense-organ  has  its  special  area  in  the  cortex 
by  which  it  is  represented,  or,  in  other  words,  each  sense-organ  has 
a  cortical  area  of  representation. 

Since  the  muscles  are  remote  from  the  brain  and  since  they  contract 
in  response  to  the  discharge  of  nerve  impulses  from  the  cells  of  the 
cortical  motor  areas,  it  may  be  said  that  the  activities  of  the  motor  areas 
are  represented  by  the  contractions  of  the  muscles;  in  other  words,  that 
the  cortical  motor  areas  have  areas  of  representation  in  the  general 
skeletal  musculature.  It  is  usually  stated,  however,  in  the  reverse 
way:  viz.,  that  the  muscle  movements  have  areas  of  representation 
in  the  cortex. 

,The  cortex  of  the  cerebrum  may  therefore  be  compared  to  a  mosaic 
made  up,  partially  at  least,  of  sensor  and  motor  areas  which  respectively 


548  TEXT-BOOK  OF  PHYSIOLOGY. 

represent  sense-organs  and  motor  organs,  and  which  hear  a  definite 
anatomic  and  physiologic  relation  one  to  the  other.  Their  cooperation 
is  essential  to  the  normal  performance  of  all  forms  of  cerebral  activity. 

A  knowledge  of  the  situation  of  these  areas,  the  order  of  their  devel- 
opment, the  effects  that  arise  from  their  stimulation  or  follow  their 
destruction,  are  matters  of  the  highest  importance  in  the  study  of  cere- 
bral activity  and  indispensable  to  the  physician  in  the  localization  of 
lesions  which  manifest  themselves  in  perversions  or  abolition  of  sensa- 
tions and  in  convulsive  seizures  or  paralyses. 

The  Sensor  Areas. — The  sensor  areas  which  should  theoretically 
be  present  in  the  cortex  are  primarily  those  which  receive  and  translate 
into  conscious  sensations  nerve  impulses,  developed  by  changes  going 
on  in  the  body  itself;  and  secondarily  those  which  receive  and  translate 
into  conscious  sensations  the  nerve  impulses  developed  in  the  special 
sense-organs  by  the  impact  of  the  external  or  objective  world.  In  the 
former  areas,  are  received  the  nerve  impulses  that  come  from  the  skin, 
mucous  membranes,  muscles,  viscera,  etc.,  and  give  rise  to  cutaneous, 
muscle,  and  visceral  sensations.  In  the  latter  areas  are  received  the 
nerve  impulses  that  come  from  the  sense-organs  and  give  rise  to  tactile, 
gustatory,  olfactory,  auditory,  and  visual  sensations.  A  number  of  such 
sense  areas  may  be  predicated:  e.  g.,  areas  of  cutaneous  and  muscle 
sensibility,  of  gustatory,  olfactory,  auditory,  and  visual  sensibility. 

The  Motor  Areas. — The  motor  areas  which  should  theoretically 
be  present  in  the  cortex  are  those  which  in  consequence  of  the  dis- 
charge of  nerve  impulses  excite  contraction  of  special  groups  of  mus- 
cles and  which,  from  their  coordinate  and  purposive  character,  are 
conventionally  termed  volitional.  Five  such  general  motor  areas 
may  be  predicated:  e.  g.,  one  for  the  muscles  of  the  head  and  eyes, 
one  for  the  muscles  of  the  face  and  associated  organs,  and  others  for 
the  muscles  of  the  arm,  leg,  and  trunk.  They  are  usually  designated 
as  head  and  eye,  face,  arm,  leg,  and  trunk  motor  areas. 

The  existence  and  anatomic  location  of  these  areas  in  the  cortex 
of  animals  have  been  determined  by  the  employment  of  two  methods 
of  experimentation:  viz.,  stimulation  and  destruction  or  extirpation; 
the  first  by  means  of  the  rapidly  repeated  induced  electric  currents, 
the  second  by  the  electric  cautery  and  the  knife.  If  the  stimulation 
or  excitation  of  any  given  area  is  followed  by  contraction  and  its  de- 
struction by  paralysis  of  muscles,  it  is  assumed  that  the  area  is  motor 
in  function — is  a  center  of  motion.  If  the  stimulation  of  a  given  area 
is  attended  by  phenomena  which  indicate  that  the  animal  is  experienc- 
ing sensation,  and  its  destruction  by  a  loss  of  this  capability  or  the  loss 
of  a  special  sense,  it  is  assumed  that  the  area  is  sensor  in  function- 
is  an  area  of  special  sense.  The  animals  generally  employed  for  ex- 
periments of  tin's  character  are  dogs  and  monkeys,  though  other  animals 
have  frequently  been  employed  by  different  investigators.  Of  all 
animals,  the  monkey  is  the  mosl  frequently  selected,  as  the  configura- 
tion of   the  brain  in  its  general    outlines  more  closely  resembles  that 


THE  CEREBRUM. 


549 


of  man  than  does  the  brain  of  any  other  animal.  The  results  therefore 
which  are  obtained,  there  is  every  reason  to  believe,  are  the  results, 
in  their  general  outlines,  that  would  follow  stimulation  of  the  human 
brain  if  this  were  possible  under  the  same  conditions.  Indeed,  the 
clinic  symptoms  which  arise  during  the  development  of  pathologic 
processes,  and  the  phenomena  which  occur  during  surgic  procedures 
for  the  removal  of  growths  and  pathologic  cortical  areas,  justify  the 
conclusion  that  the  chart  of  the  motor  and  sensor  areas  of  the  monkev 
brain  may  be  transferred  to  the  human  brain  without  introducing  any 
serious  errors. 

The  Sensor  Areas  of  the  Monkey  Brain. — From  experiments 
made  on  the  brains  of  monkeys,  Ferrier,  Schafer,  Horsley,  and  many 
others  have  mapped  out,  though  not  with  a  high  degree  of  defmite- 


Fig.  249. — Diagram  of  the  Motor  and  Sensor  Areas  on  the  Lateral  Surface 
OF  the  Monkey  Brain. — {After  Horsley  and  Schafer.) 


ness  and  certainty,  the  sensor  areas,  stimulation  of  which  gives  rise  to 
sensation,  destruction  to  loss  of  sensation.  A  diagrammatic  repre- 
sensation  of  these  areas  is  shown  in  Fig.  249  and  Fig.  250. 

The  tactile  area  or  area  of  tactile  perception  has  not  been  accurately 
or  definitely  located.  Ferrier  assigned  it  to  the  hippocampal  region. 
Schafer  and  Horsley  assigned  it  to  the  limbic  lobe,  and  especially  to 
that  portion  known  as  the  gyrus  fornicatus,  as  destruction  of  this  con- 
volution was  followed  by  hemianesthesia  of  the  opposite  side  of  the 
body  which  was  more  or  less  marked  and  persistent.  These  observers 
conclude  that  the  limbic  lobe  "is  largely  if  not  exclusively  concerned 
in  the  appreciation  of  sensation,  painful  and  tactile."  Other  ex- 
perimenters question  this  conclusion  and  locate  the  area  near  to.  if  not 
within,  the  Rolandic  area.     The  difference  of  opinion  regarding  the 


55o  TEXT-BOOK  OF  PHYSIOLOGY. 

location  and  probable  limitation  of  the  area  of  tactile  sensibility  renders 
necessary  additional  and  more  conclusive  experiments. 

The  olfactory  and  gustatory  areas  or  areas  of  olfactory  and  gusta- 
tory perception  have  been  located  in  the  uncinate  gyrus  and  the  adjacent 
region,  though  their  exact  limits  have  not  been  determined  by  the  ex- 
periments thus  far  performed. 

The  auditory  area  or  area  of  auditory  perception  was  located  by 
Ferrier  in  the  upper  two-thirds  of  the  superior  temporo-sphenoidal 
convolution.  Bilateral  cauterization  of  this  region  gave  rise  to  com- 
plete deafness,  which  endured  to  the  time  of  the  animal's  death,  more 
than  a  vear  later.  Unilateral  destruction  of  this  region  gave  rise  to 
deafness  in  the  opposite  ear  only.     The  results  of  experiments  made 


Fig.  250. — Diagram  of  the  Motor  and  Sensor  Areas  on  the  Mesial  Surface 
of  the  Monkey  Brain. — (After  Horsley  and  Schajer.) 


subsequently  by  other  observers  would  indicate  that  the  auditory  area 
is  somewhat  more  extended  than  that  designated  by  Ferrier,  as  ap- 
parently animals  recovered  their  hearing,  to  some  extent  at  least,  after 
complete  recovery  from  the  operation.  The  limit  or  extension  of  the 
area  is,  however,  uncertain. 

The  visual  area  or  area  of  visual  perception  has  been  located  in 
the  occipital  lobe,  though  in  this,  as  in  the  previous  instances,  its  exact 
limits  have  not  been  positively  determined.  Experimenters  also  are 
not  in  accord  as  to  the  relative  functions  of  its  different  parts.  Ferrier 
located  this  area  in  the  occipital  lobe  and  that  adjacent  portion  of  the 
parietal  lobe  on  the  outer  surface  known  as  the  angular  gyrus.  He 
found  that  extirpation  of  the  angular  gyrus  alone  was  followed  by  a 
temporary  blindness  of  the  opposite  eye,  which  was,  however,  not 


THE  CEREBRUM.  551 

hemiopic  in  character.*  He  also  found  that  destruction  of  the  occip- 
ital lobe  together  with  the  angular  gyrus  gave  rise  to  a  more  or  less 
enduring  hemianopsia,  in  addition  to  the  transient  blindness  of  the 
opposite  eye.  From  these  and  similar  facts  he  concluded  that  the  an- 
gular gyrus  is  the  area  of  representation  for  the  macular  or  central 
.region  of  the  retina,  and  the  occipital  lobe  for  the  corresponding  halves 
of  the  peripheral  portions  of  the  retina. 

It  was,  however,  found  by  Munk,  Schaf  er,  and  others  that  the  angular 
gyrus  was  not  concerned  in  any  way  with  vision;  that  extirpation  of  the 
occipital  lobe  alone,  especially  if  the  line  of  division  be  carried  a  little 
further  forward  on  the  mesial  and  inferior  surfaces,  was  followed 
by  homonymous  hemiopia  (loss  of  retinal  function  on  the  same  side), 
and  therefore  homonymous  hemianopsia.  Additional  experiments 
lead  to  the  conclusion  that  the  area  for  macular  vision  is  near  the  an- 
terior extremity  of  the  calcarine  fissure,  while  the  area  for  peripheral 
vision  is  in  the  posterior  portion  of  the  mesial  surface  and  for  a  variable 
distance  on  the  outer  surface.  Moreover,  there  is  reason  to  believe  that 
the  area  for  macular  vision  is  in  relation  with  homonymous  halves  of 
the  two  maculae  luteae.  The  supposed  error,  the  assignment  of  macu- 
lar vision  to  the  angular  gyrus,  has  been  attributed  to  destruction  of 
the  fibers  of  the  optic  radiation,  which  in  their  course  to  the  occipital 
lobe  pass  close  to  this  gyrus. 

The  Motor  Areas  of  the  Monkey  Brain.— From  experiments 
made  on  the  brains  of  monkeys  Ferrier  mapped  out  a  number  of  areas 
stimulation  of  which  give  rise  to  muscle  contractions  on  the  opposite 
side  of  the  body  which  are  so  purposive  and  coordinate  in  character 
that  they  may  be  regarded  as  identical  with  those  produced  volitionally. 
Destruction  of  these  areas  is  followed  by  paralysis.  The  results  of 
Ferrier' s  earlier  work  are  represented  in  Fig.  251,  the  descriptive  text 
to  which  renders  them  intelligible.  In  a  general  way  it  may  be  said 
that  the  upper  third  of  the  anterior  and  posterior  central  convolutions 
presides  over  the  movements  of  the  leg  of  the  opposite  side  of  the  body; 

*  In  a  consideration  of  this  subject  certain  facts  connected  with  visual  perception,  both 
in  physiologic  and  pathologic  conditions,  must  be  kept  in  mind.  Thus,  visual  sensation 
may  arise  from  stimulation  of  either  the  central  portion,  the  macula,  or  the  peripheral 
portion  of  the  retina  or  both.  In  the  first  instance  the  vision  is  termed  central  or  macular; 
in  the  second  instance,  peripheral  or  retinal.  Macular  vision  is  clear,  sharp,  and  distinct; 
retinal  vision  progressively  indistinct  from  the  center  toward  the  periphery.  Division  of 
one  optic  tract  is  followed,  in  consequence  of  the  partial  decussation  of  the  optic  nerve- 
fibers  at  the  chiasma,  by  a  loss  of  function  in  the  outer  two-thirds  of  the  retina  of  the 
same  side,  both  in  the  central  (macular)  as  well  as  in  its  peripheral  portions,  and  the 
inner  one-third  of  the  retina  of  the  opposite  side.  To  this  condition  the  term  hemiopia 
has  been  applied.  As  a  result  of  this  want  of  functional  activity  of  these  retinal  portions 
on  the  side  of  the  lesion,  rays  of  light  emanating  from  objects  situated  in  the  opposite 
side  of  the  field  of  vision  will  not  be  perceived  when  both  eyes  are  directed  to  the  fixation 
point.  To  this  "  blindness  "  in  the  opposite  half  of  the  field  of  vision  the  name  hemianopsia 
is  given.  In  the  lesion  under  consideration  (division  of  one  optic  tract)  the  hemianopsia 
is  bilateral,  and  as  it  affects  the  corresponding  portions  associated  in  normal  vision  it  is  of 
the  homonymous  variety.  Division  of  the  right  optic  tract  is  followed  by  left  lateral  homon- 
ymous hemianopsia,  indicative  of  the  fact  that  objects  in  the  held  of  vision  to  the  Left  of 
the  binocular  fixation  point  are  invisible. 


55^ 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  middle  third  over  the  movements  of  the  arm;  the  inferior  third  over 
the  movements  of  the  face  and  tongue.  Collectively  these  areas  are 
known  as  the  motor  area  or  motor  zone;  and  as  it  is  ranged  along  the 
Rolandic  fissure,  it  is  sometimes  termed  the  Rolandic  area. 

The  experiments  of  Horsley  and  Schafer  added  additional  facts 
and  enabled  them  to  construct  a  new  diagrammatic  representation  of 

the  motor  area  and  more  ac- 
curately define  the  special 
areas  upon  the  lateral  and 
mesial  aspects  of  the  brain 
of  the  monkey.  The  bound- 
aries of  the  general  and 
special  areas,  as  determined 
by  these  observers,  will  be 
readily  apparent  from  an 
examination  of  Fig.  249. 
Their  experiments  have  en- 
abled them  also  to  subdivide 
the  general  into  special  areas 
as  follows: 

1.  The  head  area  or  area  for 
visual  direction  into 
areas  excitation  of  which 
causes  "opening  of  the 
eyes,  dilatation  of  the 
pupils  and  turning  the 
head  to  the  opposite 
side  with  conjugate  de- 
viation of  the  eyes  to 
that  side." 

2.  The  leg  area  maybe  sub- 
divided into  (a)  an  area 
both  on  the  lateral  and 
mesial  surfaces  which 
presides  over  the  move- 
ments of  the  hip  and 
thigh;  (b)  an  area  in  the 
posterior  part  which 
presides  over  the  move- 

ments of  the  legs  and 
toes;  (c)  an  area  in  the  paracentral  lobule  for  the  movements  of 
the  hallux  or  great  toe. 

3.  The  trunk  area,  situated  largely  on  the  mesial  surface,  may  be  sub- 

divided into  an  anterior  and  a  posterior  area,  which  respectively 
preside  over  the  movements  of  the  spinal  column  as  arching  and 
rotation,  and  the  movements  of  the  pelvis  and  tail. 

4.  The  arm  area  maybe  subdivided  as  follows:  (a)  an  area  superiorly 


Fig.  251.  —  Left  Hemisphere  of  Monkey, 
Showing  Details  of  Motor  Areas  Indicated 
by  the  Movements  Following  Stimulation  of: 
1.  Superior  parietal  lobule;  exciting  advance  of  the 
hind  limb.  2.  Top  of  ascending  frontal  and 
parietal  convolutions;  flexion  and  outward  rotation 
of  thigh;  flexion  of  toes.  3.  On  ascending  frontal 
convolution  near  semilunar  sulcus;  movements 
of  hind  limb,  tail  and  extremity  of  trunk.  4.  On 
adjacent  margins  of  ascending  frontal  and  parietal 
convolution;  adduction  and  extension  of  arm, 
pronation  of  hand.  5.  Top  of  ascending  frontal 
near  superior  frontal  convolution;  forward  extension 
of  arm.  a,  b,  c,  d.  On  ascending  parietal;  move- 
ments of  various  muscles  of  the  forearm.  6. 
Ascending  frontal  convolution;  flexion  of  forearm 
and  supination  of  hand  which  is  brought  toward 
mouth.  7.  Retraction  and  elevation  of  corner 
of  mouth.  8.  Elevation  of  nose  and  lip.  9  and  10. 
Opening  mouth  and  motions  of  tongue,  n.  Re- 
traction of  angle  of  mouth.  12.  Middle  and 
superior  frontal  convolutions;  movements  of  head 
and  eyelids.  13  and  13'.  Anterior  and  posterior 
Jim  I  is  of  angular  gyrus;  movements  of  eyeballs. 
14.  Superior  .temporo-sphenoidal  convolution,  ear 
pricked  and  head  moved.  15.  Movement  of  lip 
and  nostril. — (Ferrier.) 


THE  CEREBRUM.  553 

which  controls  the  movements  of  the  shoulder;  (b)  an  area  pos- 
teriorly and  below  this,  which  controls  the  movements  of  the  elbow; 
(c)  an   area   anteriorly  and   below  the  preceding,  governing  the 
movements  of  the  wrist  and  fingers;  (d)  an  area  posteriorly  and 
below  governing  the  movements  of  the  thumb. 
5.    The   face   area  may  be  divided  into  an  upper  part,  comprising 
about  one-third,  and  a  lower  part,  comprising  the  remaining  two- 
thirds.     In  the  upper  part  are  areas  governing  the  movements 
of  the  opposite  angle  of  the  mouth  and  of  the  lower  face.     In  the 
lower  part   anteriorly  there  is  an  area  governing  the  movements 
of  the  vocal  membranes  or  bands  (the  laryngeal  area) ;  posteriorly 
areas  governing  the  opening  and  closing  of  the  mouth,  the  protru- 
sion and  retraction  of  the  tongue. 
Electric  stimulation  of  the  sensor  areas  is  attended  by  certain 
motor  reactions  which  vary  in  accordance  with  the  area  stimulated. 
Thus,  when  the  electrodes  are  applied  to  different  portions  of  the 
occipital  lobe  the  eyeballs  are  conjugately  turned  upward,  downward, 
or  laterally  and  to  the  opposite  side;  when  placed  on  the  upper  por- 
tion of  the  superior  temporal  convolution,  the  ear  is  pricked  up  or 
retracted,  the  head  is  turned  to  the  opposite  side  and  the  pupils  are 
dilated;  when  placed  on  the  hippocampal  convolution,  there  is  move- 
ment of  torsion  of  the  nostril  and  lips  of  the  same  side. 

Ferrier  assumed  that  these  movements  were  the  result  of  the  origin- 
ation of  subjective  sensations  and  not  an  evidencet  that  the  area  in 
question  is  a  motor  area,  in  the  sense  that  this  term  is  applied  to  the 
areas  of  the  Rolandic  region,  especially  as  their  destruction  is  not 
followed  by  paralysis  of  any  of  the  corresponding  muscles.  This 
interpretation  is  supported  by  the  experiments  of  Schafer,  which  showed 
that  the  contraction  of  the  eye-muscles  which  followed  stimulation  of 
the  occipital  lobe  took  place  between  0.2  and  0.3  second  later  than 
when  the  frontal  lobe  was  stimulated;  and  that  as  the  motor  reaction 
takes  place  after  extirpation  of  the  frontal  region,  the  route  of  the 
efferent  impulse  cannot  be  to  and  through  the  frontal  lobe,  but  prob- 
ably through  some  lower  center.  The  same  facts  hold  true  for  the 
reactions  of  the  ear-muscles  following  stimulation  of  the  temporal 
lobe. 

The  view  that  the  cortex  of  the  cerebrum  can  be  divided  into 
separate  and  independent  though  physiologically  related  motor  and 
sensor  areas  has  been  questioned  in  recent  years,  and  a  somewhat 
different  interpretation  given  to  the  facts.  It  is  believed  by  many 
physiologists  and  neurologists  that  the  so-called  motor  and  sensor  areas 
are  so  closely  related  that  it  is  almost  impossible  to  distinguish  one 
from  the  other  either  anatomically  or  physiologically.  Thus  the  Rolandic 
region  is  believed  to  be  both  motor  and  sensor  in  function,  the  former, 
however,  being  more  predominant  in  the  pre-central,  the  latter  in  the 
post-central,  convolution.  As  these  two  functions  are  so  intimately 
blended  and  their  anatomic  substrata  so  difficult  of  separation,  it   is 


554  TEXT-BOOK  OF  PHYSIOLOGY. 

thought  the  term  sensori-motor  should  be  employed  as  more  descriptive 
and  more  in  accordance  with  the  facts  to  the  entire  Rolandic  region. 

This  view  has  been  strengthened  by  the  results  of  the  embryologic 
investigation  of  Flechsig,  which  show  that  different  nerve-tracts  be- 
come medullated  or  receive  their  myelin  investment  at  successively 
later  periods  and  that  the  tracts  which  first  become  myelinated  and 
are  hence  first  functionally  active,  belong  to  the  afferent  system. 
Among  the  first  to  undergo  myelinization  are  three  tracts  numbered 
by  Flechsig  i,  2  and  3,  which  arise  largely  from  the  median  nucleus 
of  the  thalamus  and  the  medial  lemniscus  and  pass  to  the  anterior 
and  posterior  convolutions,  to  the  para-central  lobule  and  foot  of 
the  superior  frontal  convolution,  and  to  the  foot  of  the  third  frontal 
convolution  respectively.  It  is  these  fibers  which  convey  nerve  im- 
pulses to  the  cortex  and  furnish  information  regarding  changes  taking 
place  in  the  body  itself  and  thus  lead  to  the  performance  of  muscle 
movements.  This  area  is  therefore  primarily  a  sensor  area,  an  area 
for  body-feelings,  cutaneous,  tactile,  muscle,  and  visceral,  and  second- 
arily a  motor  area.  The  afferent  fibers  to  this  region  become  myel- 
inated during  the  ninth  month  of  intra-uterine  life,  the  efferent  fibers 
from  it  become  mvelinated  during  the  third  month  of  extra-uterine 
life. 

By  the  same  method  of  reasoning  the  gustatory,  olfactory,  audi- 
tory, and  visual  sense  areas  are  to  be  regarded  as  sensori-motor  in 
character,  for  embryologic  investigations  show  that  subsequently 
to  the  myelinization  of  the  afferent  tracts  connecting  the  sense-organs 
with  the  cortex,  efferent  nerve-tracts  arise  from  or  near  to  the  same 
centers  and  undergo  myelinization.  In  other  words,  these  areas  are 
primarily  sensor  and  secondarily  motor,  and  therefore  should  be  termed 
sensori-motor.  In  Flechsig's  own  terminology  each  cortico-petal  or 
afferent  tract  is  accompanied  by  a  cortico-fugal  or  efferent  tract. 

In  this  view  sensations,  or  the  mental  processes  the  outcome  of 
sensations,  are  the  immediate  cause  of  the  movements  of  the  muscles 
connected  with  both  the  sense-organs  and  skeletal  structures.  Though 
this  interpretation — viz.,  the  coincidence  of  sensor  and  motor  areas — ap- 
pears more  in  accordance  with  the  facts  than  the  earlier  view,  it  must  be 
admitted  that  there  are  many  facts  both  of  a  physiologic  and  pathologic 
character  which  it  is  difficult  to  harmonize  with  it. 

The  Motor  Area  of  the  Chimpanzee  Brain. — In  a  series  of 
experiments  made  by  Sherrington  and  Grunbaum  on  the  brain  of  the 
chimpanzee  it  was  discovered  that  the  so-called  motor  area  was  not 
so  widely  distributed  as  in  the  monkeys  generally,  but  was  confined 
almost  exclusively  to  the  convolution  just  in  front  of  the  fissure  of 
Rolando,  as  it  was  impossible  to  obtain  any  movement  on  direct  stim- 
ulation of  the  convolution  just  behind  it.  All  points  on  the  surface  of 
the  pre-central  convolution,  including  the  portion  forming  the  wall 
of  the  Rolandic  fissure  itself,  were  found  to  be  excitable  and  productive 
of  movement  when  stimulated.     The  sequence  of  representation  from 


THE  CEREBRUM. 


555 


below  upward  is  similar  to  that  observed  in  the  monkey.  One  pecul- 
iarity, however,  was  the  location  of  the  area  for  conjugate  deviation 
of  the  eyeballs  to  the  opposite  side.  This  is  situated  far  forward  in  the 
middle  and  inferior  frontal  convolutions,  and  separated  from  the-  areas 
in  the  pre-central  convolution  by  a  region  apparently  inexcitable. 
These  facts  are  of  great  interest  and  value  in  the  assignment  of  the  motor 
areas  in  the  cortex  of  the  human  brain,  as  in  its  development  and  con- 
figuration the  chimpanzee  brain  more  closely  resembles  the  human 
brain  than  does  the  monkey's. 

The  Localization  of  Sensor  and  Motor  Areas  in  the  Human 
Brain. — The  observation  of  clinical  symptoms  and  their  interpreta- 
tion by  postmortem  findings,  the  phenomena  observed  during  surgical 


CONCRETE   CONCEPT 


Fig.  252. — The  Areas  and  Centers  of  the  Lateral  Aspect  of  the  Human  Hemi- 
cerebrum. — (C.  K.  Mills.) 


procedures,  and  the  results  of  embryologic  investigations,  point  to  the 
conclusion  that  corresponding  areas  both  for  sensations  and  move- 
ments exist  in  the  cerebral  cortex  of  the  human  brain,  though  it  is 
probable  that  their  locations  do  not  in  all  respects  coincide  with 
those  characteristic  of  the  monkey  or  even  the  ape  brain.  In  the  fol- 
lowing diagrams  (Figs.  252  and  253),  the  sensor  and  motor  areas  are 
at  least  provisionally  located,  in  accordance  with  recent  observations. 
They  are  represented  as  limited  or  bounded  by  a  serrated  line  to  in- 
dicate, as  suggested  by  Mills,  that  they  are  not  sharply  defined,  but 
that  they  interfuse  or  interdigitate  with  surrounding  regions. 

The  Sensor  Areas. — The  sensor  areas  occupy  regions  correspond- 
ing in  a  general  way  with  those  of  the  monkev  brain. 


556 


TEXT-BOOK  OF  PHYSIOLOGY, 


The  cutaneous  and  muscle  sense  areas  have  been  assigned  to  the 
post-central,  a  portion  of  the  super  and  sub-parietal  convolutions 
on  the  lateral  aspect,  and  to  portions  of  the  frontal  convolution  and 
of  the  callosal  convolution  on  the  mesial  aspect.  It  is  also  probable 
that  the  tactile  (cutaneous)  area  may  be  assigned,  though  in  less 
degree,  to  the  pre-central  convolution,  the  general  motor  area.  This 
is  in  accordance  with  the  embryologic  investigations  of  Flechsig,  who 
concludes  that  the  entire  Rolandic  region  is  to  be  regarded  as  sensor 
as  well  as  motor  in  function,  and  names  it  the  area  of  body  feelings, 
or  the  somesthetic  area. 

The  clinic  and  postmortem  evidence  as  to  the  extent  of  the  area 
of  tactile  sensibility  and  its  coincidence  with  the  motor  area  is  sorae- 


Fig.  253. — The  Areas  and  Centers  of  the  Mesial  Aspect  of  the  Human  Hemi- 
cerebrum. — (C.  K.  Mills.) 


what  contradictory,  and  in  some  respects  apparently  in  opposition 
to  the  view  of  Flechsig.  Thus,  Dr.  C.  K.  Mills,  whose  skill  in  inter- 
preting the  phenomena  of  disease  is  well  known,  states  in  this  con 
nection  in  his  work  on  nervous  diseases  that  "innumerable  cases  have 
been  reported  of  lesions  of  the  motor  cortex  without  the  slightest  im- 
pairment of  sensibility."  In  several  cases  of  excision  of  the  human 
cortex  in  the  Rolandic  region  by  surgical  operations  careful  studies 
of  the  patients  failed  to  show  any  impairment  of  sensation.  Other 
competent  observers,  however,  have  reported  a  number  of  cases  in 
lii<  h  anesthesia  more  or  less  pronounced  and  persistent  has  accom- 
panied lesions  of  the  motor  area.  The  explanation  of  these  contra- 
dictory observations  is  not  apparent. 

The  olfactory  area  has  been  assigned  to  the  uncinate  convolution, 


THE  CEREBRUM.  557 

the  anterior  part  of  the  callosal  convolution,  and  the  posterior  part  of  the 
base  of  the  frontal  lobe.  Lesions  in  this  region  are  frequently  accom- 
panied by  subjective  olfactory  sensations. 

The  gustatory  area  has  been  assigned  to  the  collateral  convolu- 
tion. 

The  auditory  area  has  been  assigned  to  the  posterior  portion  of  the 
super  temporal  convolution  and  to  the  retro-insular  convolutions, 
the  island  of  Reil.  Unilateral  destruction  of  this  region  is  followed 
by  only  a  partial  loss  of  hearing  in  the  opposite  ear  (owing  to  the  par- 
tial decussation  of  the  cochlear  nerve),  which,  however,  may  be  re- 
covered from  after  a  time,  owing  probably  to  a  compensatory  activity 
of  the  insular  convolutions.  Bilateral  disease  of  this  region  is  followed 
by  complete  deafness.  Within  this  area  there  is  a  smaller  region, 
disease  of  which  is  accompanied  by  word-deafness  only,  the  patient 
being  unable  to  distinguish  the  tone  intervals  between  words  and  syl- 
lables and  therefore  hearing  only  confused  noises.  Object  hearing 
has  also  a  separate  area  of  representation. 

The  visual  area  has  been  assigned  to  a  triangular  shaped  area  on 
the  mesal  surface  of  the  occipital  lobe,  which  includes  the  gray  matter 
above  and  below  the  calcarine  fissure  (the  cuneus  and  upper  part  of 
the  lingual  lobe),  and  to  the  gray  matter  of  the  first  occipital  convolution 
on  the  lateral  aspect  of  the  occipital  lobe.  Focal  lesions  of  this  area 
on  one  side  are  followed  by  lateral  homonymous  hemianopsia,  which, 
however,  does  not  involve,  as  a  rule,  the  fovea  or  macula.  It  is,  there- 
fore, the  area  of  homonymous  half-retinal  representation.  The  loca- 
tion of  the  area  for  macular  or  central  vision  is  uncertain.  Henschen 
locates  it  in  the  anterior  part  of  the  area  near  the  extremity  of  the  cal- 
carine fissure,  and  asserts  that  in  each  area  both  maculse  are  represented. 
From  experiments  made  on  monkeys  Schafer  locates  it  in  the  same 
region.  Beyond  the  limits  of  this  visual  area  and  on  the  lateral  aspect 
of  the  parietal  lobe  there  is  a  region  (the  supra-marginal  convolution 
and  angular  gyrus)  in  which  impressions  of  words  and  letters  seen  have 
their  representation.  Destruction  of  this  area  by  diseases  is  followed 
by  word-  and  perhaps  letter-blindness,  the  patient  being  unable  to 
recognize  words  and  letters  seen  because  of  failure  to  revive  the  mem- 
ory images  of  words  and  letters.  The  areas  for  visual  sensations  and 
optic  memory  pictures  are  therefore  separate,  a  fact  which  has  led  to 
a  division  of  the  visual  area  into  a  lower  and  a  higher  area. 

It  was  stated  in  a  previous  paragraph  that  electric  stimulation  of 
the  sensor  areas  of  the  monkey  brain  is  attended  by  certain  motor 
reactions  which  vary  with  the  area  stimulated.  Corresponding  areas 
are  believed  to  be  present  in  the  human  brain  and  that  their  stimula- 
tion would  be  followed  by  similar  motor  reactions.  Their  location  is 
shown  in  Figs.  252  and  253,  and  named  visual,  auditory,  olfactory, 
and  gustatory  motor. 

The  stereo  gnostic  area  or  area  of  stereo  gnostic  perception,  by  which 
objects  are  recognized  through  their  form  independent  of  vision  and 


558 


TEXT-BOOK  OF  PHYSIOLOGY. 


by  the  sense  of  touch  alone,  has  been  located  in  the  super  parietal 
convolution  and  the  precuneus  (Mills).  The  existence  of  such  an 
area  is  rendered  probable  by  the  fact  that  cases  have  been  recorded 
in  which  there  was  a  loss  of  this  power  (astereognosis)  uaccompanied 
by  either  sensor  or  motor  disturbances.  Postmortem  investigations 
showed  that  in  these  cases  there  was  a  destruction  of  the  superior 
parietal  convolution. 

Equilibratory,  intonation,  and  orientation  areas  have  been  pro- 
visionally located  in  the  sphenotemporal  lobe. 

The  Motor  Area. — The  general  motor  area  (Fig.  254)  is  represented 
as  occupying  the  pre-central  convolution,  the  base  of  the  super-frontal 
convolution,  both  on  its  lateral  and  mesial  aspects,  and  the  paracentral 


Fig.  254. — Scheme  of  the  Motor  Area  of  the  Human  Beain  and  its  Subdivisions. 
—(Ajter  Mills.) 


lobule.  The  exclusion  of  the  post-central  convolution  from  the  motor 
area  is  in  accordance  with  the  embryologic  researches  of  Flechsig,  which 
indicate  that  the  efferent  fibers  which  compose  the  pyramidal  tract  come 
from  the  region  anterior  to  the  central  fissure,  and  with  the  experiments 
of  Sherrington  and  Griinbaum  on  the  brain  of  the  chimpanzee,  which 
demonstrate  that  the  post-central  convolution  is  absolutely  inexcitable 
to  electric  stimulation.  It  is  quite  probable  that  with  the  growth  of  the 
brain  in  size  and  complexity,  the  motor  area  has  come  to  occupy  a  posi- 
tion somewhat  farther  forward  in  the  human  brain  than  in  the  monkey 
brain. 

This  general  area  is  also  capable  of  subdivision  into^areas  of  vari- 
able size,  in  which  the  movements  of  the  face  and  associated  structures, 


THE  CEREBRUM.  559 

the  head  and  eyes,  the  arm,  trunk,  and  leg,  are  represented.  (Fig. 
254.)  The  sequence  of  their  representation  from  below  upward  is 
similar  to  that  observed  in  the  monkey  and  chimpanzee.  In  each  of 
these  five  main  areas  there  are  yet  smaller  areas  in  which  the  move- 
ments of  localized  regions  of  the  body  are  in  part  represented  and 
which  are  indicated  in  diagram  (Fig.  254)  by  corresponding  words. 
The  words  in  the  areas  marked,  eyes  and  head,  face,  arm,  trunk,  and 
leg,  indicate  the  location  of  nerve-cells  which  through  the  discharge 
of  nerve  impulses  excite  to  contraction  the  muscles,  which  impart  to 
the  regions  indicated  by  these  words,  their  characteristic  movements. 
A  localized  irritative  lesion  of  any  one  of  these  areas  gives  rise  to 
convulsive  movements  of  the  muscles  of  the  opposite  side  of  the  body, 
similar  in  character  to  those  resulting  from  electric  stimulation  of  the 
corresponding  areas  of  the  monkey  and  ape  brains.  Destruction  of 
these  areas  from  the  growth  of  tumors,  softening,  etc.,  is  followed  by 
paralysis  of  the  muscles.  Electric  stimulation  of  these  areas  of  the 
human  brain  for  the  purpose  of  localizing  obscure  irritative  lesions 
prior  to  surgical  procedures  on  the  brain  gives  rise  to  the  same  con- 
vulsive movements. 

Language. — The  succession  of  motor  acts  by  which  ideas  are 
expressed,  is  known  as  language,  which  may  be  divided  into  (1)  artic- 
ulate or  spoken,  and  (2)  written. 

The  expression  of  ideas  both  by  words  and  signs  depends  primarily 
on  the  power  of  reviving  the  images  or  memories  of  words  and  letters 
heard  and  seen;  and  secondarily  on  the  power  of  reviving  the  images 
or  memories  of  the  muscle  movements  which  were  previously  em- 
ployed in  an  effort  to  imitate  or  reproduce  the  words  (speech)  or  the 
verbal  signs  (writing). 

Clinico-pathologic  investigations  have  shown  that  words  and 
letters  heard  and  seen  have  areas  of  representation  in  the  cortex,  the 
former  in  the  general  auditory  area,  the  latter  in  the  supra-marginal 
convolution  and  angular  gyrus  (Fig.  252).  Destruction  of  these 
areas  is  followed  by  word-deafness  and  word-blindness  respectively. 
The  same  methods  of  investigation  have  shown  that  the  muscle  move- 
ments employed  to  reproduce  the  words  and  the  verbal  signs  also  have 
areas  of  representation  in  the  cortex;  the  former  in  the  sub-frontal 
convolution  (Fig.  252),  and  probably  in  the  adjacent  region,  the  island 
of  Reil,  on  the  left  side  in  the  great  majority  of  people;  the  latter  in  front 
of  the  arm  region  of  the  general  motor  area.  Destruction  of  these 
areas  is  followed  in  the  first  instance  by  a  loss  of  the  power  of  executing 
the  movements  of  the  muscles  employed  in  speech,  and  in  the  second 
instance,  of  those  employed  in  writing. 

These  different  areas  are  connected  with  one  another  by  associa- 
tion fibers,  and,  taken  collectively,  constitute  the  language  zone  Their 
situation  and  relations  are  shown  in  Fig.  255.  In  this  figure  the 
dotted  lines  coming  from  the  ear  (a)  and  the  eye  (v)  represent 
the  auditory  and  visual  tracts  through  which  nerve  impulses   pass 


560 


TEXT-BOOK  OF  PHYSIOLOGY. 


to  the  auditory  (A)  and  the  visual  centers  (V)  respectively.  Similar 
lines  coming  from  the  muscles  involved  in  speech  and  writing  might 
also  be  represented  to  indicate  the  paths  of  the  nerve  impulses  to  the 
motor  speech  (M)  and  the  motor  writing  center  (E).  The  continuous 
lines  on  the  surface  of  the  cortex  represent  nerve-fibers  which  associate 

the  auditory  and  visual  centers 
with  the  speech  and  writing  cen- 
ters and  with  higher  psychic 
centers  (O  O)  as  well.  The 
dotted  lines  coming  from  the 
speech  and  writing  centers  repre- 
sent the  tracts  through  which 
nerve  impulses  pass  to  the  muscle 
of  the  larynx,  tongue,  mouth,  and 
lips,  and  to  the  muscles  of  the 
hand.  The  anatomic  and  physi- 
ologic association  of  the  various 
areas  is  essential  to  the  registra- 
tion of  the  impressions  made  on 
the  ear  and  eye  and  for  the  ex- 
pression of  the  ideas  evolved 
from  them  by  words  (speech) 
and  signs  (writing).  Their  col- 
lective action  is  essential  to  the 
acquisition  of  language.  De- 
struction of  any  part  of  this 
cerebral  mechanism  is  attended 
by  an  impairment  or  a  total  loss 
either  in  the  power  of  obtaining 
auditory  images  of  words  heard 
and  visual  images  of  words  seen, 
or  in  the  power  of  expressing 
ideas  by  speech  and  writing.  To 
this  pathologic  condition  the 
term  aphasia  has  been  given. 

Aphasia. — It  was  discovered 
by  Bouillaud  that  a  destructive 
lesion  of  the  third  frontal  convo- 
lution on  the  left  side  was  accom- 


Fig.  255. — Diagram  Showing  the  Re- 
lation of  the  Centers  op  Language  and 
their  Principal  Associations.  A.  Audi- 
tory center.  V.  Visual  center.  M.  Motor 
speech  center.  E.  Motor  writing  center. 
O    O.   Intellectual  center. — (After  Grasset.) 


panied  by  a  partial  or  complete  loss  of  the  faculty  of  articulate  speech, 
the  power  to  express  ideas  with  words.  To  this  condition  the  term 
aphasia  was  given.  Though  of  limited  application  ctymologically, 
the  word  is  now  em  ployed  in  a  wider  sense  to  signify  "partial  or  com- 
plete loss  of  the  power  of  expression  or  comprehension  of  the  con- 
ventional signs  of  language,"  words  cither  spoken  or  written,  due  to 
ie  ions  of  different  portions  of  the  cortex,  and  especially  on  the  left 
side. 


.THE  CEREBRUM.  561 

Aphasias  are  of  many  degrees  and  kinds,  though  they  may  be  in- 
cluded in  the  two  general  divisions,  motor  and  sensor. 

Motor  aphasia  may  be  either  ataxic  or  agraphic.  In  ataxic  aphasia 
the  patient  is  unable  to  express  or  communicate  his  thoughts  by  spoken 
words,  owing  to  an  inability  to  execute  those  movements  of  the  mouth, 
tongue,  etc.,  necessary  for  speech  without  there  being  any  paralysis 
of  these  muscles.  The  lesion  is  usually  in  the  third  frontal  convolution 
and  most  frequently  associated  with  right  hemiplegia.  In  agraphic 
aphasia  the  patient  is  unable  to  communicate  his  ideas  by  writing  through 
an  inability  to  execute  the  necessary  movements,  though  retaining  his 
mental  processes.  In  this  form  of  aphasia  the  lesion  is  in  the  writing 
area.  These  two  forms  of  motor  aphasia  are  not  infrequently  asso- 
ciated. 

Sensor  aphasia  or  amnesia  may  be  either  visual  or  auditory.  In 
visual  aphasia  or  amnesia  the  patient  is  unable  to  recognize  a  letter 
or  word,  printed  or  written  (though  capable  of  seeing  other  objects), 
a  condition  known  as  letter-  or  word-blindness.  It  is  usually  associated 
with  lesions  in  the  neighborhood  of  the  supra-marginal  convolution. 
In  auditory  aphasia  or  amnesia  the  patient  cannot  understand  articu- 
late or  vocal  speech,  though  capable  of  hearing  and  understanding  other 
sounds,  through  an  inability  to  distinguish  tone  intervals  of  words  and 
letters — a  condition  known  as  word-deafness.  It  is  associated  with 
lesions  of  the  auditory  area. 

Paraphasia  is  an  inability  to  recall  the  proper  words  to  associate 
with  ideas  and  necessary  to  their  expression. 

Concept  aphasia  is  the  inability  to  recall  the  names  of  objects. 
It  is  associated  with  lesions  of  the  cortex  of  the  mid-temporal  or  third 
temporal  convolution  (Mills).  This  area  is  known  as  the  concept  or 
naming  area. 

Bilateral  Representation. — Though  highly  specialized  move- 
ments, such  as  those  performed  by  the  arms  and  hands,  legs  and  feet, 
have  their  areas  of  representation  on  one  side  of  the  cerebrum  only, 
and  that,  opposite  to  the  side  of  the  movement,  less  highly  specialized 
movements,  such  as  the  masticatory,  phonatory,  respiratory  and 
various  trunkal  movements,  which  require  for  their  performance  the 
cooperation  of  muscles  on  both  sides  of  the  body,  have  their  areas  of 
representation  on  both  sides  of  the  cerebrum;  the  area  of  either  side 
exciting  to  action  the  muscles  on  both  sides  of  the  body.  In  the  case 
of  specialized  movements  the  representation  is  unilateral;  in  the  case 
of  the  more  general  movements  the  representation  is  bilateral. 

Association  Centers. — The  sensor  and  motor  areas  to  which 
specific  functions  have  been  assigned  do  not  constitute  more  than 
one-third  of  the  total  cerebral  cortex.  There  yet  remain  large  regions 
to  which  it  has  not  been  possible  to  assign  specific  functions  based  on 
physiologic  experiments.  Three  or  four  such  regions  separated  by 
the  sensor  and  motor  centers  are  to  be  recognized  on  the  lateral  and 
mesial  aspects  of  the  hemisphere.  In  Fig.  256  the  location,  extent, 
36 


,-6: 


TEXT-BOOK  OF  PHYSIOLOGY. 


and  names  of  these  regions  are  represented.  The  fibers  which  are 
found  in  these  regions  belong  almost  exclusively  to  the  association 
system,  and  become  medullated  at  a  later  period  than  do  the  fibers  of 
the  projection  system;  moreover,  from  the  method  of  their  medulliza- 


Motor  and  tactile  area. 


Parietal  association  area 


-*-^S§ 


w?mm* 


\  1  V      J  :&■?:} 


Frontal 

association 

area. 


Occipito-temporal 
association  area. 


Auditory  area. 


Motor  and  tactile  area. 


Parietal  association  area 
(Precuneus). 


Frontal 

association 

area. 


Occipito-temp 
association  area 


Olfactory  lobe. 
Olfactory  tract. 

Olfactory  area. 


Gyrus  hippocampus. 


Fig.  256. — Diagrams  to  show  the  Position  and  the  Relation  of  the  Association 
and  Projection  Areas.     The  Projection  Areas  are  Dotted. — (After  Flechsig.) 

tion  it  would  appear  that  many  of  these  fibers  grow  out  directly  from 
the  sensor  centers  into  these  regions  and  become  related  to  the  nerve- 
cells  of  their  convolutions,  while  others  grow  out  from  adjacent  as 
well  as  distant  convolutions.  From  histologic  and  pathologic  evidence 
these  regions  were  termed  by  Flechsig  association  centers  or  areas,  im- 
plying the  idea  that  through  the  intervention  of  their  cell  mechanisms 


THE  CEREBRUM.  563 

the  sense  areas  are  indirectly  associated  anatomically  and  physiolog- 
ically, and  together  constitute  a  mechanism  by  which  sensations  are 
associated  and  elaborated  into  concrete  forms  of  knowledge  or  related 
to  definite  forms  of  movement. 

It  has  been  assumed  by  Flechsig  that  the  frontal  association  center, 
from  its  connections  with  the  sensor  and  motor  areas  of  the  Rolandic 
region,  the  olfactory,  and  perhaps  other  regions,  is  engaged  in  asso- 
ciating and  registering  body  sensations  and  volitional  acts,  and  that  the 
knowledge  thus  gained  has  reference  largely  to  the  personality  of  the 
individual;  that  the  parieto-occipital  association  area,  from  its  relation 
to  the  visual,  auditory,  and  tactile  sense  areas,  is  engaged  in  associating 
and  registering  visual,  auditory,  and  tactile  sensations,  and  that  the 
knowledge  thus  gained  has  reference  mainly  to  the  external  world. 
These  assumptions  in  a  general  way  are  supported  by  the  phenomena 
of  disease.  In  certain  lesions  of  the  frontal  lobe  the  symptoms  indicate 
a  loss  or  change  of  ideas  regarding  personality  rather  than  of  the 
objective  world,  while  the  reverse  is  true  in  disease  of  the  parieto- 
occipital lobe. 


The  Intra-cranial  Circulation. — The  circulation  within  the 
cranium  presents  certain  peculiarities  which  distinguish  it  from  that 
in  other  parts  of  the  body.  These  peculiarities  reside  in  part  in  the 
anatomic  arrangement  of  the  blood-vessels,  in  the  probable  absence  of 
vaso-motor  nerves  to  the  blood-vessels,  and  in  greater  part  in  the  fact 
that  the  brain  and  its  blood-vessels  are  contained  in  a  box  with  rigid, 
unyielding  and  closed  walls. 

The  Blood-supply. — As  stated  in  a  previous  paragraph  the 
arteries  supplying  the  brain  with  blood  are  four  in  number  viz. :  The 
two  internal  carotids  and  the  two  vertebrals.  These  four  arteries 
anastomose  very  freely  at  the  base  of  the  brain,  the  anastomosis 
constituting  the  circle  of  Willis.  From  this  circle  there  arise  the 
anterior,  middle  and  posterior  cerebral  arteries  which  are  distributed 
to  the  cortex  and  the  underlying  white  matter.  The  basal  ganglia, 
the  capsule  and  adjacent  white  matter  are  supplied  by  a  number  of 
branches  which  arise  from  the  circle  of  Willis  or  from  the  three  cerebral 
arteries  immediately  after  their  origin.  From  the  distribution  of 
these  two  sets  of  vessels  they  have  been  named  the  cortical  and  the 
central  ganglionic  respectively. 

The  venous  blood  is  returned  by  a  system  of  vessels  which  present 
characteristics  of  physiologic  interest.  These  vessels  consist  of  large 
sinuses  formed  by  folds  of  the  dura  mater  or,  as  at  the  base  of  the  cranium, 
by  the  dura  mater  and  the  bone.  These  sinuses,  from  the  very  nature 
of  the  tissues  which  enter  into  their  formation,  have  rigid  walls  and  will 
therefore  withstand  any  pressure  to  which  they  may  be  subjected  under 
physiologic  conditions.  The  same  obtains  at  their  points  of  exit  from 
the  cranium    where  a  free  outflow  is  in  consequence  always  assured. 


564  TEXT-BOOK  OF  PHYSIOLOGY. 

The  various  sinuses  have  opening  into  them,  the  veins  which 
return  the  blood  from  the  cortex  and  subjacent  white  matter,  and 
from  the  inner  structures  of  the  brain.  Neither  sinuses  nor  veins 
have  valves  and  most  of  the  veins  which  empty  into  the  superior 
longitudinal  sinus  have  their  mouths  directed  forward,  hence  the  blood 
discharged  from  these  veins  must  flow  against  the  current  in  the  sinus. 
The  venous  blood  leaves  the  cranium  mainly  by  way  of  the  internal 
jugular  veins  which  are  direct  continuations  of  the  lateral  sinuses. 

The  Intra-cranial  Lymph  Spaces. — In  order  to  understand  the 
phenomena  attending  the  circulation  of  blood  through  the  cranium 
it  is  necessary  to  take  into  consideration  an  important  fact,  viz. :  that 
the  brain  and  spinal  cord  are  surrounded  on  all  sides  by  a  relatively 
large  and  continuous  lymph  space.  This  space  which  is  found 
between  the  arachnoid  and  the  pia  mater  is  filled  with  a  liquid,  the 
so-called  cerebrospinal  fluid,  which  being  interposed  between  the 
brain  and  the  skull  on  the  one  hand  and  the  spinal  cord  and  the  vertebras 
on  the  other  hand,  acts  as  a  water  cushion  protecting  these  delicate 
organs  from  the  injury  which  might  result  from  sudden  jars.  The 
ventricles  of  the  brain  are  also  filled  with  cerebrospinal  fluid  which  is 
in  communication  with  that  in  the  subarachnoid  space  through  the 
foramen  of  Magendie  and  the  foramina  of  Key  and  Retzius.  The 
cerebrospinal  fluid  may  also  penetrate  into  the  perineural  lymph 
spaces  surrounding  the  cranial  and  spinal  nerves.  The  quantity  of 
the  cerebrospinal  fluid  is  relatively  small,  amounting  to  from  60  to 
80  c.c. 

The  Mechanism  of  the  Intra-cranial  Circulation. — As  pre- 
viously stated,  by  virtue  of  the  physical  relations  existing  between  the 
blood,  the  brain,  the  cerebrospinal  fluid  and  rigid  walls  of  the  cranium, 
the  flow  of  the  blood  through  the  brain  and  cranial  cavity,  is  attended 
by  certain  phenomena  which  are  peculiar  to  this  region  and  present 
in  no  other  situation. 

Taking  as  a  point  of  departure  the  conditions  during  the  diastole 
of  the  arteries,,  the  relations  of  these  structures  are  somewhat  as 
follows:  the  cerebrospinal  fluid  occupies  all  the  available  lymph 
space,  but  under  a  pressure  approximately  equal  to  that  in  the  large 
veins  and  hence  not  materially  above  that  of  the  atmosphere;  the 
pressure  in  the  arteries,  capillaries  and  veins  presents  the  usual  values 
in  these  different  regions  of  the  vascular  apparatus;  the  brain  presents 
a  volume  which  may  be  termed  diastolic. 

With  the  occurrence  of  the  succeeding  cardiac  systole,  the  cerebral 
vessels,  receiving  an  additional  volume  of  blood,  expand  and  occasion  a 
corresponding  increase  in  the  volume  of  the  brain,  which  is  accom- 
plished by  a  partial  displacement  of  the  cerebrospinal  fluid  into 
extra-cranial  lymph  spaces.  Because  of  the  fact  that  the  displacement 
of  the  cerebrospinal  fluid  is  insufficient  to  permit  of  the  complete 
expansion  of  the  brain,  there  is  developed  in  the  intra-cranial  lymph 
spaces  a  counter  pressure  (the  so-called  intra-cranial  pressure)  which 


THE  CEREBRUM.  565 

would  keep  pace  with  and  finally  equalize  the  rising  pressure  in  the 
arteries.  In  consequence  of  this,  the  brain  tissue,  it  is  believed,  would 
be  subjected  to  a  pressure  sufficiently  great  to  interfere  with  its  activi- 
ties, even  to  the  point  of  unconsciousness.  If  this  is  not  to  occur  the 
maximum  expansion  of  the  arteries,  and  hence  the  brain,  must  be 
checked  and  controlled.  This  is  accomplished  in  the  following  way: 
As  the  brain  approaches  that  degree  of  expansion  permitted  bv  the 
displacement  of  the  cerebrospinal  fluid,  it  begins  to  exert  a  com- 
pression of  the  pial  veins.  This  compression  by  narrowing  the 
lumen  of  the  veins  diminishes  their  capacity  and  hence  increases  the 
pressure  of  their  contained  blood  until  it  is  equivalent  to  the  pressure 
exerted  by  the  brain  against  the  veins.  At  this  moment  the  pressures 
in  the  arterioles,  capillaries  and  veins  approximate  each  other  in  value. 

From  these  factors  it  will  be  seen  that  the  circulation  through  the 
brain  approximates  a  circulation  through  a  system  of  rigid  tubes. 
The  result  is  an  increase  in  the  velocity  of  the  outflow  and  a  diminution 
of  the  blood-pressure.  As  an  additional  result  the  pulse  wave  of  the 
arterial  system  is  transmitted  to  the  blood  of  the  large  veins  and  sinuses 
which  therefore  exhibit  normally  pulsations  synchronous  with  those 
of  the  arteries.  The  rise  of  the  pressure  in  the  cerebral  veins  is 
regarded  therefore  as  the  factor  which,  by  limiting  brain  expansion, 
checks  the  rise  of  the  intra-cranial  pressure  beyond  physiologic  limits. 
With  the  diastole  of  the  heart  and  arteries,  the  former  relation  of  the 
blood,  brain,  cerebrospinal  fluid  and  cranial  walls  is  regained.  Be- 
cause of  this  change  of  relation  with  each  heart-beat,  the  brain  pulsates 
synchronously  with  the  arteries. 

The  brain  differs  from  other  organs,  also,  in  that  normally  its 
volume  is  more  influenced  in  a  positive  direction  by  the  expiratorv 
rise  of  venous  pressure  than  by  the  inspiratory  rise  of  general  arterial 
pressure.  Thus  the  rise  of  pressure  in  the  thoracic  veins  which  occurs 
with  each  expiratory  act,  causes  a  damming  back  of  the  venous  blood 
in  the  sinuses  and  pial  veins,  resulting  in  a  further  increase  in  the 
volume  of  the  brain  and  in  the  intra-cranial  pressure.  The  reverse 
takes  place  in  inspiration. 

It  has  been  ascertained  experimentally  that  the  intra-cranial 
pressure  may  vary  considerably  and  consciousness  still  be  preserved. 
Hill  found  it  to  be  40  to  50  mm.  of  Hg.  in  the  convulsions  of  strychnia 
poisoning  and  a  little  less  than  zero  in  a  patient  standing  erect. 

The  Regulation  of  the  Volume  of  Blood  Entering  the  Brain.— 
li  is  generally  believed  that  the  cerebral  vessels  are  not  provided  with 
vaso-motor  nerves.  Every  attempt  to  prove  their  existence  either  by 
physiologic  or  histologic  methods  has  thus  far  failed  of  convincing 
proof.  In  the  absence  of  vaso-motor  nerves,  the  regulation  of  the 
circulation  in  the  brain  must  necessarily  be  dependent  on  changes 
affecting  the  arterial  and  venous  pressures  in  other  regions  of  the 
body. 

The  most   effective   factor  in  increasing  or  decreasing  the  blood- 


566  TEXT-BOOK  OF  PHYSIOLOGY. 

supply  to  the  brain  resides  in  the  power  of  the  vaso-motor  center  to 
cause  a  contraction  or  dilatation  of  the  cutaneous  and  splanchnic  vessels. 
Thus  if  the  vaso-motor  center  declines  in  its  tonus  from  any  cause  what- 
ever, there  is  a  relaxation  of  the  blood-vessels  in  one  or  both  of  these 
regions,  an  increase  in  the  volume  of  the  blood  flowing  into  them,  and 
in  consequence,  a  decrease  in  the  volume  of  the  blood  flowing  through 
the  brain.  If  on  the  contrary  the  vaso-motor  center  is  increased  in 
its  tonus,  the  reverse  conditions  prevail  in  the  cutaneous  and  splanch- 
nic vessels  and  the  quantity  of  blood  flowing  into  the  brain  is  increased. 
Thus  in  an  indirect  way  the  vaso-motor  center,  by  bringing  about  a 
rise  or  a  fall  in  the  general  arterial  pressure,  regulates  the  blood-supply 
to  the  brain,  and  controls  its  amount  in  accordance  with  its  needs. 

Brain  Activity  and  Brain  Repose  or  Sleep. — Brain  activity  is 
characterized  by  an  active  consciousness,  the  development  of  sensations, 
ideas,  feelings,  and  the  exercise  of  volitional  power  (which  manifests 
in  muscle  movement)  and  is  the  result  of  a  physiologic  condition  of 
the  body  at  large.  For  the  manifestation  of  brain  activity  it  is  essential, 
that  the  irritability  of  the  brain  cells  and  more  especially  of  those  com- 
posing in  large  measure  the  cerebral  cortex  be  maintained  at  a  normal 
physiologic  level,  so  that  they  may  respond  in  the  manner  peculiar 
to  them,  to  the  action  of  nerve  impulses  reflected  through  afferent 
nerves  from  all  regions  of  the  body.  Here  as  elsewhere  throughout 
the  body,  the  irritability  depends  on,  and  is  maintained  by,  the  presence 
of  blood  flowing  into  and  out  of  the  brain  in  varying  quantity  from 
moment  to  moment,  with  a  given  velocity  and  under  a  definite  pressure. 
So  long  as  these  conditions  are  maintained  in  the  strictly  physiologic 
condition,  so  long  will  the  brain  respond  to  stimuli  by  the  development 
of  sensations.  The  avenues  through  which  nerve  impulses  pass  to  the 
cortical  cells  are  those  beginning  in  the  special  and  general  sense 
organs  of  the  body  in  contact  with  the  external  world,  viz. :  the  eyes, 
ears,  nose,  tongue,  and  skin.  The  maintenance  of  these  structures 
in  a  strictly  physiologic  condition  is  also  one  of  the  essential  conditions 
for  brain  activity. 

Judging  from  the  changes  in  the  character  and  composition  of  the 
blood  which  occur  during  its  passage  through  the  brain  capillaries, 
there  is  coincidently  with  brain  activity  an  active  metabolism,  which 
eventuates,  at  the  end  of  a  variable  number  of  hours,  in  the  decline 
of  the  irritability,  a  reduction  of  functional  activity,  and  the  establish- 
ment of  the  condition  of  fatigue.  The  irritability  of  the  sense  organs, 
especially  of  the  eyes  and  ears,  in  all  probability  declines  in  a  similar 
manner.  These  structures  pass  into  the  condition  of  fatigue  and 
become  less  responsive  to  external  stimuli.  The  result  of  all  these 
conditions  is  a  less  active  stimulation  of  the  brain  cells,  which  in  con- 
nection with  other  factors  predisposes  to 

Brain  Repose  or  Sleep. — Brain  repose  or  sleep  is  characterized 
by  a  greater  or  less  degree  of  unconsciousness,  the  non-development 
of  sensations,  ideas,  feelings  and  volitional  acts,  and  is  the  result  of  a 


THE  CEREBRUM.  567 

diminution  in  the  physiologic  activities  of  the  body  at  large  and  more 
especially  of  the  brain,  sense  organs  and  spinal  cord.  Coincident  with 
the  cessation  of  brain  activity  and  the  onset  of  sleep,  there  is  a 
diminution  in  the  rate  and  force  of  the  heart  beat,  and  in  the  frequency 
and  depth  of  the  respiratory  movements,  and  a  relaxation  of  the  skeletal 
muscles,  especially  those  employed  in  voluntary  movements. 

The  sense  organs  are  in  part  protected  from  the  action  of  external 
stimuli.  The  eyeball  is  so  turned,  that  its  anterior  pole  is  directed 
far  upward  under  the  eyelid,  while  the  pupil  is  markedly  diminished 
in  size,  and  in  consequence  the  entrance  of  light  largely  prevented. 
The  ear  is  protected  against  the  reception  of  sounds  of  ordinary 
pitch  by  an  increased  tension  of  the  tympanic  membrane.  The 
nose  and  mouth  are  less  responsive  to  various  stimuli  because  of  the 
dryness  of  their  mucou's  membranes  from  diminished  secretion.  The 
skin  appears  to  be  less  sensitive  to  mechanic  pressure  and  other  forms 
of  stimulation. 

In  addition  to  the  foregoing  phenomena,  experimental  investigations 
have  shown,  that  there  is  a  shunting  of  a  portion  of  the  blood  stream 
from  the  brain  to  other  regions  of  the  body,  especially  to  the  skin  and 
perhaps  to  the  abdominal  viscera  as  well,  whereby  it  becomes  incapable 
of  functionating  physiologicly.  The  fact  that  the  brain  receives  a 
lessened  quantity  of  blood  during  sleep  has  been  shown  by  trephining 
the  skull  and  inserting  in  the  orifice  a  glass  plate  through  which  the 
circulatorv  conditions  of  the  brain  can  be  observed.     In  the  wakinsr 

-  O 

condition  the  blood-vessels  on  the  surface  of  the  brain  are  prominent, 
and  turgid  with  blood  and  the  whole  organ  completely  fills  the  cranial 
cavity,  indicating  that  the  blood-vessels  in  the  interior  of  the  brain  are 
in  a  similar  condition.  With  the  onset  of  sleep  the  larger  blood-vessels 
begin  to  diminish  in  size,  the  smaller  vessels  disappear  from  view, 
the  brain  tissues  become  pale  and  the  volume  of  the  brain  shrinks. 
During  the  continuance  of  deep  sleep,  this  anemic  condition  persists. 
As  the  period  of  sleep  approaches  its  termination,  the  smaller  blood- 
vessels again  fill  with  blood,  the  surface  of  the  brain  flushes,  and  in  a 
very  short  time  the  former  circulatory  conditions  return,  the  volume 
of  the  brain  increases  and  the  waking  state  is  reestablished. 

The  fact  that  the  skin  receives  an  increased  volume  of  blood  during 
sleep,  has  been  shown  by  inserting  an  arm  or  leg  in  a  plethysmograph 
by  which  means  a  record  of  any  change  in  volume  can  be  obtained. 
Howell  thus  succeeded  in  obtaining  graphic  records  in  the  variations 
of  the  volume  of  the  arm  during  sleep.  These  records  disclosed 
the  fact  that  with  the  onset  of  sleep  the  volume  of  the  arm  gradually 
increased  in  size  until  it  attained  a  maximum  which  was  from  one  to 
two  hours  after  the  beginning  of  sleep.  After  this  period  the  volume 
remains  practically  the  same  for  several  hours,  diminishing  ■  as  the 
intensity  of  sleep  diminishes,  and  the  waking  state  is  approached. 
Just  previous  to  the  return  of  consciousness  there  is  a  rapid  diminution 
in  the  volume  of  the  arm.     If  it  be  accepted  that  the  enlargement  of 


568  TEXT-BOOK  OF  PHYSIOLOGY. 

the  cutaneous  vessels  is  followed  by  a  diminution  in  size  of  the  cerebral 
vessels,  it  follows  that  the  former  condition  stands  to  the  latter  in  the 
relation  of  cause  and  effect,  whereby  a  portion  of  the  blood  is  diverted 
from  the  brain  to  the  skin.  It  also  naturally  follows  that  the  with- 
drawal of  the  blood  from  the  brain  to  the  skin  and  possibly  other 
regions  as  well,  is  the  fundamental  condition  for  brain  repose. 

The  Intensity  of  Sleep. — Observations  of  individuals  during 
sleep  show,  that  the  intensity  or  the  deepth  of  sleep  varies  from  hour 
to  hour.  Attempts  have  been  made  to  estimate  the  intensity  by 
measuring  the  intensity,  or  the  loudness  of  a  sound  caused  in  several 
ways,  that  is  necessary  to  awaken  the  sleeper.  Accepting  this  criterion 
it  may  be  stated  from  the  results  of  many  experiments,  that  sleep 
increases  in  intensity  or  depth  and  reaches  its  maximum  between  the 
first  and  second  hours,  after  which  it  rapidly  decreases  until  the  end 
of  the  third  hour,  when  consciousness  is  so  nearly  restored,  that 
but  a  very  slight  stimulus  is  required  to  awaken  the  sleeper.  It  is 
during  the  latter  period  when  the  brain  is  reviving,  that  dreams  arise 
the  element  of  which  are  formed  of  previous  sensations. 

The  Causes  of  Sleep. — Different  theories  have  been  proposed  to 
account  for  the  causes  of  sleep,  none  of  which  have  been  wholly  satis- 
factorv.  From  all  the  facts  which  have  been  presented  it  would  appear 
that  one  cause  is  a  decline  in  the  irritability  of  the  nerve-cells  of  the 
brain  and  associated  sense  organs,  and  the  development  of  fatigue 
conditions,  the  result  of  prolonged  activity. 

A  second  cause  is  the  withdrawal  of  a  large  portion  of  the  blood 
from  the  brain  on  the  presence  of  which,  here  as  elsewhere,  normal 
activity  depends.  As  to  whether  the  diminished  activity  of  the  brain 
is  the  cause  of,  or  the  result  of  the  withdrawal  of  the  blood  there  has 
been  much  difference  of  opinion.  Howell  has  offered  a  plausible 
explanation  for  the  withdrawal  of  the  blood  from  the  brain  to  t he- 
cut  aneous  vessels,  based  on  the  activity  of  the  vaso-motor  center. 
He  assumes  that  for  a  variable  number  of  hours,  corresponding  to 
the  usual  waking  state,  this  center  possesses  a  certain  average  tonus, 
due  in  all  probability  to  reflex  influences,  by  virtue  of  which  it  main- 
tains a  certain  average  contraction  of  the  cutaneous  vessels.  But 
at  the  end  of  this  period  it,  too,  becomes  fatigued,  declines  in  irritability, 
becomes  less  responsive  to  reflex  influences,  and  hence  loses  its  control 
over  the  vessels.  As  a  result  they  dilate  and  thus  reduce  the  amount 
•of  blood  flowing  to  the  brain  to  a  level  insufficient  to  maintain  its 
activity,  after  which  sleep  supervenes.  During  sleep  the  irritability 
and  tonus  of  the  center  are  restored  when  its  control  of  the  blood- 
vessels  is  regained.  Unless  the  brain  in  its  functional  activities  differs 
from  all  other  organs  of  the  body,  it  may  be  inferred  that  cessation  of 
activity  or  repose  is  the  result  partly  of  fatigue  and  partly  of  a  diminu- 
i  ion  of  the  blood-supply. 


CHAPTER  XXIT. 
THE  CEREBELLUM. 

The  cerebellum  is  situated  in  the  inferior  fossae  of  the  occipital 
bone,  beneath  the  posterior  lobes  of  the  cerebrum,  from  which  it 
is  separated  by  the  tentorium  cerebelli,  a  semilunar  fold  of  the  dura 
mater.  It  is  partially  divided  into  hemispheres  by  a  longitudinal 
fissure,  more  apparent  on  the  inferior  surface,  though  united  by  a 
central  lobe,  the  vermiform  process.  Each  hemisphere  is  connected 
with  the  cerebrum,  the  pons,  medulla  and  spinal  cord  by  three  bundles 
of  nerve-fibers  known  respectively  as  the  superior,  middle,  and  inferior 
peduncles.  The  surface  of  the  cerebellum  presents  a  series  of  lobes  and 
fissures  of  which  the  former  have  received  more  or  less  fanciful  names. 
A  section  of  the  cerebellum  shows  that  it  is  composed  of  gray  matter 
externally  and  white  matter  internally.  The  general  appearance 
presented  on  section  is  shown  in  Fig.  257. 

Structure  of  the  Gray  Matter. — The  gray  matter  consists 
mainlv  of  nerve-cells  of  varying  size  and  shape,  which  are  arranged  in 
two   layers:  viz.,  an   outer   or   molecular  and  an  inner  or  granular. 

The  molecular  layer  consists  of  stellate  and  multipolar  cells  of 
small  size,  from  which  dendrites  and  axons  pass  horizontally  and 
vertically.  The  granular  layer  consists,  as  its  name  implies,  of 
granular  shaped  cells  and  large  stellate  cells.  These  cells  are  character- 
ized by  the  possession  of  dendrites  and  axons,  the  course  and  relation 
of  which  have  not  been  clearly  determined. 

The  inner  border  of  the  molecular  layer  presents  a  series  of 
large  cells  originally  described  by  Purkinje  and  known  by  his  name. 
From  the  outer  end  of  the  cell-body  one  or  more  dendrites  emerge  which 
soon  divide  and  subdivide  into  a  number  of  branches  which  pass 
toward  the  cerebellar  surface.  The  general  arrangement  of  these 
dendrites  gives  to  the  entire  cell  a  tree-like  appearance  (Fig.  258). 
From  the  inner  end  of  the  cell  an  axon  emerges  which  passes  centrally 
into  the  white  matter. 

Structure  of  the  White  Matter. — The  white  matter  consists  of 
nerve-fibers  which  are  arranged  in  association  and  projection  systems. 

The  Association  System. — The  libers  which  compose  this  system 
are  of  variable  lengths  and  unite  adjacent  as  well  as  distant  regions 
of  the  cerebellar  cortex.  They  doubtless  associate  them  both  anatom- 
ically and  physiologically. 

The  Projection  System. — The  fibers  composing  this  system  con- 
nect the  cerebellar  cortex  with  certain  structures  in  the  cerebrum, 
pons,  medulla,  and  spinal  cord.  They  may  be  divided  into  efferent 
and  afferent  systems. 

569 


57° 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  efferent  fibers  have  their  origin  in  the  cells  of  Purkinje  and 
the  dentate  nucleus.  Some  of  these  fibers  emerge  from  the  cere- 
bellum in  the  superior  peduncles  through  which  they  pass  toward  -and 
beneath  the  corpora  quadrigemina  to  terminate  around  the  cells  of 
the  red  nucleus.  As  they  approach  this  nucleus  some  of  the  fibers 
cross  the  median  line  and  decussate  with  those  coming  from  the  op- 
posite side,  while  others  pursue  a  straight  direction,  terminating  on  the 

same  side.  Through  the 
intervention  of  fibers 
which  arise  from  the 
red  nucleus  and  ascend 
to  the.  cerebral  cortex, 
the  cortex  is  thus  con- 
nected with  both  sides 
of  the  cerebellum,  though 
chiefly  with  the  opposite 
side. 

Efferent  fibers  also 
leave  the  cerebellum  by 
the  middle  peduncle  and 
pass  directly  to  the  nu- 
cleus ponds,  around  the 
cells  of  which  their  term- 
inals arborize.  Efferent 
fibers  also  descend  the 
inferior  peduncles  and 
constitute  the  tract 
known  as  the  Lowenthal 
and  Marchi  tract,  situ- 
ated in  the  antero-lateral 
region  of  the  spinal  cord 
in  its  upper  part. 

The  afferent  fibers 
come  from  a  variety  of 
sources.  Those  found 
in  the  superior  peduncles  come  from  the  red  nucleus;  those  in  the 
middle  peduncles  from  the  nucleus  pontis  of  the  opposite  side,  having 
crossed  or  decussated  at  the  raphe  near  the  anterior  surface  of  the 
pons;  those  contained  in  the  inferior  peduncles  are  the  most  abundant 
and  important,  and  are  represented  by  (i)  the  direct  cerebellar  tract, 
which  terminates  in  the  superior  vermis  after  decussation;  (2)  the 
anterior  and  posterior  arcuate  fibers,  the  former  coming  from  the 
gracile  and  cuneate  nuclei  of  the  opposite  side,  the  latter  from  the 
same  side,  which  also  pass  to  the  superior  vermis;  (3)  the  acustico- 
cerebellar  tract,  composed  of  fibers  the  axons  of  the  sensory  end 
nuclei  (Deiters)  of  the  vestibular  portion  of  the  auditory  nerve.  It 
is  probable  that  all  these  libers  decussate  prior  to  their  final  termination. 


Fig.  257. — View  of  Cerebellum  in  Section,  and 
of  Fourth  Ventricle,  with  the  Neighboring 
Parts. — {From  Sappey.)  1.  Median  groove  fourth 
ventricle,  ending  below  in  the  calamus  scriptorius, 
with  the  longitudinal  eminences  formed  by  the  fas- 
ciculi teretes,  one  on  each  side.  2.  The  same  groove, 
at  the  place  where  the  white  streaks  of  the  auditory 
nerve  emerge  from  it  to  cross  the  floor  of  the  ventricle. 
3.  Inferior  peduncle  of  the  cerebellum,  formed  by  the 
restiform  body.  4.  Posterior  pyramid;  above  this  is 
the  calamus  scriptorius.  5,  5.  Superior  peduncle  of 
cerebellum,  or  processus  e  cerebello  ad  testes.  6,  6. 
Fillet  to  the  side  of  the  crura  cerebri.  7,  7.  Lateral 
grooves  of  the  crura  cerebri.  8.  Corpora  quadri- 
gemina.— {After  Hirschfeld  and  Leveille.) 


THE  CEREBELLUM. 


57i 


The  cerebellum  through  this  system  of  efferent  and  afferent  fibers 
is  brought  into  relation  with  many  different  regions  of  the  cerebrum, 
pons,  medulla,  and  spinal  cord.  Each  half  of  the  cerebellum  is  con- 
nected with  the  foregoing  structures  of  the  same  side,  but  more  espe- 
cially of  the  opposite  side. 

THE  FUNCTIONS  OF  THE  CEREBELLUM. 

From  the  observations  of  the  results  of  experimental  lesions,  from 
analysis  of  clinico-pathologic  facts,  and  from  its  comparative  anatomic 
development  in  different  animals,  the  deduction  has  been  drawn  that 
the  cerebellum  coordinates  and  har- 
monizes the  action  of  those  muscles 
the  activities  of  which  are  necessary  to 
the  maintenance  of  body  equilibrium 
both  during  station  and  progression. 

By  equilibrium  of  the  body  is  un- 
derstood a  condition  which  may  be 
maintained  for  a  variable  length  of 
time  without  displacement,  and  if 
possible  only  so  long  as  a  line  passing 
through  the  center  of  gravity  falls 
within  the  base  of  support.  The  sup- 
port offered  by  the  earth  to  the  feet 
neutralizes  and  counteracts  the  force 
of  gravity.  In  station,  when  the  body 
is  in  the  erect  or  military  position, 
the  arms  by  the  side,  the  center  of 
gravity  lies  between  the  sacrum  and 
the  last  lumbar  vertebra,  and  the  line 
of  gravity  falls  between  the  feet  and 
within  the  base  of  support.  The  en- 
tire skeleton  for  the  time  being  is  rendered  fixed  and  rigid  at  all  its 
joints  by  the  combined  action  of  the  muscles  connected  with  it.  That 
this  position  may  be  maintained  all  the  different  groups  of  antagonistic 
but  cooperative  muscles  must  be  accurately  coordinated  in  their  ac- 
tions. Any  failure  in  this  respect  is  at  once  attended  by  a  disturbance 
of  the  equilibrium  and  displacement. 

In  progression,  walking,  running,  dancing,  etc.,  the  body  is  trans- 
lated from  point  to  point  by  the  alternate  action  of  the  legs.  Whether 
the  direction  of  the  translation  be  linear  or  curvilinear,  as  the  legs 
change  their  position  from  moment  to  moment,  the  center  of  gravity 
also  changes,  and  at  once  the  equilibrium  is  menaced.  If  it  is  to  be 
maintained  and  displacement  prevented  there  must  be  a  prompt 
readjustment  in  the  relation  of  all  parts  of  the  body  so  that  the  line 
of  gravity  falls  again  within  the  base  of  support.  The  more  com- 
plicated the  movements  of  progression,  or  the  narrower  the  base  of 
support,  the  greater  is  the  clanger  to  the  equilibrium,  and  hence  the 


Fig.  258. — Section  of  Cerebellar 
Cortex.  A.  Outer  or  molecular 
layer.  B.  Inner  or  granular  layer. 
C.  White  matter,  a.  Cell  of  Purk- 
inje.  b.  Small  cells  of  inner  layer 
c.  Dendrites  of  these  cells,  d.  A 
similar  cell  lying  in  the  white  matter. 
— (Stirling.) 


572  TEXT-BOOK  OF  PHYSIOLOGY. 

necessity  for  rapid  and  compensatory  changes  in  coordinated  muscle 
activity.  All  movements  of  this  character,  in  man  at  least,  are  pri- 
marilv  volitional  and  require  for  their  performance  the  constant  exercise 
of  the  attention.  With  frequent  repetition  they  gradually  come  to 
be  performed  independently  of  consciousness  and  fall  into  the  cate- 
gorv  of  secondary  or  acquired  reflexes. 

Though  coordinating  power  is  exhibited  by  the  spinal  cord,  medulla, 
and  basal  ganglia,  it  is  only  in  the  cerebellum  that  this  power  attains 
its  highest  development  and  differentiation.  To  it  is  assigned  the 
power  of  selecting  and  grouping  muscles,  not  in  any  restricted  part, 
but  in  all  parts  of  the  body,  and  coordinating  their  actions  in  such  a 
manner  as  to  preserve  the  equilibrium. 

The  Results  of  Experimental  Lesions. — If  the  cerebellum  in 
its  totality,  coordinates  and  harmonizes  the  action  of  the  muscles 
on  the  opposite  sides  of  the  body,  any  derangement  of  its  structure  or 
its  connections  with  the  cord,  medulla,  pons,  or  basal  ganglia  should 
at  once  be  followed  by  incoordination  of  muscles  and  a  want  of  har- 


x      '4a.  ■' 


<      ^ 


Fig.   259. — Attitude  Assumed  After  Destruction  of  the  Left  Half  of  the 
Cerebellum. — (Morat  and  Doyon,  after  Thomas.) 

mony  in  their  action.  Experimental  lesions  of  the  cerebellum  are 
attended  by  such  results.  The  phenomena  observed  are  many  and 
complex.  They  differ  in  extent  and  character  in  different  animals 
and  in  accordance  with  the  extent  and  location  of  the  lesion,  though 
the  note  of  incoordination  runs  through  them  all. 

Removal  of  one  lateral  half  of  the  cerebellum  in  the  dog  is  followed 
by  an  inability  to  maintain  the  equilibrium  necessary  to  the  erect  posi- 
tion. On  attempting  to  stand,  the  animal  at  once  falls  toward  the  side  of 
the  lesion,  the  muscles  of  which  at  the  same  time  contract  and  give 
to  the  body  a  distinctly  curved  condition  (Fig.  259).  The  anterior 
limbs  are  extended  to  the  opposite  side.  On  making  efforts  to  re- 
gain the  standing  position,  the  animal  may  roll  over  around  the  long 
axis  of  its  body.  Conjugate  deviation  of  the  eyes  is  frequently  ob- 
1  r ■  1  :d  as  well  as  nystagmus. 

After  a  few  days  the  symptoms  partially  subside  and  the  animal 
acquires  the  power  of  sitting  on  the  abdomen  when  the  anterior  limbs 
are  widely  extended  (Fig.  260).  As  the  days  go.  by  the  improve- 
ment continues,  and  the  animal  recovers  the  power  of  walking,  though 


THE  CEREBELLUM. 


573 


Fig.  260. — Attitude  in  Repose  after 
the  Complete  Removal  of  the  Cere- 
bellum BUT  DURING  THE  PERIOD  OF  RES- 
TORATION of  Function. — {Moral  and  Doyon, 
after  Thomas.) 


each  step  is  attended  with  tremor  and  oscillations  of  the  body.  Any 
change  in  the  center  of  gravity  such  as  results  when  one  leg  is  lifted 
may  result  in  a  fall  toward  the  side  of  the  lesion,  owing  to  an  inability 
to  promptly  bring  about  the  necessary  compensatory  muscle  actions. 
With  time  the  animal  continues  to  improve  in  its  power  of  adjustment, 
though  it  never  completely  recovers  it.  Movements  of  progression 
are  apt  to  be  characterized  by 
stiffness  and  accompanied  by 
tremor  suggestive  of  volitional 
efforts. 

Total  removal  of  the  cerebel- 
lum is  followed  by  a  different 
train  of  symptoms.  The  extensor 
muscles  apparently  preponderate 
in  their  action,  for  the  limbs  are 
extended  and  abducted,  the  head 
and  neck  are  retracted,  and 
opisthotonos  is  established.  In 
time  these  effects  also  partially 
subside,  though  all  attempts  at 
walking  are  permanently  accom- 
panied by  tremor  and  oscillations.  The  characteristic  effect  which 
follows  section  of  the  peduncles  is  again  incoordination,  manifesting 
itself  in  deviation  of  the  head,  eyes,  inability  to  walk,  tremor  on 
exertion,  etc.  The  effects  vary,  however,  according  to  the  peduncle 
divided.  Section  of  the  middle  peduncle  gives  rise  to  the  most  pro- 
nounced effects.  The  head  and  the  anterior  part  of  the  body  are  at 
once  drawn  toward  the  pelvis  on  the  side  of  the  section.     A  voluntary 

effort   on   the   part   of    the 


animal  causes  it  to  lose  all 
control  of  its  muscles  and 
the  body  is  rotated  in  the 
direction  of  its  longitudinal 
axis  from  40  to  60  times  a 
minute  before  it  comes  to 
rest.  According  as  the 
lesion  is  made  from  behind 
or  before,  the  rotation  is 
from  or  to  the  side  of  the 
section.  In  time  these 
symptoms  subside,  though  the  animal  never  completely  recovers. 

The  partial  recovery  of  the  power  of  coordination,  observed  after 
removal  of  a  portion  or  the  whole  of  the  cerebellum,  indicates  that 
the  centers  in  the  cord,  medulla,  pons,  and  cerebrum  endowed  with 
corresponding  though  less  developed  power,  develop  compensatory 
activity  and  acquire  to  some  extent  the  capabilities  of  the  cerebellum 
itself  (Fig.  261). 


Fig.    261. — Progression    after    Destruction 
of  the  Vermis. — {Moral  and  Doyon,  after  Thomas.) 


574  TEXT-BOOK  OF  PHYSIOLOGY. 

Clinico- pathologic  facts  partly  corroborate  the  results  of  phys- 
iologic investigations.  In  various  forms  of  uncomplicated  cerebellar 
disease,  vertigo,  tremor  on  making  voluntary  efforts,  difficulty  in  main- 
taining the  erect  position,  unsteadiness  in  walking,  opisthotonos, 
pleurothotonos,  are  among  the  symptoms  generally  observed. 

Comparative  anatomic  investigations  reveal  a  remarkable  correspond- 
ence between  the  development  of  the  cerebellum  and  the  complexity 
of  the  movements  exhibited  by  animals.  In  those  animals  whose 
movements  are  complex  and  require  for  their  performance  the  coopera- 
tion of  many  groups  of  muscles  the  cerebellum  attains  a  much  greater 
development  in  reference  to  the  rest  of  the  brain  than  in  animals  whose 
movements  are  relatively  simple  in  character.  This  relative  increase 
in  the  development  of  the  cerebellum  is  found  in  many  animals,  such 
as  the  kangaroo,  the  shark,  the  swallow,  and  the  predaceous  birds 
generally. 

The  Coordinating  Mechanism.  —Though  it  is  not  known  how 
the  cerebellum  selects  and  coordinates  groups  of  muscles  for  the  per- 
formance of  any  complex  movement,  it  is  known  that  its  activity  is 
largely  reflex  in  origin  and  excited  by  impulses  reflected  to  it  from 
peripheral  organs.  In  this  as  in  other  forms  of  reflex  activity  the 
mechanism  involves  (i)  afferent  nerves,  e.  g.,  cutaneous,  muscle,  optic, 
and  vestibular,  and  their  related  end-organs,  tactile  corpuscles,  muscle 
spindles,  retina,  and  semicircular  canals,  all  indirectly  connected  with 
(2)  the  cerebellar  centers;  (3)  efferent  nerves  indirectly  connected 
with  (4)  the  general  musculature  of  the  body.  Both  station  and  pro- 
gression are  directly  dependent  on  the  development  and  transmission 
of  afferent  impulses  from  the  previously  mentioned  peripheral  sense- 
organs  to  the  cerebellum.  Tactile,  muscle,  visual,  and  labyrinthine 
impressions  and  sensations  not  only  cooperate  in  the  development 
and  organization  of  the  motor  adjustments  necessary  to  the  main- 
tenance of  the  equilibrium  and  locomotive  coordination,  but  even  after 
their  organization  they  are  necessary  to  the  excitation  of  cerebellar 
activity.  The  manner  in  which  they  lead  to  the  development  of  this 
capability  on  the  part  of  the  cerebellum  is  conjectural.  Their  ever- 
present  influence  is  shown  by  the  effects  which  follow  their  removal, 
as  the  following  facts  indicate. 

The  prevention  of  the  development  of  tactile  impulses  by  freezing 
or  anesthetizing  the  soles  of  the  feet,  and  the  blocking  of  normally  de- 
veloped impulses  through  destruction  of  afferent  pathways  in  diseases 
of  the  spinal  cord  lead  at  once  to  marked  impairment  in  the  coordinat- 
ing power.  The  removal  of  the  skin  from  the  hind  legs  of  the  frog, 
previously  deprived  of  its  cerebrum,  destroys  its  coordinating  power, 
which  it  would  otherwise  possess  in  a  high  degree. 

The  blocking  in  consequence  of  destructive  lesions  of  the  spinal 
cord,  of  the  impulses,  which  come  from  the  muscles,  tendons,  etc., 
and  which  inform  us  of  the  activity  and  the  degree  of  activity  of  our 
muscles,  the  location  of  the  limbs,  the  amount  of  effort  necessary 


THE  CEREBELLUM.  575 

to  produce  a  given  movement,  etc.,  also  gives  rise  to  much  incoor- 
dination. A  blocking  of  both  tactile  and  muscle  impulses  frequently 
exists  in  degeneration  or  sclerosis  of  the  posterior  columns  of  the 
spinal  cord.  The  coordinating  power  is  so  much  impaired  in  this 
disease  that  the  patient  is  unable  to  maintain,  without  strained  effort, 
the  erect  position  and  especially  if  the  directive  power  of  the  eyes  be 
removed  by  closure  of  the  lids.  Walking  becomes  extremely  difficult; 
the  gait  is  irregular  and  jerky,  and  equilibrium  is  maintained  only 
by  keeping  the  eyes  fixed  on  the  ground  in  front  and  by  artificially 
increasing  the  basis  of  support  by  the  use  of  canes. 

An  interference  with  the  development  of  the  customary  visual 
impressions  which  in  a  measure  maintain  the  sense  of  relation  of  the 
individual  to  surrounding  objects  also  gives  rise  to  equilibratory  dis- 
turbances. A  rapid  change  in  the  relation  of  the  individual  to  sur- 
rounding objects  or  the  reverse;  a  change  in  the  direction  of  one 
optic  axis  from  the  use  of  a  prism  or  from  paralysis  of  an  eye  muscle; 
the  destruction  of  an  eye; — these  and  similar  conditions  frequently 
give  rise  to  such  marked  disturbances  of  the  equilibratory  power  that 
displacement  is  difficult  to  prevent. 

An  interference  with  the  development  of  the  so-called  labyrin- 
thine impressions  by  destruction  of  the  semicircular  canals  gives  rise 
to  the  most  remarkable  disturbances  in  this  respect.  Section  of  one 
horizontal  canal*  in  the  pigeon  is  followed  by  oscillations  of  the  head 
in  a  horizontal  plane  around  a  vertical  axis.  Bilateral  section  so 
increases  these  oscillations  that  the  pigeon  is  unable  to  maintain 
equilibrium  and  forced  to  fall  and  turn  continuously  around  the 
vertical  axis.  Bilateral  section  of  the  posterior  vertical  canals  gives 
rise  to  oscillations  around  a  horizontal  axis  which  frequently  become 
so  exaggerated  as  to  eventuate  in  the  turning  of  backward  somersaults, 
head  over  heels.  Similar  phenomena  follow  division  of  the  superior 
vertical  canals. 

Bilateral  destruction  of  both  sets  of  canals  is  attended  by  extra- 
ordinary disturbances  in  the  equilibrium.  From  the  moment  of  the 
operation  the  animal,  the  pigeon,  loses  all  control  of  its  motor  mechan- 
isms. It  can  neither  maintain  a  fixed  attitude  nor  execute  orderly 
movements  of  progression;  its  activity,  continuous  and  uncontrollable, 
is  characterized  by  spinning  around  a  vertical  axis,  turning  somer- 
saults, dashing  itself  against  surrounding  objects  until  life  is  en- 
dangered. If  the  animal  be  protected  from  injury,  these  disturbances 
gradually  subside,  and  in  the  course  of  a  few  months  the  equilibratory 
power  is  so  far  regained  that  standing  and  walking  at  least  become 
possible.  In  this  condition,  however,  the  coordinating  power  is 
directly  dependent  on  visual  impulses,  for  with  the  closure  of  the 
eyes  all  the  previous  motor  disturbances  at  once  recur.  These  and 
similar  facts  indicate  that  the  semicircular  canals  are  the  peripheral 

*  The  physiologic  anatomy  of  the  semicircular  canals  is   described    in    the   chapter 
devoted  to  the  ear,  to  which  the  reader  is  referred. 


576  TEXT-BOOK  OF  PHYSIOLOGY. 

sense-organs  from  which  come  the  nerve  impulses  most  essential  to 
the  excitation  of  the  cerebellar  coordinative  centers  in  their  control  of 
equilibrium  and  of  progression. 

The  cerebellum  may  therefore  be  regarded  as  the  essential,  most 
highly  differentiated  portion  of  the  coordinating  mechanism  con- 
cerned in  the  maintenance  of  equilibrium,  during  both  station  and 
progression.  The  manner  in  which  the  cerebellum  accomplishes 
this  result  is  unknown,  though  it  is  certain,  from  the  foregoing  facts, 
that  its  special  mode  of  activity  is  dependent  on  the  excitatory  action 
of  nerve  impulses  reflected  from  a  variety  of  peripheral  sense-organs. 


CHAPTER  XXIII. 
THE  CRANIAL  NERVES. 

The  nerve-trunks  which  serve  as  channels  of  communication 
between  the  encephalon  and  the  structures  of  the  head,  the  face, 
and  in  part  the  organs  of  the  thorax  and  abdomen,  pass  through  for- 
amina in  the  walls  of  the  cranium,  and  for  this  reason  are  termed 
cranial  nerves. 

According  to  the  classification  now  generally  adopted,  there  are 
twelve  cranial  nerves  on  either  side  of  the  median  line,  which,  enu- 
merated from  before  backward,  are  as  follows  (Fig.  262) : 

First  or  Olfactory.  Seventh  or  Facial. 

Second  or  Optic'  Eighth  or  Auditory. 

Third  or  Oculo-motor.  Ninth  or  Glosso-pharyngeal. 

Fourth  or  Patheticus.  Tenth  or  Pneumogastric  or  Vagus. 

Fifth  or  Trigeminal.  Eleventh  or  Spinal  Accessory. 

Sixth  or  Abducens.  Twelfth  or  Hypoglossal. 

The  cranial  nerves  may  be  classified  physiologically  in  accordance 
with  their  functional  manifestations  into  three  groups,  viz. : 

1.  Nerves  of  Special  Sense:    e.g.,  Olfactory,  Optic,  Auditory,  Gustatory  (Glosso- 

pharyngeal). 

2.  Nerves  of  General   Sensibility:    e.g.,   Large  root  of   the  Trigeminal,    Glosso- 

pharyngeal, and  Pneumogastric. 

3.  Nerves  of  Motion :  e.g.,  Oculo-motor,  Patheticus,  the  small  root  of  the  Trigeminal, 

Abducens,  Facial,  Spinal  Accessory,  and  Hypoglossal. 

Though  this  classification  in  the  main  holds  true,  it  must  be  borne  in 
mind  that  modern  investigations  have  demonstrated  that  the  glosso- 
pharyngeal and  pneumogastric  nerves  contain  even  at  their  junction 
with  the  medulla  oblongata  a  number  of  efferent  or  motor  fibers, 
and  to  this  extent  are  mixed  nerves. 

The  Origins  of  the  Cranial  Nerves. — In  accordance  with  modern 
views  as  to  the  origins  of  nerves  in  general,  it  may  be  stated  that— 

The  nerves  of  special  sense  have  their  origin  respectively  in  the 
neuro-epithelial  cells  in  the  mucous  membrane  of  the  olfactory  region 
of  the  nose,  in  the  ganglion  cells  of  the  retina,  in  the  cells  of  the  spiral 
ganglion  of  the  cochlea  and  the  ganglion  of  Scarpa,  and  in  the  cells 
of  the  petrous  and  jugular  ganglia.  From  the  cells  of  these  ganglia 
dendrites  pass  peripherally  to  become  associated  with  specialized 
end-organs,  while  axons  pass  centrally  in  well-defined  bundles  to 
become  related  by  means  of  their  end-tufts  with  primary  basal  ganglia. 

The  nerves  of  general  sensibility  have  their  origin  in  the  ganglia 
on  their  trunks,  and  in  this  respect  resemble  the  spinal  nerves.  From 
the  ganglion  cell  there  emerges  a  short  axon  process  which  soon 
divides  into  a  central  and  a  peripheral  branch.     The  former  passes 

37  577 


78 


TEXT-BOOK  OF  PHYSIOLOGY. 


toward  and  into  the  gray  matter  located  beneath  the  floor  of  the  fourth 
ventricle,  where  its  end-tufts  arborize  about  nerve-cells.  The  latter 
(the  peripheral  branch)  passes  toward  the  general  periphery  to  be 
distributed  to  skin  and  mucous  membranes  (Fig.  263). 

The  nerves  of  motion  have  their  origin  in  the  nerve-cells  in  the  gray 
matter  beneath  the  aqueduct  of  Sylvius  and  beneath  the  flo6r  of  the 
fourth  ventricle  (Fig.  264).  The  axons  emerging  from  these  cells 
course  peripherally  to  be  distributed  to  skeletal  muscles.     In  some  of 

the  motor  nerves,  and  in  some  sen- 
sory nerves  as  well,  there  are  to  be 
found  efferent  fibers  of  smaller  size 
which  have  a  similar  origin  and 
which  become  related  through  the 
intervention  of  sympathetic  ganglia 
(peripheral  neurons)  with  visceral 
muscles  and  glands.  These  nerves 
have  been  termed  autonomic  nerves. 
The  Cortical  Connections  of 
the  Cranial  Nerves. — Each  of 
these  three  groups  of  cranial  nerves 
has  special  connections  with  the 
cerebral  cortex. 

The  nerves  0]  special  sense  for 
the  most  part  terminate  in  primary 
basal  ganglia,  around  the  cells  of 
which  their  central  end-tufts  arbor- 
ize. From  these  cells  axons  arise 
which  pass  upward  and  directly  or 
indirectly  come  into  physiologic  re- 
lation with  sensor  nerve-cells  in  the 
cerebral  cortex. 

The  nerves  of  general  sensibility 
terminate  in  the  gray  matter  be- 
neath the  floor  of  the  fourth  ventri- 
cle, around  the  nerve-cells  of  which 
their  end-tufts  arborize.  These 
groups  of  nerve-cells  are  known  as  sensor  end-nuclei.  Though  once 
regarded  as  the  centers  of  origin  of  the  sensor  nerves,  they  are  now 
regarded  as  the  centers  of  origin  of  axons  which  pass  upward  to  the 
cortex  of  the  cerebrum,  where  they  also  come  into  physiologic  relation 
with  sensor  nerve-cells. 

The  axons  in  both  of  these  classes  of  nerves  thus  originate  in  the 
cells  of  the  central  nerve  system  and  continue  upward  to  the  cere- 
brum, the  primary  afferent  path. 

The  motor  nerves  which  have  their  origin  in  the  cells  of  the  gray 
matter  beneath  the  aqueduct  of  Sylvius  and  beneath  the  floor  of  the 
fourth    ventricle   are    in   physiologic   relation   with    nerve-cells   in   the 


Fig.  262. — Superficial  Origin  of 
the  Cranial  Nerves  from  the  Base 
of  the  Encephalon.  i.  Olfactory.  2 
Optic.  3.  Motor  oculi.  4.  Patheticus. 
5.  Trigeminal.  6.  Abducens.  7.  Facial. 
7'.  Nerve  of  Wrisberg.  8.  Auditory.  9. 
Glosso-pharyngeal.  10.  Pneumogastric. 
11.  Spinal  accessory.  12.  Hypoglossal. — 
'  Morat  and  Doyon.) 


THE  CRANIAL  NERVES. 


579 


motor  region  of  the  cortex  through  descending  axons  contained  in  the 
pyramidal  tract,  the  end-tufts  of  which  arborize  around  the  nerve- 
cells.  The  efferent  path  beginning  in  the  cerebral  cortex  is  thus 
continued  by  the  motor  nerves  to  the  general  periphery. 

The  three  groups  of  nerves,  those  of  special  sense,  of  general 
sensibility,  and  the  motor  nerves,  are  neurons  of  the  first  order;  the 


Fig.  263. — Ganglia  of  Origin  of  the 
Sensor  Cranial  Nerves,  i.  Trigem- 
inal (ganglion  of  Gasser).  2.  Nerve  of 
Wrisberg.  3.  Auditory  (ganglion  of 
Scarpa).  4.  Glosso-pharyngeal  (gang- 
lion of  Andersch).  5.  Pneumogastric 
(ganglion  plexiformis). — {After  Moral  and 
Doyon.) 


Pig.  264. — Nuclei  of  Origin  of  the 
Motor  Cranial  Nerves,  i.  Motor 
oculi.  2.  Patheticus.  3.  Motor  root  of 
the  trigeminal.  4.  Abducens.  5.  Fa- 
cial. 6.  Mixed  nucleus  for  efferent  fibers 
of  the  glosso-pharyngeal  vagus  and  spinal 
accessory.  7.  Ffypoglossus.  S.  Spinal 
accessory,  q.  Spinal  nerves. — {After 
Moral  and  Doyon.) 


nerve-cells  and  fibers  which  constitute  the  cerebral  connections  are 
neurons  of  the  second  order.  It  is  probable  that  the  sensor  cells  in 
the  cerebral  cortex  are  neurons  of  a  third  order. 

FIRST  PAIR.     THE  OLFACTORY. 

The  first  cranial  nerve,  the  olfactory,  is  situated  in  the  upper 
third  of  the  nasal  fossa,  in  the  regio  olfactoria.  It  consists  of  from  20 
to  30  branches,  the  fibers  of  which  are  non-medullated. 

Origin. — The  olfactory  nerve  is  composed  of  centrallv  coursing 


58o 


TEXT-BOOK  OF  PHYSIOLOGY. 


axons  which  have  their  origin  in  the  central  ends  of  bipolar,  rod- 
shaped,  or  spindle-shaped  nerve-cells  interspersed  among  the  epithelial 
cells  covering  the  mucous  membrane  in  the  regio  olfactoria;  the 
peripheral  ends  of  these  cells  give  off  a  number  of  dendrites  which  are 
spread  out  to  form  a  delicate  feltwork  over  the  surface  of  the  mucous 
membrane.  From  their  origin  the  axons  gradually  converge  to  form 
bundles  which  ascend  to  the  cribriform  plate  of  the  ethmoid  bone, 
through  the  foramina  of  which  they  pass  to  become  related  by  their 

end-tufts  with  structures  in 
the  gray  matter  of  the  olfac- 
tory bulb  (Fig.  265). 
6  Cortical  Connections. 

—The  olfactory  bulb  and 
.  olfactory  tract,  formerly 
called  the  olfactory  nerve, 
are  portions  of  the  cere- 
brum (the  olfactory  lobe) 
which  arise  embryologically 
by  a  protrusion  of  the  walls 
of  the  cerebral  cavity.  The 
bulb  is  oval-shaped  and 
consists  of  both  gray  and 
white  matter.  It  rests  on 
the  cribriform  plate  of  the 
ethmoid  bone  and  is  em- 
braced by  the  olfactory 
nerves.  As  seen  on  sagittal 
section,  there  is  just  be- 
neath the  surface  a  layer  of 
large  pyramidal  and  spin- 
dle-shaped cells  (termed 
also  mitral  cells),  each  pro- 
vided with  an  apical  and 
The  apical  dendrite  passes  toward  the  surface 
or  basket-like  expansion  which  interlaces  with 


Fig.  265. — The  Relation  oe  the  Olfactory 
Nerves  to  the  Oleactory  Tract,  i.  Ol- 
factory nerve-cell.  2.  Axon  process.  3.  Epi- 
thelial cells.  4.  Glomerulus.  5.  Mitral  cells. 
6.  Centrally  coursing  axons  of  the  olfactory 
tract. — (Morat  and  Doyon.) 


two  lateral  dendrites. 

and  ends  in  a  brush 

the  end-tufts  of  the  olfactory  nerves,  forming  what  are  known  as  the 

olfactory  glomerules.     The  lateral  dendrites  end  free. 

The  axons  of  the  pyramidal  cells  pass  toward  the  center  of  the 
bulb  and  bend  at  right  angles,  after  which  they  pursue  a  horizontal 
direction  toward  and  into  the  olfactory  tract.  This  tract  is  about  five 
centimeters  in  length,  prismatic  in  shape  on  cross-section  and  divisible 
into  a  ventral  and  a  dorsal  portion.  It  emerges  from  the  posterior 
extremity  of  the  bulb,  passes  backward  to  the  posterior  part  of  the 
anterior  lobe,  where  it  divides  into  three  roots:  viz.,  a  lateral  or  external, 
a  mesial  or  internal,  a  middle  or  dorsal.  The  fibers  of  the  lateral  and 
mesial  roots  are  derived  almost  exclusively  from  the  ventral  portion 
of  the  tract,  the  fibers  of  which  come  from  the  mitral  cells  in  the  bulb. 


THE  CRANIAL  NERVES. 


58i 


The  lateral  root-fibers  pass  outward  into  the  fossa  of  Sylvius  and  come 
into  relation  with  nerve-cells  in  the  inferior  extremity  of  the  gyrus 
hippocampus  and  the  gyrus  uncinates.  The  mesial  fibers  pass  inward 
and  come  into  relation  with  nerve-cells  in  the  pre-callosal  part  at  least 
of  the  gyrus,  fornicatus.  The  fibers  thus  far  considered  are  undoubt- 
edly true  olfactory  fibers,  pursuing  a  centripetal  direction,  carrying 
nerve  impulses  from  the  olfactory  cells  to  the  cerebrum  (Fig.  266). 

Histologic  and  embryologic  methods  of  research  have  shown  that 
some  of  the  fibers  in  the 
olfactory  tract  are  cen- 
trifugal in  direction. 
They  originate  in  the 
olfactory  cortical  areas, 
pass  toward  the  peri- 
phery as  far  as  the  an- 
terior commissure,  where 
they  cross  to  become  the 
dorsal  root,  enter  the 
olfactory  tract,  and 
finally  terminate  in  the 
bulb.  This  tract  serves 
to  connect  the  cortex 
v  ith  the  bulb  of  the  op- 
posite side,  and  carries 
impulses  from  the  cortex 
to  the  bulb.  The  two 
opposite  cerebral  olfac- 
tory areas  are  also  united 
by  commissural  fibers 
which  decussate  at  the 
anterior  commissure. 

Function. — The 
function  of  the  olfactory 
system  in  its  entirety,  is 
the  transmission  of  nerve 
impulses  from  its  origin 
in  the  olfactory  region  of 

the  nose  to  the  cerebral  cortex,  where  they  evoke  sensations  of  odor. 
The  stimulus  to  its  excitation  is  the  impact  and  chemic  action  of 
gaseous  or  volatile  organic  matter  on  the  dendrites  of  the  olfactory  cells. 
The  sensitiveness  of  the  olfactory  end-organ  to  the  action  of  many  suit- 
stances  is  remarkable,  responding,  for  example,  to  the  T2000 ¥o~  °f  a 
gram  of  oil  of  roses  and  to  the  276^00Tr  of  a  gram  of  mercaptan. 

Division  or  destruction  of  the  olfactory  path  at  any  point  is  followed 
by  an  abolition  of  the  sense  of  smell  on  the  corresponding  side.  De- 
structive lesions  of  the  hippocampal  and  uncinate  gyri  are  followed 
by  similar  results. 


Fig.  266. — Olfactory  Lobe  of  the  Human  Brain. 
— Bu.  Olfactory  bulb.  T.  Tract.  Tr.o.  Trigone.  R. 
Rostrum  of  corpus  callosum.  p.'  Peduncle  of  corpus 
callosum,  passing  into  G.  s.,  gyrus  subcallosus  (diagonal 
tract,  Broca).  Br.  Broca's  area.  F.p.  Fissura  prima. 
F.s.  Fissura  serotina.  C.a.  Position  of  anterior  com- 
missure. L.t.  Lamina  terminalis.  Ch.  Optic  chiasma. 
T.o.  Optic  tract,  p.  olf.  Posterior  olfactory  lobule  (or 
anterior  perforated  space),  m.r.  Mesial  root.  l.r. 
Lateral  root  of  tract.— (His.) — (After  Qiiain.) 


<82 


TEXT-BOOK  OF  PHYSIOLOGY. 


SECOND  PAIR.     THE  OPTIC. 

The  second  cranial  nerve,  the  optic,  consists  of  centrally  coursing 
axons  of  neurons,  which  connect  the  essential  part  of  the  organ  of 
vision,  the  retina,  with  sensory  end-nuclei  or  ganglia  situated  at  the 
base  of  the  cerebrum. 

Origin. — The  axons  which  constitute  the  optic  nerve  have  their 
origin  in  the  ganglionic  cells  in  the  anterior  part  of  the  retina.  Through 
their  dendrites  these  cells  are  brought  into  relation  posteriorly  with 
successive  layers  of  cells  which  collectively  constitute  the  retina. 
Though  the  retina  is  said  to  consist  of  ten  or  eleven  layers,  it  may  be 
reduced  practically  to  three,  viz.  (Fig.  267): 

1.  The  layer  of  visual  cells. 

2.  The  layer  of  bipolar  cells. 

3.  The  layer  of  ganglionic  cells. 
The  visual  cells  present  peripherally  modified 

dendrites,  known  as  the  rods  and  cones;  centrally 
they  give  off  an  axon  which  after  a  short  course 
terminates  in  an  end-tuft.  The*  bipolar  cells  also 
possess  dendrites  and  an  axon;  the  former  interlace 
with  the  end-tufts  of  the  visual  cell  axon,  the  latter 
with  the  dendrites  of  the  ganglion  cell.  The  retina 
may  be  regarded  therefore  as  the  peripheral  end- 
organ  in  which  the  optic  nerve  originates.  From 
their  origin  the  axons  turn  backward,  at  the  same 
time  converging  to  form  a  distinct  bundle  which 
passes  through  the  chorioid  coat  and  sclera.  After 
emerging  from  the  eyeball  the  nerve-bundle  (the 
optic  nerve)  passes  backward  as  far  as  the  sella 
turcica,  traversing  in  its  course  the  orbit  cavity  and 
the  optic  foramen.  At  the  sella  turcica  there  is  a 
union  and  partial  decussation  in  man  and  other 
mammals  of  the  two  nerves,  forming  the  optic 
chiasm* 

Decussation  of  the  Optic  Nerves. — The  ex- 
tent to  which  the  fibers  from  each  eye  decussate  at  the  chiasm  is  a 
subject  of  dispute  but  the  results  of  various  methods  of  research  would 
seem  to  indicate,  that  the  fibers  from  the  nasal  third  of  the  retina  of 
the  left  eye  cross  in  the  chiasm,  to  unite  with  the  fibers  from  the  tem- 
poral two-thirds  of  the  retina  of  the  right  eye.  In  a  similar  manner 
the  libers  from  the  nasal  third  of  the  retina  of  the  right  eye  cross  in 
the  chiasm,  and  unite  with  the  fibers  from  the  temporal  two-thirds  of 

*  Though  the  foregoing  is  the  usual  method  of  stating  the  origin  and  course  of  the  optic 
nerve,  nevertheless  morphologically  the  true  optic  nerve  lies  wholly  within  the  retina  and 
compo  ed  of  the  visual  cells  there  found.  The  remainder  of  the  visual  system  from 
and  including  the  ganglion  cells  of  the  retina  to  the  optic  basal  ganglia,  is  the  optic  tract, 
being  no  anatomi*  or  physiologic  distinction  between  the  optic  nerve  so  called  and 
the  optic  tract.  Both  are  outgrowths  from  the  brain  and  hence  possess  properties  which 
differentiate  them  from  other  cranial  nerves. 


Fig.  267. — Reti- 
nal Cells,  s',  z'. 
Visual  cells  with 
their  peripheral  ter- 
minations. 5.  Rods. 
2.  Cones,  b.  Bi- 
polar cells,  g.  Gan- 
glion cells  from  which 
arise  the  axons  of  the 
optic  nerve. 


THE  CRANIAL  NERVES. 


583 


the  retina  of  the  left  eye  (Fig.  268).  Posterior  to  the  chiasm  the  crossed 
and  uncrossed  fibers  form  the  so-called  optic  tracts,  which  after  winding 
around  the  crura  cerebri  enter  the  optic  basal  ganglia.  Transection  of 
the  optic  nerve  shows  that  it  is  composed  of  an  enormous  number  of 
non-medullated  nerve-fibers,  estimated  by  Salzer  at  from  450,00c  to 
800,000,  enclosed  in  a  sheath  of  the  dura  mater. 

The  visual  fibers  comprising  the  optic  nerve  may  be  physiologicly 
divided  into  two  classes,  (a)  those  coming  from  the  peripheral  portion  of 
the  retina,  and  (b)  those  com- 
ing from  that  central  area  VISUAL  HOD  VISUAL  HELD 
known  as  the  macula  lutea. 
The  retinal  fibers  are  by  far 
the  more  abundant,  and  make 
up  the  major  portion  of  the 
nerve;  the  macular  fibers  are 
less  abundant.  An  examina- 
tion of  a  cross-section  of  the 
optic  nerve  shows  the  presence 
of  a  wedge-shaped  tract  occu- 
pying the  center  of  the  nerve 
and  which  is  regarded  as  com- 
posed of  the  macular  fibers 
At  the  chiasm  this  bundle  of 
fibers  undergoes  a  partial  de- 
cussation similar  to  that  of  the 
fibers  coming  from  the  more 
peripheral  portions  of  the 
retina.  In  the  left  optic  tract, 
therefore,  fibers  from  at  least 
four  different  regions  are  to  be 
found:  viz.,  the  two-thirds  of 
the  temporal  side  of  the  left 
retina,  the  temporal  half  of 
the  left  macula,  the  nasal  third 
of  the  right  retina,  and  the 
nasal  half  of  the  right  macula. 
Corresponding  fibers  are  to  be 
found  in  the  right  optic  tract. 

As  the  optic  tract  passes  around  the  crus  cerebri  it  divides  into  a  lateral 
or  outer,  and  a  mesial  or  inner  bundle,  which  then  terminate  in  the 
optic  basal  ganglia.  The  fibers  of  the  lateral  bundle  are  traceable 
into  the  lateral  or  external  geniculate  body  (the  pre-geniculum) ,  the 
pulvinar  of  the  optic  thalamus,  and  the  anterior  quadrigeminal  body 
(the  pre-geminum).  These  are  in  all  probability  the  true  visual 
fibers.  The  fibers  of  the  mesial  bundle  are  traceable  into  the  internal 
geniculate  body  (the  post-geniculum)  and  the  posterior  quadrigeminal 
body  (the  post-geminum). 


Fig.  268. — Diagram  Illustrating  Left 
Homonymous  Lateral  Hemianopsla  from 
a  Lesion  of  the  Right  Optic  Tract  or 
the  Right  Cuneus.  The  Shaded  Lixes 
in  the  Visual  Fields  Indicate  the 
Darkened  Area. 


584  TEXT-BOOK  OF  PHYSIOLOGY. 

Cortical  Connections. — After  entering  the  basal  ganglia  the 
visual  fibers  terminate  in  end-tufts  which  arborize  around  nerve-cells. 
From  these  cells  new  axons  arise  which  ascend  through  the  posterior 
part  of  the  internal  capsule,  at  the  same  time  curving  backward  to  form 
the  optic  radiation  of  Gratiolet,  and  terminate  finally  around  nerve- 
cells  in  the  gray  matter  of  the  cuneus  and  in  the  gray  matter  bordering 
the  calcarine  fissure,  both  situated  on  the  mesal  aspect  of  the  occipital 
lobe. 

Centrifugal  Fibers  of  the  Optic  Nerve. — All  the  fibers  previously 
alluded  to  have  been  afferent  or  centripetal  in  direction;  but  the  optic 
nerve  also  contains  efferent  or  centrifugal  fibers  which  come  from 
nerve-cells  in  the  basal  ganglia  and  ramify  around  special  cells,  the 
amacrine  cells,  in  the  retina.  Their  function  is  unknown.  It  has 
been  suggested  that  they  regulate  the  vascular  supply  to  the  retina. 
Centrifugally  coursing  fibers  also  connect  the  visual  areas  of  the  cortex 
with  the  basal  ganglia. 

Function. — The  function  of  the  visual  apparatus  in  its  entirety 
is  the  transmission  of  nerve  impulses  from  the  retina  to  the  cerebral 
cortex  where  they  evoke  the  sensations  of  light  and  its  different 
qualities — colors.  The  specific  physiologic  stimulus  to  the  retinal 
visual  cells  is  the  impact  of  the  undulations  of  the  ether.  In  general 
it  may  be  said  that,  at  least  for  the  same  color,  the  intensity  of  the 
objective  undulation  or  vibration,  determines  the  intensity  of  the  sen- 
sation. 

Pupillary  Fibers. — The  optic  nerve  also  contains  nerve-fibers 
somewhat  larger  in  caliber  than  the  usual  visual  fibers,  which  are 
supposed  to  form  the  afferent  path  for  those  nerve  impulses  which 
excite  reflexly  a  contraction  of  the  sphincter  pupillce  muscle,  thus  vary- 
ing the  size  of  the  pupil.  These  fibers,  termed  pupillary  fibers,  come 
from  all  portions  of  the  retina  but  most  abundantly  from  the  posterior 
pole  in  and  around  the  macula.  The  existence  of  these  fibers  is. 
confirmed  by  pathologic  findings.  In  a  manner  similar  to  that  of  the 
visual  fibers  they,  too,  undergo  a  decussation  in  the  optic  chiasm,  so 
that  in  the  optic  tract  there  are  pupillary  fibers  which  come  from  the 
temporal  side  of  the  eye  of  the  corresponding  side,  and  fibers  which 
come  from  the  nasal  side  of  the  eye  of  the  opposite  side  (Fig.  272). 
The  central  termination  of  these  fibers  is  not  positively  known. 

Hemiopia  and  Hemianopsia. — Division  of  the  optic  nerve 
between  the  eyeball  and  the  optic  chiasm  is  followed  by  complete 
blindness  in  the  eye  of  the  corresponding  side.  Owing  to  the  partial 
decussation  of  the  fibers  in  the  chiasm,  division  of  an  optic  tract  is  fol- 
lowed by  a  loss  of  sight  in  the  outer  two-thirds  of  the  eye  of  the  same  side 
and  in  the  inner  third  of  the  eye  of  the  opposite  side.  To  this  loss  of 
visual  power  in  the  retina  the  term  hemiopia  is  given.  In  consequence  of 
this  loss  of  visual  power  in  the  retina  there  is  a  corresponding  obscura- 
tion or  total  obliteration  of  nearly  one-half  of  the  visual  field,  to  which 
the  term  hemianopsia  is  given.     Jf,  for  example,  the  right  optic  tract 


THE  CRANIAL  NERVES. 


58; 


is  divided  there  will  be  hemiopia  in  the  outer  two-thirds  of  the  right 
eye  and  the  inner  third  of  the  left  eye,  with  left  lateral  hemianopsia, 
and  as  the  portions  of  the  retina  which  are  affected  are  associated  in 
vision  the  loss  of  the  visual  fields  is  spoken  of  as  homonymous  hemi- 
anopsia (Fig.  268).  A  destructive  lesion  of  the  cerebral  visual  area, 
the  cuneus  and  the  adjacent  gray  matter  on  the  right  side,  is  also 
followed  by  left  lateral  hemianopsia.* 

The  existence  of  an  homonymous  hemianopsia  becomes  evident 
when  the  individual  is  directed  to  focus  the  vision  on  an  object  placed, 
directly  in  front  and  with  its 
center  in  the  median  plane  of 
the  body,  when  if  the  lesion  be 
on  the  right  side,  the  left  half 
of  the  object  will  be  invisible. 
The  reason  for  this  will  be  ap- 
parent on  reference  to  Fig.  269. 
All  the  light  rays  emanating 
from  the  left  half  of  the  object 
fall  on  the  retina  on  the  side  of 
the  injury,  and  hence  there  will 
be  no  sensation.  If,  however, 
the  object  be  moved  to  the  right 
without  change  in  the  position 
of  the  head,  the  entire  object 
will  be  visible,  as  all  the  rays 
fall  on  the  normal  side.  If,  on 
the  contrary,  the  object  be 
moved  to  the  left,  it  will  be  in- 
visible for  the  opposite  reason. 

Hemianopsia  may  be  the 
result  of  either  destruction  of 
the  optic  tract  or  of  the  cortical 

visual  area.  The  seat  of  lesion  in  any  given  case  is  indicated  by  a 
peculiarity  of  the  iris  reflex  pointed  out  by  Wernicke,  which  will  be 
referred  to  in  connection  with  the  consideration  of  the  oculo-motor 
nerve. 

THIRD  PAIR.     THE  OCULO-MOTOR. 

The  third  cranial  nerve,  the  oculo-motor,  consists  of  some  15,000 
peripherally  coursing  nerve-fibers  which  serve  to  bring  the  nerve-cells 
from  which  they  arise  into  relation  with  a  large  portion  of  the  general 
musculature  of  the  eye. 

Origin. — The  axons  composing  the  third  nerve  arise  from  a  series 

*  It  should  be  borne  in  mind  that  in  both  instances  the  retina  itself  is  unaffected.  The 
impact  of  light  generates,  as  usual,  nerve  impulses  which  proceed  as  far  backward  as  the 
point  of  division  or  destruction.  In  consequence  those  portions  of  the  cerebral  cortex 
stimulation  of  which  evokes  the  sensation  of  light  remain  unaffected  and  the  individual 
does  not  become  aware  through  sensation,  of  the  presence  of  a  luminous  body  in  the  left 
side  of  the  visual  field. 


Fig.  269. — Diagram  to  Show  the  Exist- 
ence of  Hemianopsia.  The  lesion  is  sup- 
posed to  be  in  the  right  optic  tract. 


;S6 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  seven  or  eight  groups  of  nerve-cells,  located  in  the  gray  matter 
beneath  the  floor  of  the  aqueduct  of  Sylvius.  From  each  of  these 
groups  or  nuclei,  bundles  of  axons  emerge,  which  after  a  short  course 
unite  to  form  the  common  trunk.     The  large  majority  of  the  fibers  in 

the  nerve  come  directly  from  the  nuclei 
of  the  same  side;  the  remainder  come 
from  a  group  of  cells  on  the  opposite 
side  of  the  median  line.  There  is 
thus  a  partial  decussation  of  its  fibers 
(Fig.  270). 

The  different  groups  of  cells,  the 
nuclei  of  origin,  are  arranged  in  a  serial 
manner.  The  anatomic  arrangement 
of  these  nuclei  would  indicate  that 
each  nucleus  is  related  to  an  individual 
member  of  the  eye-group  of  muscles. 
Clinical  observation  and  the  investiga- 
tion of  the  results  of  pathologic  pro- 
cesses have  not  only  shown  that  this 
is  the  case,  but  have  succeeded  in 
locating  the  position  of  the  nucleus 
for  any  given  muscle.  Though  there 
is  some  difference  of  opinion  in  regard 
to  the  exact  location  of  one  or  two  of 
the  nuclei,  the  tabulation  subjoined  is 
approximately  correct. 

Enumerating  them  from  before 
backward,  the  nuclei  occur  in  the 
following  order: 

The  sphincter  pupilke. 


Fig.  270. — Diagrammatic  View  of 
the  Situation  and  Relation  of 
the  Nuclei  of  Origin  of  the 
oculo-motor  and  patheticus 
(Trochlearis)  Nerves.  The  oculo- 
motor nuclei  consist  of  an  anterior 
nucleus,  the  Edinger-Westphal  nucleus 
(a  and  b),  and  a  posterior  nucleus; 
the  posterior  nucleus  has  a  dorsal,  a 
ventral,  and  a  mesial  portion;  the  de- 
cussation of  fibers  from  the  dorsal 
portion  of  the  posterior  nucleus  is  also 
shown.  The  decussation  of  the  fibers 
of  the  fourth  nerve  is  also  represented. 
— (Edinger.) 


2.  The  tensor  chorioideas  (the  accom- 
modation nucleus). 

3.  The  convergence  nucleus,  a  com- 
mon nucleus  for  the  conjoint  ac- 
tion of  the  two  internal  recti 
muscles. 

4.  The  superior  rectus. 

5.  The  inferior  rectus. 

6.  The  levator  palpebral. 

7.  The  inferior  oblique. 

Cortical  Connections. — The  oculo-motor  nuclei  are  in  histologic 
and  physiologic  relation  with  the  motor  area  of  the  cerebrum.  Nerve- 
cells  in  the  cortex  give  off  axons  which,  entering  the  pyramidal  tract, 
descend  through  the  internal  capsule,  and  the  crus  cerebri,  from  which 
they  cross  to  the  opposite  side.  The  end-tufts  arborize  around  the 
nuclei  of  the  oculo-motor  nerve  with  the  exception  of  the  nucleus  for 
the  iris  sphincter. 


THE  CRANIAL  NERVES. 


537 


Distribution. — After  their  origin  the  axons  converge  to  form  a 
common  trunk,  which  emerges  from  the  base  of  the  encephalon,  on 
the  inner  side  of  the  cms  cerebri,  in  front  of  the  pons  Varolii.  The 
nerve  then  passes  forward  through  the  sphenoid  fissure  into  the  orbit 
cavity,  where  it  divides  into  a  superior  and  an  inferior  branch.  The 
former  is  distributed  to  the  superior  rectus  and  the  levator  palpebral 
muscles;  the  latter  is  distributed  to  the  internal  and  inferior  recti  and 
inferior  oblique  muscles  (Fig.  271). 

From  the  inferior  branch  a  short  bundle  of  fibers  passes  to  the 
ciliary  or  ophthalmic  ganglion,  where  they  terminate,  arborizing  around 
the  ganglion  cells.  These  fibers  are  smaller  in  size  than  those  con- 
stituting the  bulk  of  the  nerve  and  belong  to  the  system  known  as 
the  autonomic.  These  cells 
give  origin  to  new  axons,  the 
ciliary  nerves,  which  enter  the 
eyeball,  pass  forward  between 
the  sclera  and  chorioid  coat, 
and  terminate  in  the  ciliary 
muscle  and  the  sphincter  of  the 
pupil.  The  ciliary  nerves  are 
not  portions  of  the  third  nerve 
proper,  but  peripheral  sym- 
pathetic neurons.  As  the 
ciliary  ganglion  receives  fila- 
ments from  the  cavernous 
plexus  of  the  sympathetic  and 
filaments  which  become  a  part 
of  the  trigeminal  nerve,  it  is 
probable  that  the  ciliary 
nerves  contain  not  only  motor, 
but  vaso-motor  and  sensor 
fibers  as  well. 

Properties. — Stimulation  of  the  nerve  near  its  exit  from  the  en- 
cephalon is  followed  by  contraction  of  the  muscles  to  which  it  is  dis- 
tributed with,  the  following  results,  viz.: 

1.  Diminution  in  the  size  of  the  pupil. 

2.  Accommodation  of  the  eye  for  near  vision. 

3.  Elevation  of  the  upper  eyelid. 

4.  Internal  deviation  and  rotation  upward  and  inward  of  the  anterior 

pole  of  the  eye,  combined  with  a  small  amount  of  torsion  toward 

the  mesial  line,  due  to  preponderating  action  of  the  internal  rectus 

and  inferior  oblique  muscles. 
Division  of  the  nerve  either  experimentally  or  as  a  result  of  com- 
pression from  a  pathologic  cause  is  followed  by  a  relaxation  of  the 
muscles,  with  the  following  effects,  viz.: 
1.   Dilatation  of  the  pupil,  the  iris  responding  neither  to  light  nor  to 

efforts  of  accommodation. 


Fig.  271. — Intra-orbital  Portion  of  the 
Third  Nerve,  i.  Optic  nerve.  2.  Third 
nerve.  3.  Superior  branch.  4.  Inferior  branch. 
5.  Abducens.  6.  Trifacial.  7.  Ophthalmic 
branch  divided.  8.  Nasal  branch.  9.  Ciliary 
ganglion.  10.  Motor  branch  to  this  ganglion 
from  the  inferior  branch  of  the  third  nerve.  11. 
Sensory  fibers.  12.  Sympathetic  fibers.  13. 
Ciliary  nerves. — (Sappey.) 


588  TEXT-BOOK  OF  PHYSIOLOGY. 

2.  Loss  of  the  accommodative  power. 

3.  Falling  of  the  upper  eyelid  (ptosis). 

4.  External  deviation  and   rotation  downward   and   outward   of   the 

anterior  pole  of  the  eyeball  combined  with  a  small  amount  of  tor- 
sion toward  the  mesial  line  due  to  the  unopposed  action  of  exter- 
nal rectus  and  the  superior  oblique  muscles. 

5.  Double  vision  or  diplopia.     The  image  of  the  eye  of  the  paralyzed 

side  is  projected  to  the  opposite  side  of  the  true  image  and  to  the 
upper  part  of  the  visual  field.  Owing  to  the  slight  mesial  torsion 
the  false  image  is  inclined  away  from  the  true  image. 

6.  Immobility  and  slight  protrusion  of  the  eyeball. 

Function. — The  function  of  the  third  nerve  is  to  transmit  nerve 
impulses  from  the  nuclei  of  origin  to  all  the  muscles  of  the  eye  except 
the  external  rectus  and  superior  oblique  and  excite  them  to  activity. 
The  majority  of  the  ocular  movements,  the  power  of  accommodation, 
the  variations  in  the  size  of  the  pupil  in  accordance  with  variations 
in  the  intensity  of  the  light,  the  power  of  convergence  of  the  visual 
axes,  are  all  excited  by  the  transmission  of  nerve  impulses  by  the  con- 
stituent fibers  of  the  nerve  from  their  related  nuclei.  This  is  made 
evident  by  the  effects  which  follow  stimulation  and  division  of  the 
nerve  or  lesions  of  the  nuclei  themselves. 

The  central  nuclei  can  be  excited  to  activity  (1)  by  nerve  impulses 
descending  the  motor  tract,  from  the  cerebral  cortex,  (2)  by  nerve 
impulses  coming  through  various  afferent  nerves.  This  holds  true 
more  especially  for  the  sphincter  pupillse  nucleus. 

The  Iris  Reflex  or  the  Pupillary  Reflex. — These  are  terms 
applied  to  the  variations  in  the  size  of  the  pupil  that  follow  variations 
in  the  intensity  of  the  light.  In  the  absence  of  light  the  pupil  widely 
dilates,  due  largely  to  the  relaxation  of  the  sphincter  pupillce  muscle 
and  partly  to  a  contraction  of  the  radiating  fibers  of  the  iris  which 
collectively  constitute  the  dilatator  pupilla  muscle.  With  the  entrance 
of  light  into  the  eye,  the  pupil  diminishes  in  size,  in  consequence  of  the 
contraction  of  the  sphincter  pupillce  caused  by  a  stimulation  of  the 
peripheral  ends  of  .the  pupillary  fibers  of  the  retina,  the  degree  of  con- 
traction depending  within  limits  on  the  intensity  of  the  light. 

The  action  of  the  sphincter  pupillae  muscle  is  therefore  a  reflex 
action  and  involves  the  usual  mechanism,  viz. :  A  receptive  surface, 
the  retina;  afferent  nerves,  the  pupillary  fibers  in  the  optic  nerve; 
an  emissive  center,  the  sphincter  nucleus  of  the  motor  oculi  center; 
efferent  nerves,  including  fibers  in  the  trunk  of  the  motor  oculi  and 
in  the  ciliary  nerves;  and  a  responsive  organ,  the  muscle.  (See  Fig. 
272.)  That  this  is  the  mechanism  involved  in  this  reflex,  is  shown 
by  the  fact  that  when  any  portion  of  it  is  destroyed,  the  reflex  con- 
tractions of  the  sphincter  are  impaired  or  abolished. 

As  slated  in  a  preceding  paragraph  the  central  termination  of  the 
afferenl  pupillary  fibers  concerned  in  this  reflex  is  not  positively  known. 
No  one  has  as  yet  succeeded   in  tracing  these  fibers  directly  to  the 


THE  CRANIAL  NERVES. 


589 


sphincter  nucleus.  Experimental  and  pathologic  data  apparently 
disprove  the  probability  of  their  terminating  in  the  anterior  corpora 
quadrigemina.  It  has  been  shown,  however,  that  as  the  optic  tract 
approaches  its  termination  the  visual  and  the  pupillary  fibers  separate 
and  it  has  been  assumed  that  the  latter  come  into  anatomic  relation 
with  some  intercal- 
ated system  which  in 
turn  is  connected  with 
the  sphincter  nucleus. 
As  to  the  situation, 
origin  and  course  of 
this  system  nothing 
positively  is  known. 
There  is  some  evi- 
dence for  the  view 
that  these  two  sys- 
tems are  associated  by 
commissural  fibers. 

The  contraction  of 
the  sphincter  and  a 
diminution  in  the  size 
of  the  pupil,  may  be 
direct,  as  when  the 
light  which  enters  one 
eye  causes  a  reflex 
contraction  of  the 
sphincter  of  one  and 
the  same  side;  or  it 
may  be  indirect  or 
consensual,  as  when 
the  light,  which  enters 
one  eye  only,  causes 
a  contraction  of  the 
sphincter  not  only  in 
the  eye  of  the  same, 
but  in  the  eye  of  the 
opposite  side  also.  It 
is,  however,  highly 
probable  that  all  re- 
flex    contractions    of 


Pupillary  fibers 
in  Optic  Ti 

J)irecL. 

Crossed, 


'ht-Corp.Quad- 
Fostga/ig/ioriic     fibers. 

\Sup.Ceroical  Ganglion. 
Pnganylivnic  fibers. 


■■/-Tiwracic  A  (//(. 


Trail seitio/i  of  Spinal  Co/ri 


Fig.  272. — Diagram  Designed  to  Show  the  Mechanism 
of  the  Iris  Reflex.  The  central  termination  of  the 
pupillary  fibers  is  hypothetical. 


the  sphincter  muscles  are  consensual,  that  is,  bilateral  reflex  actions 
because  of  the  decussation  of  the  pupillary  fibers  at  the  chiasm.  Con- 
traction of  both  pupils  also  occurs  as  an  associated  movement  in  the 
convergence  of  the  eyes  during  accommodation. 

The  dilatation  of  the  pupil  is,  however,  not  due  exclusively  to  the 
relaxation  of  the  sphincter  pupillas  muscle,  but  partly  to  the  contraction 
of  the  dilatator  pupillas  muscle,  which  is  kept  normally  in  a  state  of 


5QO  TEXT-BOOK  OF  PHYSIOLOGY. 

tonic  contraction  by  impulses  emanating  from  a  nerve-center  in  the 
medulla  oblongata. 

The  axons  which  arise  in  this  center  pass  down  the  cord,  emerge 
through  the  first  thoracic  nerve,  and  then  ascend  to  the  superior 
cervical  ganglion  (see  Fig.  272),  in  which  their  terminal  branches 
arborize  around  its  nerve-cells.  From  these  cells  new  axons  of  the 
sympathetic  system  arise  which  pass  successively  to  the  ophthalmic 
division  of  the  fifth  nerve,  the  nasal  nerve,  the  long  ciliary  nerve  and 
he  iris. 

Experimental  research  renders  it  highly  probable  that  the  dilatator 
center  is  in  a  state  of  continuous  activity  and  the  dilatator  muscle  in 
a  state  of  tonic  contraction.  Whatever  the  normal  stimulus  may  be, 
the  center  is  increased  in  activity  by  dyspneic  blood,  by  severe  muscle 
exercise,  by  emotional  excitement,  and  by  stimulation  of  various 
sensor  nerves.  That  the  efferent  pathway  just  alluded  to  transmits 
the  impulses  to  the  iris  is  shown  by  the  fact  that  division  in  any  part 
of  the  course  is  followed  by  narrowing,  stimulation  by  active  dilatation 
of  the  pupil. 

The  variations  in  the  size  of  the  pupil,  though  largely  a  reflex 
act  under  the  control  of  the  oculo-motor  nerve,  are  nevertheless  partly 
due  to  the  active  cooperation  of  the  dilatator  nerves  and  their  related 
muscle.  The  size  of  the  pupil  necessary  from  moment  to  moment 
for  the  admission  of  just  that  amount  of  light  essential  to  the  formation 
and  perception  of  a  distinct  image  is  the  result  of  two  nicely  adjusted 
and  delicately  balanced  forces. 

Wernicke's  Hemianopic  Pupillary  Reaction. — It  was  stated 
on  page  585  that  a  modification  of  the  pupillary  reaction  is  observed  in 
some  cases  of  hemianopsia,  which  indicates  approximately  the  seat  of 
the  lesion.  This  reaction,  or  inaction  as  it  is  sometimes  called,  is 
present  when  the  lesion  is  along  the  course  of  the  optic  tract  between 
the  chiasma  and  the  anterior  quadrigeminal  body.  In  a  case  of  left 
lateral  hemianopsia,  the  lesion  being  in  the  right  optic  tract,  the  method 
of  testing  for  the  reaction  is  as  follows :  The  eye  of  the  left  side  is  first 
carefully  shielded  from  the  light.  A  fine  ray  of  light  is  then  projected 
into  the  right  eye  in  such  a  manner  that  it  falls  entirely  on  the  non- 
sensitive  (the  temporal)  side  of  the  retina.  There  will  be  an  absence 
of  the  usual  pupillary  response,  or  rather  the  pupil  remains  inactive; 
but  if  the  light  is  gradually  directed  towards  the  sensitive  (the  nasal) 
side  of  the  retina,  there  will  come  a  moment,  as  the  central  line  is  crossed 
and  the  light  falls  on  the  sensitive  side,  when  the  usual  pupillary  response 
manifests  itself,  viz. :  a  contraction  of  the  sphincter  pupillae  and  a 
diminution  in  the  size  of  the  pupil.  The  explanation  of  these  facts 
will  become  apparent  from  an  examination  of  Fig.  272  in  which  the 
course  of  the  pupillary  fibers  is  shown  and  especially  if  it  be  accepted 
that  these  fibers  at  their  central  terminations  decussate  or  are  in  relation 
either  directly  or  indirectly  with  the  sphincter  centers. 

The  eye  of  the  right  side  is  then  in  turn  shielded  from  the  light 


THE  CRANIAL  NERVES. 


591 


and  the  same  method  of  examination  is  carried  out.  In  this  case, 
however,  the  light  is  projected  first  on  the  nasal,  which  is  the  non- 
sensitive  side  of  the  retina;  there  will  again  be  no  response  in  the  pupil. 
But  if  the  light  is  gradually  directed  towards  the  sensitive  (the  temporal) 
side,  there  will  come  a  moment,  as  the  central  line  is  crossed  and  the 
light  falls  on  the  sensitive  portion  of  the  retina,  when  the  usual  pupillarv 
response  manifests  itself.  The  course  of  the  pupillary  fibers  in  this 
instance  will  also  become  apparent  from  an  examination  of  Fig.  272. 
It  is  evident,  however,  that  in  either  case  a  bilateral  pupillary  reaction 
will  follow  stimulation  of  the  sensitive  side  of  either  eye  because  of  the 
central  decussation  of  the  pupillary  fibers. 

FOURTH  PAIR.     THE  PATHETICUS. 

The  fourth  cranial  nerve,  the  patheticus,  consists  of  peripherally 
coursing  axons  which  serve  to  bring  the  cells  from  which  they  arise 
into  relation  with  the  superior 
oblique  muscle. 

Origin. — The  axons  of  this 
nerve  arise  from  a  group  of  cells 
located  beneath  the  aqueduct  of 
Sylvius  just  posterior  to  the  last 
nucleus  of  the  third  nerve.  xAiter 
emerging  from  the  nucleus  the 
nerve-fibers  pass  downward  for 
a  short  distance,  then  curve  dor- 
sally  around  the  aqueduct  of 
Sylvius,  and  enter  the  valve  of 
Vieussens,  where  they  completely 
decussate  with  the  nerve-fibers  of 
the  opposite  side. 

Cortical  Connections. — 
The  nucleus  of  the  pathetic 
nerve  is  in  histologic  and  physi- 
ologic connection  with  the  motor 
area  of  the  cerebral  cortex. 
Nerve-cells  in  this  region  give  off 
axons  which  enter  the  pyramidal 
tract  and  descend  through  the 
internal  capsule  and  the  crus 
cerebri,  after  which  they  cross  to 

the  opposite  side.  Their  end-tufts  arborize  around  the  cells  of  the 
nuclei  already  described. 

Distribution. — After  its  decussation  the  nerve-trunk  emerges 
just  below  the  posterior  quadrigeminal  body,  crosses  the  superior 
cerebellar  peduncle,  and  winds  around  the  crus  cerebri  to  the  anterior 
border  of  the  pons  Varolii.  It  then  enters  the  orbit  cavity  through 
the  sphenoid  fissure  and  finally  terminates  in  the  superior  oblique 


Fig.  273. — Distribution  of  the 
Patheticus.  I.  Olfactory  nerve.  II.  Op- 
tic nerves.  III.  Motor  oculi  communis.  IV. 
Patheticus,  by  the  side  of  V  the  ophthalmic 
branch  of  the  fifth,  and  passing  to  the  supe- 
rior oblique  muscle.  VI.  Motor  oculi  ex- 
ternus.  i.  Ganglion  of  Gasser.  2,  3,  4, 
5,  6,  7,  8,  9,  10.  Ophthalmic  division  of 
the  fifth  nerve,  with  its  branches. — (Hirscli- 
jeld.) 


592 


TEXT-BOOK  OF  PHYSIOLOGY. 


muscle.  In  its  course  the  nerve  receives  filaments  from  the  cavernous 
plexus  of  the  sympathetic  and  the  ophthalmic  division  of  the  trigeminal 
(Fig.  273). 

Properties. — Stimulation  of  the  nerve-trunk  is  followed  by  spas- 
modic contraction  of  the  superior  oblique  muscle,  the  anterior  pole 
of  the  eyeball  being  turned  downward  and  outward,  combined  with 
slight  torsion  away  from  the  middle  line. 

Division  of  the  nerve  is  followed  by  a  relaxation  or  paralysis  of 

the  muscle.  In 
consequence  of  the 
now  unopposed  ac- 
tion of  the  inferior 
oblique  muscle,  the 
anterior  pole  of  the 
eyeball  is  turned 
upward  and  inward 
with  slight  torsion 
toward  the  middle 
line.  The  diplopia 
consequent  upon 
this  paralysis  is 
homonymous,  the 
images  appearing 
one  above  the 
other.  The  image 
of  the  paralyzed 
eye  is  below  that  of 
the  normal  eye  and 
its  upper  end  in- 
clined toward  that 
of  the  normal  eye. 
Function. — The  function  of  this  nerve  is  to  transmit  nerve  im- 
pulses to  the  superior  oblique  muscle  and  to  excite  it  to  contraction. 


3  4 

Fig.  274. — Scheme  oe  Origin  and  Constitution  of  the 
Trigeminal  Nerve,  i.  Centrally  coursing  fibers.  2,  3, 
4.  Peripherally  coursing  fibers  of  the  cells  of  the  ganglion  of 
Gasser.  R,  N.  Nuclei  of  origin  of  the  efferent  fibers.  6. 
Motor  root.     Central  terminations  of  the  large  root. 


FIFTH  PAIR.     THE  TRIGEMINAL. 


The  fifth  cranial  nerve,  the  trigeminal,  consists  of  both  afferent  and 
efferent  axons  which  for  the  most  part  are  separate  and  distinct. 
The  afferent  axons  constitute  by  far  the  major  portion,  the  efferent 
fibers  the  minor  portion,  of  the  nerve. 

Origin  of  the  Afferent  Axons. — The  afferent  axons  have  their 
origin  in  the  monaxonic  cells  in  the  ganglion  of  Gasser,  which  rests 
on  the  apex  of  the"  petrous  portion  of  the  temporal  bone.  The  cells 
of  this  ganglion  give  origin  to  a  short  process  which  soon  divides  into 
two  branches,  one  of  which  passes  centrally,  the  other  peripherally 
(Fig.  274).  The  centrally  directed  branches  collectively  form  the  so- 
called  large  or  sensor  root;  the  peripherally  directed  branches  collec- 


THE  CRANIAL  NERVES. 


593 


tively  constitute  the  three  main  divisions  of  the  nerve:  viz.,  the  oph- 
thalmic, the  superior  maxillary,  and  the  inferior  maxillary.  Branches 
of  the  carotid  plexus  of  the  sympathetic  enter  the  nerve  in  the  neighbor- 
hood of  the  ganglion  of  Gasser  and  accompany  some  of  its  branches 
to  their  terminations. 

Distribution. — i.  The  Central  Branches. — The  axons  of  the 
large  root  pass  backward  into  the  pons  Varolii  on  its  lateral  aspect. 
After  entering  the  pons  each  axon  divides  into  two  branches,  one 
of  which  passes  upward  a  short  distance,  the  other  passes  downward, 
descending  as  far  as  the  second  cervical  segment.  Both  branches 
give  off  a  number  of  collaterals,  some  of  which  terminate  in  fine  end- 
tufts  around  nerve-cells  in  the  substantia  gelatinosa. 

2.  The  Peripheral  Branches. 
— The .  peripheral  axons  emerge 
from  the  peripheral  end  of  the 
ganglion  of  Gasser  in  three  dis- 
tinct and  separate  branches,  each 
of  which  is  distributed  to  a  differ- 
ent region  of  the  face  and  head. 
i.  The  ophthalmic  branch  passes 

forward  and  subdivides  into 

three    large    branches,    the 

frontal,  the  lachrymal,  and 

the    nasal.      The    ultimate 

termination  of  the  branches 

of  these  nerves  is  as  follows : 

viz.,    the    conjunctiva    and 

skin  of  the  upper  eyelid,  the 

cornea,  the  skin  of  the  fore- 
head    and     the    nose,    the 

lachrymal    gland    and    car- 
uncle,    and     the     mucous 

membrane  of  the  nose  (Fig. 

275)- 

2.  The  superior  maxillary  branch  passes  forward  through  the  foramen 

rotundum,  crosses  the  spheno-maxillary  fossa,  enters  the  infra- 
orbital canal,  and  emerges  at  the  infra-orbital  foramen.  In  its 
course  it  gives  off  a  number  of  branches  which  are  distributed 
as  follows:  viz.,  to  the  integument  and  conjunctiva  of  the  lower 
lid,  the  nose,  cheek,  and  upper  lip,  the  palate,  the  teeth  of  the 
upper  jaw,  and  the  alveolar  processes  (Fig.  276). 

3.  The  inferior  maxillary  branch   passes  through  the  foramen  ovale, 

after  which  it  subdivides  into  three  branches — the  auriculo-tem- 
poral,  the  lingual,  and  the  inferior  dental.  The  ultimate  branches 
are  distributed  as  follows:  viz.,  the  external  auditory  meatus,  the 
side  of  the  head,  the  mucous  membrane  of  the  mouth,  the  anterior 
portion  of  the  tongue,  the  arches  of  the  palate,  the  teeth  and 
38 


F16.  275. — Ophthalmic  Branch  of  the 
Fifth,  i.  Ganglion  of  Gasser.  2.  Oph- 
thalmic division  of  the  fifth.  3.  Lachrymal 
branch.  4.  Frontal  branch.  5.  External 
frontal.  6.  Internal  frontal.  7.  Supra- 
trochlear. 8.  Nasal  branch.  9.  External 
nasal.     10.    Internal  nasal. — (Hirschjeld.) 


594 


TEXT-BOOK  OF  PHYSIOLOGY 


alveolar  process  of  the  lower  jaw  and  the  integument  of  the  lower 

part  of  the  face  (Fig.  277), 
The  afferent   axons  thus  serve  to  bring  into  relation  the   skin, 
mucous  membranes  of  the  head  and  face,,  and  other  sentient  struc- 
tures, with  certain  sensor  end-nuclei  in  the  pons,  medulla  oblongata, 
and  adjoining  structures. 

Cortical  Connections. — The  afferent  portion  of  the  trigeminal 
nerve  is  brought  into  physiologic  relation  with  the  sensor  portion  of 
the  cerebral  cortex  by  means  of  nerve-fibers,  which  have  their  origin  in 
the  cells  around  which  the  terminal  branches  of  the  centrally  coursing 
fibers  arborize.     The  cells  situated  in  the  substantia  gelatinosa  give 


Fig.  276. — 1.  Superior  maxillary  nerve.  2,  3,  4,  5.  Dental  nerves.  6.  Spheno- 
palatine ganglion.  7.  \  idian  nerve.  8.  Large  superficial  petrosal.  9.  Carotid  branch 
of  large  petrosal.  10.  Oculo-motor.  11.  Superior  cervical  ganglion.  12.  Carotid 
branches  of  this  ganglion.  13.  Facial,.  14.  Glosso-pharyngeal.  15.  Jacobson's  nerve, 
and  16,  17,  iS,  19,  branches  to  the  sympathetic,  fenestra  rotunda,  Eustachian  tube.  20. 
Deep  external  petrosal.     21.  Deep  internal  petrosal. — (H'irschjeld.) 


off  axons,  which  after  a  short  course  cross  the  median  line,  enter  the 
fillet  and  then  ascend  in  the  general  sensor  tract  to  the  cortex  where 
they  in  turn  arborize  around  sensor  nerve-cells. 

Properties. — Irritative  pathlogic  lesions,  e.  g.,  pressure  by  tumors, 
aneurysms,  neuritis,  degenerative  changes  in  the  ganglion  cells,  or 
lesions  which  in  any  way  gradually  impair  the  physical  or  chemic 
integrity  of  the  nerve-fibers,  give  rise  to  a  variety  of  painful  sensations 
referable  to  the  seat  of  the  lesion  or  to  one  or  more  regions  in  the 
peripheral  distribution  of  the  nerve.  Many  of  the  various  forms  of 
trigeminal  neuralgia  are  caused  by  lesions  of  this  character.  Ex- 
posure of  the  dental  nerves  from  caries  of  the  teeth,  the  presence  of 
minute  foreign  bodies  in  the  conjunctiva,  operative  procedures  in  the 
nasal  chambers,  all  testify  to  the  extreme  sensibility  of  the  nerve. 
Division  of  the  large  root  within  the  cranium  is  followed  at  once  by 
complete  abolition  of  all  sensibility  in  the  head  and  face  to  which  its 
branches  arc  distributed.     The  skin  and  mucous  membranes,  the  eye, 


THE  CRANIAL  NERVES. 


595 


nose,  or  teeth  may  be  experimentally  injured  without  any  evidences 
of  pain  on  the  part  of  the  animal.  Various  reflexes,  e.  g.,  those  of 
mastication,  insalivation,  deglutition,  the  afferent  paths  of  which  are 
formed  in  part  by  the  fifth  nerve,  are  often  seriously  impaired.  At  the 
same  time  the  lachrymal  secretion  diminishes  and  the  pupil  contracts. 
The  same  results  are  observed  in  human  beings  in  whom  the  nerve 


v   . 


V 


JWL* 


Fig.  277. — Inferior  Maxillary  Branch  of  the  Trigeminal  Nerve,  i.  Branch 
to  the  masseter  muscle.  2.  Filament  of  this  branch  to  the  temporal  muscle.  3.  Buccal 
branch.  4.  Branches  anastomosing  with  the  facial  nerve.  5.  Filament  from  the  buccal 
branch  to  the  temporal  muscle.  6.  Branches  to  the  external  pterygoid  muscle.  7. 
Middle  deep  temporal  branch.  S.  Auriculotemporal  nerve.  9.  Temporal  branches. 
10.  Auricular  branches.  11.  Anastomosis  with  the  facial  nerve.  12.  Lingual  branch. 
13.  Branch  of  the  small  root  to  the  mylo-hyoid  muscle.  14.  Inferior  dental  nerve,  with 
its  branches  (15,  15).  16.  Mental  branch.  17.  Anastomosis  of  this  branch  with  the 
facial  nerve. — {Hirschfeld.) 


has  been  divided  for  relief  from  neuralgia.  Anesthesia  or  a  loss  of 
sensibility  may  also  be  caused  by  pathologic  lesions  of  the  nerve-trunks 
or  of  the  sensor  end-nuclei. 

Division  of  the  large  root  at  or  near  the  ganglion  of  Gasser  has 
not  infrequently  been  followed  by  an  alteration  in  the  nutrition  of  the 
eye  and  nose.  In  the  course  of  twenty-four  hours  the  eye  becomes 
vascular  and  inflamed;  the  cornea  becomes  opaque;  ulceration  sets 
in   which   may   lead   to   complete   destruction   of   the   eyeball.     The 


5g6  TEXT-BOOK  OF  PHYSIOLOGY. 

mucous  membrane  of  the  nose  becomes  swollen,  vascular,  and  liable 
to  hemorrhage  on  the  slightest  irritation.  The  degenerative  changes 
may  lead  to  a  complete  loss  of  the  sense  of  smell.  These  results  were 
formerly  attributed  to  a  loss  of  trophic  influence  which  it  was  believed 
the  nerve  exercised  over  these  structures.  Modern  experimentation 
and  various  surgical  procedures  have  demonstrated  that  the  nutritive 
disorders  are  septic  in  origin,  made  possible  by  the  anesthetic  condi- 
tion and  by  the  changed  vascular  supply  from  division  of  the  vaso- 
motor fibers  which  join  the  nerve  at  or  near  the  ganglion. 

Origin  of  the  Efferent  Axons. — The  efferent  axons  arise  for  the 
most  part  from  nerve-cells  located  in  the  gray  matter  beneath  the  upper 
half  of  the  floor  of  the  fourth  ventricle.  A  group  of  cells  known  as  the 
superior  or  accessory  nucleus,  situated  posterior  to  the  corpora  quad- 
rigemina,  give  origin  to  axons  which  descend  and  join  the  axons  from 
the  chief  motor  nucleus  (Fig.  274). 

Distribution. — From  their  origin  the  fibers  pass  forward  through 
the  pons  and  emerge  on  its  lateral  aspect,  forming  the  so-called  small 
root  of  the  fifth  nerve.  This  then  passes  forward  beneath  the'  ganglion 
of  Gasser,  leaves  the  cavity  of  the  skull  through  the  foramen  ovale, 
and  joins  the  inferior  maxillary  division  already  described.  Its 
axons  are  ultimately  distributed  to  the  muscles  of  mastication:  viz., 
the  masseter,  the  temporal,  the  external  and  internal  pterygoids,  the 
mylohyoid,  and  the  anterior  portion  of  the  digastric.  A  few  axons 
are  also  distributed  to  the  tensor  tympani  and  tensor  palati  muscles. 
The  efferent  or  peripherally  coursing  axons  thus  serve  to  bring  the 
nerve-cells  from  which  they  arise  into  relation  with  the  muscles  of 
mastication. 

Cortical  Connections. — The  nuclei  of  origin  of  the  small  root 
are  in  histologic  and  physiologic  relation  with  the  lower  third  of  the 
motor  area  of  the  cerebral  cortex.  Nerve-cells  in  this  region  give 
off  axons  which  enter  the  pyramidal  tract,  descend  through  the  in- 
ternal capsule  and  the  crus  cerebri,  after  which  they  cross  to  the 
opposite  side.  Their  end-tufts  arborize  around  the  cells  of  nuclei  in 
the  medulla  oblongata. 

Properties. — Stimulation  of  the  small  root  gives  rise  to  convulsive 
movements  of  the  muscles  of  mastication.  Division  of  the  nerve  is 
followed  by  a  paralysis  of  these  muscles.  Contraction  or  paralysis  of 
the  tensor  tympani  and  tensor  palati  muscles  would  also  be  observed 
under  the  same  conditions.  . 

Functions. — The  function  of  the  afferent  fibers  of  the  fifth  nerve 
is  the  transmission  of  nerve  impulses  from  its  peripheral  distribution 
to  (a)  the  medulla  oblongata;  (b)  through  its  afferent  cortical  tracts 
to  the  cerebral  cortex  where  they  evoke  sensations.  The  nerve  there- 
fore endows  all  the  parts  to  which  it  is  distributed  with  sensibility. 

The  function  of  the  efferent  fibers  is  the  transmission  of  nerve  im- 
pulses from  the  cells  from  which  they  take  their  origin,  to  the  muscles  of 
mastication  which  are  excited  to  activity  by  them.     The  afferent  nerves 


THE  CRANIAL  NERVES. 


597 


are  in  relation  centrally  with'  the  nuclei  of  origin  of  the  efferent  nerves, 
hence  the  latter  can  be  excited  not  only  voluntarily  but  refiexly  as  in 
the  usual  acts  of  mastication.  The  afferent  fibers  from  the  mouth 
doubtless  assist  in  the  reflex  secretion  of  saliva. 

Peripheral  stimulation  of  different  areas  in  the  distribution  of  the 
afferent  fibers,  e.  g.,  conjunctiva,  nasal  and  oral  mucous  membranes, 
teeth,  etc.,  causes  a  variety  of  reflex  activities  in  the  muscles  associated 
with  the  eyes,  face,  the  respiratory  and  cardiac  mechanisms,  which 
indicate  that  the  afferent  fibers  are  centrally  in  relation  with  a  number 
of  motor  nerve  centers. 

SIXTH  PAIR.     THE  ABDUCENS. 

The  sixth  cranial  nerve,  the  abducens,  consists  of  peripherally 
coursing  axons  which  serve  to  bring  the  nerve-cells  from  which  they 
arise  into  relation  with  the  external  rectus  muscle. 

Origin. — The  axons  arise  from 
a  group  of  cells  located  in  the  gray 
matter  beneath  the  upper  half  of 
the  floor  of  the  fourth  ventricle.  It 
is  quite  probable  that  a  few  fibers 
in  each  nerve-trunk  come  from  the 
nucleus  on  the  opposite  side  of  the 
middle  line. 

Distribution. — The  nerve- 
fibers  pass  forward  from  their 
origin  through  the  gray  and  white 
matter  and  emerge  through  the 
groove  between  the  medulla  oblon- 
gata and  the  pons  Varolii  just  ex- 
ternal to  the  anterior  pyramid.  The 
nerve  then  passes  through  the 
sphenoid  fissure  into  the  orbit 
cavity,  where  it  is  distributed  to 
the  external  rectus  muscle  (Fig. 
278).  In  its  course  the  nerve  re- 
ceives filaments  from  the  carotid 
plexus  of  the  sympathetic. 

Cortical  Connections. — The 
nucleus  of  the  sixth  nerve  is  in  histologic  and  physiologic  connection 
with  the  motor  area  of  the  cerebral  cortex.  From  nerve-cells  in  this 
region  axons  are  given  off  which  enter  the  pyramidal  tract,  descend 
through  the  internal  capsule  and  crus  cerebri,  after  which  they  cross 
to  the  opposite  side,  Avhere  their  end-tufts  arborize  around  the  cells 
of  the  nucleus  already  described. 

Properties. — Stimulation  of  the  nerve  is  followed  by  spasmodic 
contraction  of  the  external  rectus  muscle  and  external  deviation  of 
the  eyeball.     Division  of  the  nerve  is  followed  by  paralysis  or  relaxation 


Fig.  2  78. — Distribution  of  the 
Motor  Oculi  Externus  or  Abducexs. 
1.  Trunk  of  the  motor  oculi  communis, 
■with  its  branches  (2,  3,  4,  5,  6,  7).  8. 
Motor  oculi  externus,  passing  to  the  ex- 
ternal rectus  muscle.  9.  Filaments  of 
the  motor  oculi  externus  anastomosing 
with  the  sympathetic.  10.  Ciliary  nerves. 
—iHirsckfeld.) 


59S  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  muscle.  As  a  result  of  the  unopposed  action  of  the  internal 
rectus  the  anterior  pole  of  the  eyeball  is  turned  toward  the  middle 
line  (internal  strabismus).  In  consequence  of  this  deviation  there 
is  homonymous  diplopia.  The  images  are  on  the  same  level  and 
parallel.  The  image  of  the  paralyzed  eye  lies  external  to  that  of  the 
normal  eye. 

Function. — The  function  of  this  nerve  is  to  transmit  nerve  im- 
pulses to  the  external  rectus  muscle  and  excite  it  to  contraction. 

SEVENTH  PAIR.     THE  FACIAL. 

The  seventh  cranial  nerve,  the  facial,  consists  of  peripherally 
coursing  nerve-fibers,  which  serve  to  bring  the  nerve-cells  from  which 
they  arise  into  relation  with  most  of  the  superficial  muscles  of  the  head 
and  face. 

The  muscles  supplied  by  this  nerve,  as  stated  by  the  general 
anatomists,  are  as  follows:  The  occipito-frontalis,  corrugator  super- 
cilii,  orbicularis  palpebrarum,  levator  labii  superioris,  alaeque  nasi, 
zvgomatici,  the  pyramidalis  nasi,  the  compressor  nasi,  the  depressor 
alae  nasi,  levator  anguli  oris,  buccinator,  orbicularis  oris,  depressor 
anguli  oris,  depressor  labii  inferioris,  the  levator  menti,  the  posterior 
belly  of  the  digastric,  the  stylo-hyoid,  and  the  platysma  myoides. 

Origin. — The  nerve-fibers  or  axons  composing  the  seventh  nerve 
arise  for  the  most  part  from  a  nucleus  of  large  multipolar  nerve-cells 
situated  about  five  millimeters  beneath  the  upper  half  of  the  floor 
of  the  fourth  ventricle  toward  the  middle  line. 

From  this  nucleus,  which  is  about  four  millimeters  long,  axons 
emerge  which  at  first  pass  inward  and  backward  as  far  as  the  epen- 
dyma  of  the  ventricle;  they  then  turn  on  themselves,  forming  an  arch 
that  encloses  the  nucleus  of  the  sixth  nerve;  they  then  course  down- 
ward and  outward,  emerging  from  the  pons  at  its  lower  border  between 
the  olivary  and  restiform  bodies.  As  the  axons  approach  the  floor 
of  the  ventricle  collateral  branches  are  given  off  which,  crossing  the 
median  line,  arborize  around  the  nerve-cells  of  the  opposite  facial 
nucleus. 

Clinic  observations  and  histologic  investigations,  however,  render 
it  probable  that  the  fibers  distributed  to  the  occipito-frontalis,  the  cor- 
rugator supercilii,  and  the  upper  half  of  the  orbicularis  palpebrarum, 
are  derived  from  the  oculo-motor  nucleus,  and,  descending  the  posterior 
longitudinal  bundle,  enter  the  trunk  of  the  facial  as  it  turns  to  pass 
forward  through  the  pons.  It  is  also  probable,  for  similar  reasons, 
that  the  fibers  distributed  to  the  orbicularis  oris  are  derived  from  the 
hypoglossal  nucleus. 

Cortical  Connections. — The  nucleus  of  the  facial  nerve  is  in 
histologic  and  physiologic  connection  with  the  facial  region  of  the 
general  motor  area  of  the  cerebral  cortex.  From  the  cells  of  this 
region  axons  descend  through  the  pyramidal  tract,  the  internal  cap- 


THE  CRANIAL  NERVES. 


599 


sule,  and  the  cms  cerebri,  beyond  which  they  cross  to  the  opposite 
side  and  arborize  around  the  cells  of  the  nucleus  already  described. 

Distribution. — From  its  superficial  origin  the  trunk  of  the  nerve 
passes  into  the  internal  auditory  meatus  beside  the  auditory  nerve. 
After  passing  forward  and  outward  for  a  short  distance  through  the 


Fig.  279. — Superficial  Branches  of  the  Facial  and  the  Fifth. — 1.  Trunk  of  the 
facial.  2.  Posterior  auricular  nerve.  3.  Branch  which  it  receives  from  the  cervical 
plexus.  4.  Occipital  branch.  5,  6.  Branches  to  the  muscles  of  the  ear.  7.  Digastric 
branches.  8.  Branch  to  the  stylo-hyoid  muscle.  9.  Superior  terminal  branch.  10. 
Temporal  branches.  11.  Frontal  branches.  12.  Branches  to  the  orbicularis  palpe- 
brarum. 13.  Nasal  or  suborbital  branches.  14.  Buccal  branches.  15.  Inferior  ter- 
minal branch.  16.  Mental  branches.  17.  Cervical  branches.  18.  Superficial  temporal 
nerve  (branch  of  the  fifth).  19,  20.  Frontal  nerves  (branches  of  the  fifth).  21,  22,  23, 
24,  2%,  26,  27.  Branches  of  the  fifth.  28,  29,  30,  31,  32.  Branches  of  the  cervical  nerves. 
~(H  ir  sch  j  eld.) 


bone  above  and  between  the  cochlea  and  vestibule,  the  nerve  makes 
a  sharp  bend,  forming  the  genu  facialis,  turns  backward  and  enters 
the  aqueduct  of  Fallopius,  the  general  course  of  which  it  follows  as 
far  as  the  stylo-mastoid  foramen.  After  emerging  from  this  foramen 
the  nerve  passes  downward  and  forward  as  far  as  the  parotid  gland, 
within  which  it  terminates  by  dividing  into  two  main  branches,  the 


6oo 


TEXT-BOOK  OF  PHYSIOLOGY. 


temporo-facial  and  the  cervicofacial,  the  ultimate  branches  of  which 
are  distributed  as  previously  stated  to  the  superficial  muscles  of  the 
head  and  face  (Fig.    279). 

The  Pars  Intermedia  or  Nerve  of  Wrisberg.— The  facial  nerve 
at  the  genu,  the  point  where  it  turns  backward  to  enter  the  aqueduct 
of  Fallopius,  presents  a  slight  enlargement,  grayish  in  color,  and  in 
which  nerve-cells  are  contained.  This  enlargement  is  known  as  the 
geniculate  ganglion.  The  cells  of  this  ganglion,  originally  bipolar,  pre- 
sent single  axons  which  soon  divide  into  centrally  and  peripherally  cours- 
ing branches.     The  former  collectively  constitute  the  pars  intermedia 

or  nerve  of  Wrisberg,  which, 
entering  and  passing  through 
the  pons,  terminates  around 
the  sensor  end-nucleus  of  the 
glosso-pharyngeal  nerve;  the 
latter,  the  peripherally  directed 
branches,  enter  the  sheath  of 
the  facial  and  accompany  it 
as  far  as  a  point  about  five 
millimeters  above  the  stylo- 
mastoid foramen. 

From  its  mode  of  origin 
the  nerve  of  Wrisberg  can  not 
be  regarded  as  an  integral 
part  of  the  facial  nerve  proper, 
but  is  to  be  regarded  as  an 
independent  sensor  nerve.  As 
to  the  true  function  and  rela- 
tion of  this  nerve  there  is 
much  conflict  of  opinion. 
Branches  of  the  Facial. 
—In  the  aqueduct  of  Fallopius  the  facial  gives  off  the  following 
branches :  the  greater  and  lesser  petrosals,  the  stapedius,  and  the  chorda 
tympani  (Fig.  280). 

1.  The  greater  petrosal  nerve  is  given  off  near  the  geniculate  ganglion. 

It  then  passes  forward  into  the  spheno-maxillary  fossa  and  ter- 
minates in  the  spheno-palatine  ganglion  by  an  arborization  of 
its  fibers  around  the  ganglion  cells. 

2.  The  lesser  petrosal  nerve  is  given  off  at  a  point  somewhat  external 

to  the  preceding.  It  leaves  the  skull  by  a  small  foramen  and 
terminates  in  the  otic  ganglion  by  an  arborization  of  its  fibers 
around  the  ganglion  cells. 

3.  The  stapedius  branch  leaves  the  aqueduct  of  Fallopius  somewhat 

further  down  by  a  small  foramen,  enters  the  pyramid  of  the 
middle  ear,  and  is  finally  distributed  to  the  stapedius  muscle. 

4.  The  chorda  tympani  is  given  off  from  the  facial  at  a  point  about 

live  millimeters  above  the  stvlo-mastoid  foramen.     Il  then  passes 


Fig.  280. — Chorda  Tympani  Nerve,  i,  2 
3,  4.  Facial  nerve  passing  through  the  aqueduc- 
tus  Fallopii.  5.  Ganglioform  enlargement.  6. 
Great  petrosal  nerve.  7.  Sphenopalatine  gan- 
glion. 8.  Small  petrosal  nerve.  9.  Chorda  tym- 
pani. 10,  11,  12,  13.  Various  branches  of  the 
facial.  14,  14,  15.  Glosso-pharyngeal  nerve. — 
(Hirsch/eld.) 


THE  CRANIAL  NERVES.  601 

upw'ard  and  forward  and  enters  the  tympanum  through  the  iter 

chordae  posterius,  crosses  the  tympanic  membrane  between  the 

malleus   and   incus,   leaves   the   tympanum   by  the   iter   chordae 

anterius  or  canal  of  Huguier,  and  finally  joins  the  lingual  branch 

of  the  fifth.     Some  of  its  fibers  can  be  traced  to  the  dorsum  of 

the  tongue,  others  to  the  submaxillary  and  sublingual  ganglia, 

where  they  terminate  in  tufts  around  the  ganglion  cells. 

Properties. — Electric  stimulation  of  the  trunk  of  the  nerve  after 

its  emergence   from  the  stylo-mastoid  foramen  produces  convulsive 

movements  in  all  the  muscles  to  which  its  branches  are  distributed. 

The  same  results  follow  stimulation  of  the  intra-cranial  portion  of  the 

nerve  in  an  animal  recently  killed. 

Irritative  pathologic  lesions — e.  g.,  tumors,  aneurysms,  etc. — sit- 
uated along  the  course  of  the  nerve  or  at  its  nuclear  origin,  fre- 
quently give  rise  to  spasmodic  movements  of  the  facial  muscles  which 
may  be  tonic  or  clonic  in  character. 

Division  of  the  facial  nerve  after  its  emergence  from  the  stylo- 
mastoid foramen  is  followed  by  a  complete  relaxation  or  paralysis 
of  the  superficial  facial  muscles.  The  same  result  follows  compres- 
sion of  the  nerve-trunk  in  any  part  of  its  intra-cranial  course. 

The  phenomena  presented  by  an  individual  suffering  from  division 
or  compression  of  the  facial  nerve,  and  which  collectively  constitute 
facial  paralysis,  are  as  follows:  A  relaxed  and  immobile  condition  of 
the  side  of  the  face  corresponding  to  the  lesion;  separation  of  the  eye- 
lids from  paralysis  of  the  orbicularis  palpebrarum  and  the  unopposed 
contraction  of  the  levator  palpebrse  muscles;  abolition  of  the  act  of 
winking;  drooping  of  the  angle  of  the  mouth;  an  escape  of  saliva 
from  the  mouth;  contraction  of  the  muscles  and  distortion  of  the 
opposite  side  of  the  face;  on  attempting  to  laugh  or  talk  the  distortion 
of  the  face  is  increased;  during  mastication  the  food  accumulates 
between  the  teeth  and  cheek,  from  paralysis  of  the  buccinator;  articu- 
lation is  impaired  from  paralysis  of  the  orbicularis  oris  muscle,  the 
labial  sounds  especially  being  imperfectly  produced. 

Properties  of  the  Branches  Given  off  in  the  Aqueduct  of 
Fallopius. — The  great  petrosal  nerve  is  in  all  probability  composed 
of  efferent  fibers  (vaso-dilatator  and  secretor)  which  leave  the  pons 
by  way  of  the  nerve  of  Wrisberg,  or  pars  intermedia,  to  be  distributed 
around  the  cells  of  the  spheno-palatine  ganglion;  for  stimulation 
either  of  this  nerve  or  of  the  ganglion  is  followed  by  the  same  results: 
viz.,  dilatation  of  the  blood-vessels  of,  and  secretion  from,  the  mucous 
membrane  of  the  nose,  soft  palate,  upper  part  of  the  pharynx,  roof 
of  mouth,  gums,  and  upper  lip. 

The  small  petrosal  nerve  is  also  composed  of  efferent  fibers;  shortly 
after  leaving  the  facial  it  is  joined  by  a  small  nerve,  derived  from 
Jacobson's  branch  of  the  glosso-pharyngeal,  which  is  also  efferent 
in  function;  for  stimulation  of  Jacobson's  nerve  as  well  as  stimulation 
of  the  otic  eanglion  is  followed  bv  the  same  result:  viz.,  dilatation 


6o2  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  blood-vessels  of,  and  secretion  from,  the  mucous  membrane  of 
the  cheek,  lower  lip,  and  gums,  and  of  the  parotid  and  the  orbit  glands. 

The  stapedius  nerve,  distributed  directly  to  the  stapedius  muscle, 
is  motor  in  function. 

The  Chorda  Tympani. — Stimulation  of  the  chorda  tympani  nerve 
in  the  tympanic  cavity  produces  dilatation  of  the  blood-vessels  of, 
and  an  increased  production  and  discharge  of  saliva  from,  the  sub- 
maxillary and  sublingual  glands. 

Division  of  this  nerve  is  followed  by  a  contraction  of  the  blood- 
vessels and  a  diminution  of  the  secretion.  From  these  results  it  is 
certain  that  the  chorda  tympani  contains  both  vaso-dilatator  and  secre- 
tor  fibers.  Nicotin  applied  to  the  submaxillary  and  sublingual  ganglia 
abolishes  the  effects  of  stimulation  of  the  chorda  tympani.  It  does 
not  prevent  the  same  effects  when  the  ganglia  themselves  are  stimu- 
lated. It  is  clear,  therefore,  that  the  vaso-dilatator  and  secretor 
fibers  arborize  around  the  cells  of  the  ganglia  and  are  not  distributed 
directly  to  the  gland  structures.  It  is  highly  probable  that  the  efferent 
fibers  in  the  chorda  tympani  emerge  from  the  pons  by  way  of  the 
pars  intermedia,  or  nerve  of  Wrisberg. 

Division  of  the  chorda  in  the  tympanum  is  also  followed  by  a  loss 
of  taste  in  the  anterior  two-thirds  of  the  tongue.  For  this  and  other 
reasons  the  chorda  tympani  has  long  been  regarded  as  the  nerve  of 
taste  for  this  region.  The  specific  physiologic  stimulus  to  the  chorda 
tympani  nerve  is  organic  matter  in  solution  acting  on  the  peripheral 
terminations  of  the  nerve  in  the  mucous  membrane  of  the  tongue. 
The  exact  pathway  for  these  afferent  or  gustatory  fibers  beyond  the 
geniculate  ganglion  has  long  been  a  subject  of  much  discussion. 
According  to  some  observers  these  fibers  enter  the  great  petrosal 
nerve,  pass  forward  as  far  as  the  spheno-palatine  ganglion,  then  into 
the  superior  maxillary  division  of  the  trigeminal,  and  so  to  the  brain. 
According  to  others,  these  fibers  pass  into  the  pars  intermedia,  into 
the  pons,  where  they  terminate  around  the  sensor  end-nucleus  of  the 
glosso-pharyngeal.  The  evidence  for  and  against  either  of  these  two 
views  is  most  conflicting  and  insufficient  to  justify  positive  statements 
one  way  or  the  other.  To  the  writer  the  weight  of  evidence  seems  to 
favor  the  view  that  the  gustatory  fibers  have  their  origin  in  the  genicu- 
late ganglion;  that  they  pass  centrally  through  the  pars  intermedia; 
that  they  are  similar  in  function  to  the  glosso-pharyngeal;  and  that 
they  are  indeed  but  aberrant  branches  of  this  nerve. 

Functions. — The  function  of  the  facial  nerve  is  the  transmission 
of  nerve  impulses  from  the  nerve-cells  from  which  it  arises  to  the 
muscles  of  the  face.  As  these  muscles  express  ideas  and  emotions 
the  nerve  has  been  termed  the  nerve  of  expression.  Because  of  the 
presence  of  efferent  fibers  which  leave  the  main  trunk  by  way  of  the 
chorda  tympani  nerve,  it  regulates  the  caliber  of  the  blood-vessels  of  the 
submaxillary  and  sublingual  glands  and  excites  the  glands  to  activity. 
It  also  influences  hearing  by  its  action  on  the  stapedius  muscle. 


THE  CRANIAL  NERVES. 


60  • 


EIGHTH  PAIR.     THE  AUDITORY. 

The  eighth  cranial  nerve,  the  auditory,  consists  of  the  centrally 
coursing  axons  of  neurons  which  connect  the  essential  organ  of  hearing 
with  sensor  end-nuclei  in  the  pons  Varolii.  This  nerve  consists  of 
two  portions:  viz.,  a  cochlear  or  auditory  and  a  vestibular  or  equilibra- 
tory. 

Origin.— The  axons  comprising  the  cochlear  portion  have  their 
origin  in  the  bipolar  nerve-cells  of  the  spiral  ganglion  located  in  the 
spiral  canal  near  the  base  of  the 

osseous  lamina  spiralis  (Fig.  281).  ..-'' 

From  this  origin  they  pass  cen- 
trally  into  the  central  canal  of  the 
modiolus,  at  the  base  of  which 
they  emerge  in  well-defined  bun- 
dles and  enter  the  internal  audi- 
tory meatus.  Dendritic  processes 
from  these  cells  pass  peripherally 
to  terminate  on  the  ciliated  epi- 
thelial cells  of  the  organ  of  Corti. 

The  axons  comprising  the  vesti- 
bular portion  have  their  origin  in 
the  bipolar  nerve-cells  of  the 
ganglion  of  Scarpa  located  in  the 
internal  auditory  meatus.  From 
this  origin  they  pass  centrally  in 
connection  with  the  cochlear  por- 
tion. Dendritic  processes  from 
these  cells  pass  peripherally  into 
the  internal  ear,  where  they 
terminate  on  epithelial  cells  situ- 
ated on  the  inner  surface  of  the 
utricle  and  saccule  and  in  the 
ampullae  of  the  semicircular  canals. 

The  common  trunk  of  the 
auditory  nerve,  consisting  of  both 
cochlear  and  vestibular  divisions 
after  emerging  from  the  internal 
auditory  meatus,  passes  backward, 
inward,  and  downward  as  far  as 
the  lateral  aspect  of  the  pons  where 
the  two  divisions  again  separate. 

The  cochlear  nerve,  the  external  root,  passes  to  the  outer  side  of 
the  restiform  body  and  enters  the  ventral  acoustic  nucleus  and  the 
lateral  acoustic  nucleus,  around  the  cells  of  which  its  end-tufts  arborize. 
The  vestibular  nerve,  the  internal  root,  passes  on  the  inner  side  of 
the  restiform  body  to  the  dorsal  portion  of  the  pons,  where,  after 
bifurcating,   the  end-tufts  of  the  axons  arborize  around  the  dorso- 


Fig.  281. — Origin*  and  Termination 
of  the  .-Auditory  Nerve,  i.  Cochlea. 
2.  Spiral  ganglion  (Corti).  3.  Cochlear 
nerve.  4.  Ventral  acoustic  nucleus.  5. 
Lateral  acoustic  nucleus.  6.  Semi- 
circular canals.  7.  Ganglion  of  Scarpa. 
S.  Vestibular  nerve.  9.  Dorso-external 
nucleus  (D  eiters).  10.  Dorso-internal 
nucleus. — (After  Moral  and  Doyon.) 


6o4  TEXT-BOOK  OF  PHYSIOLOGY. 

internal  or  chief  auditory  nucleus  and  the  dorso-external  or  Deiters' 
nucleus.  Some  of  the  fibers  of  the  vestibular  branch  descend  through 
the  pons  and  medulla  as  far  as  the  cuneate  nucleus. 

Cortical  Connections. — The  cochlear  nerve  is  ultimately  con- 
nected with  the  cerebral  acoustic  area,  in  the  temporal  lobe  of  the 
opposite  side  through  the  intermediation  of  the  auditory  tract.  This 
tract  is  complex  and  involved.  In  a  general  way  it  may  be  said  to 
consist  in  part  of  fibers  which  come  direct  from  the  cochlear  branch. 
After  passing  through  the  ventral  nucleus  and  the  trapezoid  body 
they  cross  the  median  line,  enter  the  lemniscus  or  fillet,  and  finally 
terminate  in  the  pre-  and  post-geminal  bodies.  In  their  course  they 
give  off  collateral  branches  to  these  various  nuclei  through  which 
they  pass.  Other  fibers  taking  their  origin  from  cells  in  these  various 
nuclei  proceed  to  the  cortex  where  they  terminate. 

Properties. — Stimulation  of  the  cochlear  nerve  is  unattended  by 
either  motor  or  sensor  phenomena.  Division  of  the  nerve  is  fol- 
lowed by  a  loss  of  the  sense  of  hearing.  Irritative  pathologic  lesions 
give  rise  to  sensations  of  sound  of  varying  character  and  intensity. 
Degeneration  of  the  nerve  or  destruction  by  tumors,  etc.,  will  also 
be  followed  by  a  loss  of  the  sense  of  hearing. 

Experimental  lesions  of  the  semicircular  canals  involving  a  de- 
struction of  the  physiologic  relations  of  the  vestibular  nerve  are  followed 
by  a  loss  of  the  coordinating  and  equilibratory  power.  Disordered 
movements,  such  as  rotation  to  the  right  or  left,  somersaults  back- 
ward and  forward,  follow  destruction  of  these  canals.  Pathologic 
lesions  in  the  peripheral  distribution  of  the  nerve  are  attended  in  man 
bv  disturbances  of  equilibrium,  e.  g.,  vertigo,  a  sense  of  swaying, 
pitching,  and  staggering. 

Functions. — The  function  of  the  cochlear  nerve  is  to  convey 
nerve  impulses  from  its  origin  to  the  pons,  from  which  they  are  trans- 
mitted by  the  auditory  tract  to  the  acoustic  area  in  the  cerebral  cortex 
where  they  evoke  sensations  of  sound  and  its  different  qualities,  inten- 
sity, pitch  and  timbre.  The  specific  physiologic  stimulus  to  the 
development  of  these  impulses  is  the  impact  of  atmospheric  undulations 
on  the  tympanic  membrane,  received  and  transmitted  by  the  chain 
of  bones  to  the  structures  of  the  internal  ear — the  organ  of  Corti— 
with  which  the  peripheral  terminations  of  the  nerve  are  connected. 

The  function  of  the  vestibular  nerve  is  the  transmission  of  nerve 
impulses  to  the  pons,  whence  they  are  transmitted  to  the  cortex  of 
both  the  cerebrum  and  cerebellum  and  to  other  centers.  The  specific 
physiologic  stimulus  is  supposed  to  be  a  variation  in  pressure  in  the 
ampulla  of  the  semicircular  canals  caused  by  movements  of  the  en- 
dolymph  induced  by  changes  in  the  position  of  the  head  and  body. 
The  impulses  carried  by  the  vestibular  nerve  give  rise  renexly  to  certain 
adaptive  and  protective  movements  by  which  the  equilibrium  of  the 
body  in  both  dynamic  and  static  conditions  is  maintained. 


THE  CRANIAL  NERVES.  605 

NINTH  NERVE.     THE  GLOSSOPHARYNGEAL. 

The  ninth  cranial  nerve,  the  glosso-pharyngeal,  consists,  as  shown 
by  both  histologic  and  experimental  methods  of  research,  of  both 
afferent  and  efferent  nerve-fibers,  of  which  the  former,  however,  are 
bv  far  the  more  abundant.  Near  its  exit  from  the  cavity  of  the  skull 
the  nerve  presents  two  ganglionic  enlargements  known  as  the  petrosal 
and  jugular  ganglia. 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  serve  to 
bring  certain  end-nuclei  in  the  medulla  oblongata  into  anatomic  and 
physiologic  relation  with  portions  of  the  mucous  membrane  of  the 
tongue,  pharynx,  and  middle  ear.  The  afferent  fibers  are  axons  of 
the  monaxonic  cells  of  the  petrosal  and  jugular  ganglia.  The  single 
axon  from  each  of  these  cells  soon  divides  into  two  branches,  one 
of  which  passes  centrally,  the  other  peripherally.  The  centrally 
directed  branches  collectively  form  the  so-called  roots,  four  or  five  in 
number,  which  enter  the  medulla  between  the  olivary  and  restiform 
bodies.  The  peripherally  directed  branches  collectively  form  the 
two  main  divisions,  from  the  distribution  of  which,  to  the  tongue  and 
pharynx,  the  nerve  takes  its  name. 

Distribution. — The  axons  of  the  centrally  directed  branches 
after  entering  the  medulla  pass  toward  its  dorsal  aspect,  where  they 
bifurcate,  give  off  collateral  branches,  and  terminate  in  fine  end-tufts 
in  the  immediate  neighborhood  of  two  groups  of  nerve-cells,  the 
sensor  end-nuclei.  The  axons  of  the  peripherally  directed  branches, 
after  emerging  from  the  base  of  the  skull  through  the  jugular  foramen, 
pass  forward  and  inward  under  cover  of  the  stylorpharyngeal  muscle; 
winding  around  this  muscle  they  divide  into  terminal  branches  which 
are  distributed  to  the  mucous  membrane  of  the  posterior  one-third 
of  the  tongue,  pharynx,  soft  palate,  uvula,  and  tonsils  (Fig.  283). 

Origin  of  the  Efferent  Fibers.— The  efferent  fibers  serve  to  bring 
the  nerve-cells  from  which  they  arise  into  connection  with  a  portion 
of  the  musculature  of  the  fauces  and  pharynx.  These  nerve-cells 
are  located  in  the  lateral  portion  of  the  formatio  reticularis  at  some 
distance  below  the  floor  of  the  fourth  ventricle.  They  constitute 
the  upper  portion  of  a  collection  of  cells  known  as  the  nucleus  am- 
bi  vuus. 

Distribution. — From  this  origin  the  efferent  fibers  pass  dorsally 
to  near  the  sensor  end-nuclei,  then  turn  outward  and  forward  and 
finally  emerge  from  the  medulla  in  intimate  association  with  the 
afferent  fibers.  They  are  ultimately  distributed  to  the  stylo-pharyn- 
geus,  the  palato-glossui  and  to  the  middle  constrictor  muscles  of  the 
pharynx.  In  addition  to  the  foregoing  efferent  fibers  the  glosso- 
pharyngeal nerve  contains  at  its  emergence  from  the  medulla  both 
vaso-motor  and  secretor  fibers. 

Jacobson's  Nerve. — This  is  a  small  branch  which  leaves  the  glosso- 
pharyngeal at  the  petrous  ganglion.  After  passing  through  a  small 
canal  in  the  base  of  the  skull  it  enters  the  tympanic  cavity,  within 


606  TEXT-BOOK  OF  PHYSIOLOGY. 

which  it  gives  off  branches  to  the  great  and  lesser  petrosal  nerves,  to 
the  mucous  membrane  of  the  foramen  ovale,  the  foramen  rotundum, 
and  to  the  Eustachian  tube. 

Cortical  Connections. — The  motor  nucleus  is  doubtless  con- 
nected with  the  general  motor  area  of  the  cortex  through  fibers  de- 
scending in  the  pyramidal  tract.  The  exact  location  of  the  cortical 
area  for  the  pharynx  is  not  well  determined,  but  is  most  likely  to  be 
found  in  the  lower  part  of  the  general  motor  area  near  the  termination 
of  the  Rolandic  fissure.  The  exact  cortical  connections  of  the  afferent 
tract  are  unknown,  but  are  most  likely  to  be  found  in  the  general 
sensor  area. 

Properties. — Electric  stimulation  of  the  glosso-pharyngeal  trunk 
calls  forth  evidence  of  pain  and  contraction  of  the  stylo-pharyngeus 
and  middle  constrictor  muscles.  Peripheral  stimulation  of  the  termi- 
nals of  the  nerve  fibers  in  the  mucous  membrane  of  the  posterior  third 
of  the  tongue  with  different  kinds  of  organic  matter  in  solution,  de- 
velops nerve  impulses  which  transmitted  to  the  cortex  evoke  sensa- 
tions of  taste.  Division  of  the  nerve  abolishes  sensibility  in  the 
mucous  membrane  to  which  it  is  distributed,  impairs  the  sense  of 
taste  in  the  posterior  third  of  the  tongue,  and  gives  rise  to  paralysis 
of  the  above-mentioned  muscles. 

Stimulation  of  Jacobson's  nerve  is  followed  by  dilatation  of  the 
blood-vessels  of,  and  secretion  from,  the  mucous  membrane  of  the 
lower  lip,  cheek,  and  gums,  and  from  the  parotid  gland.  Division 
of  the  nerve  is  followed  by  the  opposite  results.  The  course  of  the 
fibers  which  give  rise  to  these  results  is  by  way  of  the  lesser  petrosal 
to  the  otic  ganglion,  around  the  cells  of  which  the  fibers  arborize. 
From  the  cells  of  this  ganglion  non-medullated  fibers  pass  to  the 
blood-vessels  and  gland  cells. 

Functions. — The  afferent  fibers  of  the  glosso-pharyngeal  trans- 
mit nerve  impulses  from  the  parts  to  which  they  are  distributed  to  the 
cerebral  cortex,  where  they  evoke  sensations  of  pain  and  sensations 
of  taste;  they  also  assist  in  all  probability  in  the  performance  of  certain 
reflexes  connected  with  deglutition.  The  efferent  fibers  transmit 
impulses  to  muscles,  exciting  them  to  activity,  and  to  the  otic  ganglion, 
which  in  turn  dilates  blood-vessels  and  excites  secretion. 

TENTH  NERVE.     THE  PNEUMOGASTRIC  OR  VAGUS. 

The  tenth  cranial  nerve,  the  pneumogastric  or  vagus,  consists, 
as  shown  by  histologic  methods  of  research,  of  both  afferent  and 
efferent  fibers,  independent  of  those  derived  in  its  course  from  adjoin- 
ing motor  or  efferent  nerves.  Near  the  exit  of  the  nerve  from  the 
cavity  of  the  cranium  it  presents  two  ganglionic  enlargements  known 
respectively  as  the  ganglion  of  the  root  (the  jugular)  and  the  ganglion 
of  the  trunk  (the  plexiform). 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  take  their 


THE  CRAXIAL  NERVES.  607 

origin  in  the  monaxonic  cells  of  the  ganglia  on  the  root  and  trunk. 
The  single  axon  from  each  of  these  cells  soon  divides  into  two  branches, 
one  of  which  passes  centrally,  the  other  peripherally.  The  centrally 
directed  branches  collectively  form  the  so-called  roots,  ten  to  fifteen 
in  number,  which  enter  the  medulla  between  the  restiform  body  and 
the  lateral  column.  The  peripherally  directed  branches  collectively 
form  a  portion  of  the  common  trunk  of  the  nerve. 

Distribution. — The  axons  of  the  centrally  directed  branches 
after  entering  the  medulla  pass  toward  its  dorsal  aspect,  where  they 
bifurcate,  give  off  collaterals,  and  terminate  in  fine  end-tufts  in  the 
immediate  neighborhood  of  two  groups  of  nerve-cells,  the  vagal 
sensor  end-nuclei. 

The  axons  of  the  peripherally  directed  branches  unite  to  form  a 
portion  of  the  common  trunk,  which,  as  it  descends  the  neck  and 
enters  the  thorax  and  abdomen,  gives  off  a  number  of  branches  which 
are  ultimately  distributed  to  the  mucous  membrane  of  the  esophagus, 
larynx,  lungs,  stomach,  and  intestine,  and  also  to  the  heart.  The 
afferent  fibers  thus  serve  to  bring  into  anatomic  and  physiologic  relation 
the  mucous  membrane  of  these  organs  with  certain  sensor  end-nuclei 
in  the  medulla  oblongata. 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  take  their 
origin  from  nerve-cells  located  in  the  lateral  portion  of  the  formatio 
reticularis  at  some  distance  below  the  floor  of  the  fourth  ventricle. 
These  cells  constitute  the  lower  portion  of  the  nucleus  ambiguus. 

Distribution. — From  their  origin  the  efferent  axons  pass  dorsally 
to  near  the  sensor  end-nuclei,  then  turn  outward  and  forward,  and 
finally  emerge  from  the  medulla  in  close  association  with  the  afferent 
branches.  They  are  ultimately  distributed  to  the  levator  palati, 
azygos  uvulae,  and  palato-pharyngeus  muscles;  to  the  superior  and 
inferior  constrictor  muscles  of  the  pharynx,  and  to  the  muscles  of  the 
esophagus;  to  the  muscle-fibers  of  the  stomach  and  perhaps  the  in- 
testines; and  to  the  non-striated  muscle-fibers  of  the  bronchial  tubes. 
Among  the  efferent  fibers  are  some  which  are  distributed  to  the  gastric 
glands  and  to  the  pancreas. 

According  to  Beevor  and  Horsley,  in  the  monkey  the  motor  fibers 
for  the  levator  palati  come  from  the  spinal  accessory  nerve. 

The  efferent  fibers  thus  serve  to  bring  the  nerve-cells  from  which 
they  arise  into  anatomic  and  physiologic  connection  with  a  portion  of 
the  musculature  of  the  alimentary  canal  and  its  diverticulum,  the 
lung. 

Communicating  Branches. — At  or  near  the  ganglia  the  vagus 
receives  communicating  branches  from  the  eleventh  nerve,  the  spinal 
accessory,  the  facial,  the  hypoglossal,  and  the  anterior  branches  of 
the  two  upper  cervical  nerves.  Owing  to  this  manifold  origin  of  the 
efferent  fibers  in  the  trunk  and  peripheral  branches  of  the  vagus, 
it  is,  in  some  instances,  difficult,  if  not  impossible,  to  determine  to 
which  of  these  nerves  a  given  muscle  contraction  is  to  be  referred. 


6o8 


TEXT-BOOK  OF  PHYSIOLOGY 


Vagal  Branches. — As  the  vagus  passes  down  the  neck  it  gives 
off  the  following  main  branches  (Fig.  282) : 
1.   The  pharyngeal  nerves  which,  after  entering  into  the  formation  of 


Fig.  282. — Distribution  of  the  Pneumogastric. — 1.  Trunk  of  the  left  pneumo- 
gastric.  2.  Ganglion  of  the  trunk.  3.  Anastomosis  with  the  spinal-  accessory.  4. 
Anastomosis  with  the  sublingual.  5.  Pharyngeal  branch  (the  auricular  branch  is  not 
shown  in  the  figure).  6.  Superior  laryngeal  branch.  7.  External  laryngeal  nerve.  8. 
Laryngeal  plexus.  9,  9.  Inferior  laryngeal  branch.  10.  Cervical  cardiac  branch.  11. 
Thoracic  cardiac  branch.  12,13.  Pulmonary  branches.  14.  Lingual  branch  of  the  fifth. 
15.  Lower  portion  of  the  sublingual.  16.  Glosso-pharyngeal.  17.  Spinal  accessory. 
18,  19,  20.  Spinal  nerves.  21.  Phrenic  nerve.  22,  23.  Spinal  nerves.  24,  25,  26,  27, 
28,  29,  30.  Sympathetic  ganglia. — (Hirschjeld.) 


the  pharyngeal  plexus,  are  distributed  to  the  mucous  membrane 
and  to  the  muscles  of  the  pharynx;  e.  g.,  superior  and  inferior 
constrictors,  the  levator  palati,  and  the  azygos  uvulae. 
2.  The  esophageal  nerves,  which   after  entering  into  the  formation  of 


THE  CRANIAL  NERVES.  609 

the  esophageal  plexus,  are  distributed  to  the  mucous  membrane, 
and  to  the  muscles  of  the  esophagus. 

3.  The  superior  laryngeal  nerve  which,  entering  the  larynx  through 

the  thyro-hyoid  membrane,  is  distributed  to  the  mucous  mem- 
brane lining  the  interior  of  the  larynx  and  to  the  crico-thyroid 
muscle.  From  the  superior  laryngeal  and  the  main  trunk  small 
branches  are  given  off  which  in  the  rabbit  unite  to  form  a  single 
nerve,  the  so-called  depressor  nerve.  (See  page  380.)  It  is  dis- 
tributed to  the  heart-muscle.  Though  this  anatomic  arrangement 
is  not  found  in  man,  there  are  many  reasons  for  believing  that 
analogous  fibers  are  present  in  the  vagus  trunk  of  man  and  other 
animals. 

4.  The  inferior  laryngeal  nerve  which  is  distributed  ultimately  to  all 

the  muscles  of  the  larynx  (except  the  crico-thyroid)  and  to  the 
inferior  constrictor  of  the  pharynx. 

5.  The  cardiac  nerves  which,  after  entering  into  the  formation  of.  the 

cardiac  plexus,  are  distributed  to  the  heart. 

6.  The  pulmonary  nerves  distributed  to  the  mucous  membrane  of  the 

bronchial  tubes  and  their  ultimate  terminations,  the  lobules  and 
air-cells,  as  well  as  to  their  non-striated  muscle-fibers. 

7.  The  gastric  and  intestinal  nerves,  distributed  to  the  mucous  mem- 

brane and  muscular  walls  of  the  stomach  and  intestines.  Other 
fibers  in  all  probability  pass  to  the  liver,  spleen,  kidney,  and 
suprarenal  bodies. 

Properties  of  the  Pneumogastric  or  Vagus  Nerve  and  its 
Various  Branches. — Faradization  of  the  vagus  nerve  close  to  the 
medulla  oblongata  gives  rise  to  sensations  of  pain  and  to  contraction 
of  the  musculature  of  a  portion  of  the  alimentary  tract:  viz.,  the 
palate,  pharynx,  esophagus,  stomach,  and  possibly  of  the  intestine 
and  of  the  pulmonary  apparatus.  Division  of  the  nerve  is  followed 
by  a  loss  of  sensibility  in  the  mucous  membrane  of  the  alimentary 
tract  and  of  the  pulmonary  apparatus,  together  with  a  loss  of  motility 
of  the  structures  above  mentioned. 

Stimulation  of  the  trunk  of  the  nerve  in  different  parts  of  its  course 
produces  a  variety  of  results  dependent  to  some  extent  on  the  presence 
of  anastomosing  branches  from  adjoining  nerves. 

The  Pharyngeal  Nerves. — Faradization  of  the  pharyngeal  nerves 
consisting  of  both  afferent  and  efferent  fibers,  gives  rise  to  sensations 
of  pain,  contraction  of  the  pharyngeal  muscles,  and  perhaps  to  vomiting. 
Division  of  these  nerves  is  followed  by  a  loss  of  sensibility  in  the  parts 
to  which  they  are  distributed  and  by  paralysis  of  the  muscles  with  a 
consequent  impairment  of  deglutition. 

The  Esophageal  Nerves. — Faradization  of  the  esophageal  nerves 
gives  rise  to  sensations  of  pain  and  to  contractions  of  the  muscle  coat 
of  the  esophagus.  Division  of  these  nerves  is  followed  by  a  loss  of  sen- 
sibility in  the  parts  to  which  they  are  distributed,  a  partial  paralysis 
of  the  muscle  coat  and  an  impairment  of  deglutition   (see  page  170). 

39 


610  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Superior  Laryngeal  Nerve. — Faradization  of  the  superior 
laryngeal  nerve  gives  rise  to  sensations  of  pain,  and  to  contraction  of 
the  crico-thyroid  muscle.  Through  reflected  impulses  it  causes  con- 
traction of  the  muscles  of  deglutition,  and  of  the  muscles  concerned 
in  the  act  of  coughing;  inhibition  of  the  inspiratory  movement  and 
arrest  of  respiration  in  the  condition  of  expiratory  standstill,  with 
perhaps  a  tetanic  contraction  of  the  expiratory  muscles,  and  con- 
traction of  the  laryngeal  muscles  with  closure  of  the  glottis.  Periph- 
eral stimulation  of  this  nerve — e.  g.,  the  contact  of  foreign  particles- 
gives  rise  to  a  similar  series  of  phenomena.  Division  of  these  nerves 
is  followed  by  a  loss  of  sensibility  in  the  laryngeal  mucous  membrane, 
paralysis  of  the  crico-thyroid  muscle  with  a  consequent  lowering  of 
the  pitch,  and  a  diminution  in  the  clearness  of  the  voice.  In  conse- 
quence of  the  loss  of  the  sensibility  there  is  an  inability  to  perceive 
the  entrance  of  foreign  bodies  into  the  larynx. 

The  Depressor  Nerve. — Stimulation  of  the  peripheral  end  of  the 
depressor  nerve  is  without  effect;  stimulation  of  the  central  end  re- 
tards and  even  arrests  the  heart's  pulsations  and  lowers  the  general 
blood-pressure.  These  two  effects,  though  associated,  are  neverthe- 
less independent  of  each  other.  If  the  vagus  nerves  be  divided  on 
both  sides  between  the  origin  of  the  depressor  and  the  origin  of  the 
cardiac  nerves,  and  the  former  stimulated,  there  will  be  a  fall  of 
pressure  without  retardation  of  the  heart.  The  effect  on  the  heart 
is  attributed  to  a  stimulation  of  the  cardio-inhibitory  mechanism  in 
the  medulla  oblongata. 

The  fall  of  general  blood-pressure  was  formerly  attributed  to  a 
sudden  dilatation  of  the  splanchnic  blood-vessels  alone,  in  conse- 
quence of  a  depression  of  that  portion  of  the  general  vaso-motor  center 
which  maintains  through  the  splanchnic  nerves  a  tonic  contraction  of 
their  walls.  It  has  been  satisfactorily  demonstrated  that  this  is  not 
the  sole  cause;  for  after  division  of  the  splanchnic  nerves,  stimulation 
of  the  depressor  causes  a  still  further  fall  of  from  30  to  40  per  cent. 
in  the  general  pressure  (Porter  and  Beyer).  Evidently,  not  any  one, 
but  all  portions  of  the  vaso-motor  center  are  subject  to  the  effects  of 
depressor  stimulation. 

The  Inferior  Laryngeal  Nerves.- — Faradization  of  the  inferior 
laryngeal  nerves  produces  effects  which  vary  in  accordance  with  the 
strength  of  the  stimulus,  with  different  animals,  and  with  the  same 
animal  at  different  periods  of  life.  In  the  adult  dog  and  in  man,  the 
glottis  is  kept  widely  open  for  respiratory  purposes  by  the  tonic  con- 
traction of  the  abductor  muscles  (the  crico-arytenoids) ;  for  phonatory 
purposes  the  glottis  is  closed  and  the  vocal  membranes  approximated 
by  the  contraction  of  the  adductor  muscles.  It  has  been  shown  that 
these  opposed  groups  of  muscles  have  independent  nerve-supplies; 
thai  two  sets  of  libers  in  the  common  trunk  can  be  separated  and 
stimulated  independently  of  each  other.  Feeble  stimulation  of  the 
common  trunk  produces  a  still  further  abduction  of  the  vocal  cords. 


THE  CRANIAL  NERVES.  611 

With  an  increase  in  the  strength  of  the  stimulus,  however,  the  reverse 
obtains:  namely,  adduction  which  increases  until  the  glottis  is  com- 
pletely closed.  Division  of  the  nerves  is  followed  by  paralysis  of  both 
the  phonatory  and  respiratory  muscles,  the  abductors  and  adductors, 
with  the  result  of  seriously  impairing  both  phonation  and  respiration 
and  not  infrequently  causing  death.  The  fibers  of  the  inferior  laryn- 
geal nerve  are  derived  from  the  eleventh  nerve,  the  spinal  accessory. 
The  Cardiac  Nerves. — Faradization  of  the  trunk  of  the  vagus  or 
of  the  peripheral  end  of  the  divided  nerve  gives  rise  to  a  diminution 
in  the  frequency  and  force  of  the  heart's  contractions;  and  if  the  stim- 
ulation be  sufficiently  powerful,  completely  arrests  it  in  the  phase  of 
diastole.  To  these  results  the  term  inhibition  is  applied.  Division 
of  the  vagi  or  of  the  cardiac  branches  is  followed  by  an  increase  in 
the  number  of  contractions  from  loss  of  inhibitor  influences.  The 
inhibitor  fibers  of  the  vagus  are  generally  believed  to  be  derived  from 
the  spinal  accessory,  though  this  has  been  questioned.  According  to 
the  recent  investigations  of  Schaternikoff .  and  Friedenthal,  they  come 
direct  in  the  vagus,  from  a  nucleus  near  the  vagal  motor  nucleus  in 
the  medulla,  the  spinal  accessory  sending  no  branches  to  the  heart. 
In  the  frog  and  other  batrachia  the  vagus  contains  also  accelerator 
or  augmentor  fibers  derived  from  the  sympathetic;  hence  stimulation, 
especially  if  feeble,  may  increase  the  heart's  action. 

The  Pulmonary  Nerves.—  The  pulmonary  nerves,  given  off  from 
the  trunk  after  its  entrance  into  the  thorax,  do  not  lend  themselves 
readily  to  experimentation.  Division  of  both  vagi  in  the  neck  above 
the  point  of  exit  of  the  pulmonary  branches  is  followed  by  a  decrease 
in  the  frequency  of  the  respiratory  acts,  with  an  increase  in  their  depth. 
At  the  same  time  there  is  a  loss  of  sensibility  of  the  mucous  membrane 
of  the  trachea  and  lungs  and  a  paralysis  of  non-striated  muscle-fibers. 
Stimulation  of  the  central  end  of  the  vagus  increases  the  frequency, 
but  decreases  the  amplitude,  of  the  respiratory  movements.  If  the 
stimulation  be  increased  in  intensity  the  respiratory  movements  in- 
crease in  frequency  until  the  inspiratory  muscles  pass  into  the  con- 
dition of  tetanus. 

Feeble  stimulation  of  the  vagus  not  infrequently  inhibits  the  in- 
spiratory movement  and  increases  the  expiratory  until  there  is  a 
complete  cessation  of  movement  in  the  condition  of  expiratory  stand- 
still. The  effect  thus  produced  is  similar  to,  if  not  identical  with, 
that  produced  by  stimulation  of  the  superior  laryngeal  nerve.  This 
would  seem  to  indicate  the  presence  in  the  vagus  trunk  of  two  sets  of 
afferent  fibers  coming  from  the  lungs  through  the  pulmonary  branches, 
one  of  which  inhibits  inspiration,  the  other  inhibits  expiration. 

Faradization  of  the  trunks  of  the  pulmonary  branches  or  stimula- 
tion of  their  peripheral  terminations  in  the  mucous  membrane  of 
the  bronchial  tubes  or  alveoli  by  the  inhalation  of  chemic  vapors 
causes  arrest  of  respiratory  movements,  a  fall  of  blood-pressure,  and 
a  re  Ilex  inhibition  of  the  heart  (Brodie). 


612  TEXT-BOOK  OF  PHYSIOLOGY. 

Gastric  Nerves. — Stimulation  of  the  peripheral  end  of  a  divided 
vagus  nerve  causes  a  distinct  contraction  of  the  right  half  of  the 
stomach  and  secretion  from  the  gastric  glands.  Division  of  the  nerve 
abolishes  the  sensibility  of  the  mucous  membrane  of  the  stomach, 
impairs  motility,  and  interferes  with  the  secretion  of  the  gastric  juice. 

Similar  experimentation  on  the  trunk  of  the  vagus  has  shown  that 
the  nerve  excites  contraction  of  the  upper  part  of  the  small  intestine 
and  of  the  gall-bladder,  the  secretion  of  the  pancreas,  the  renal  cir- 
culation, the  secretion  of  urine,  etc. 

Functions. — The  afferent  fibers  transmit  nerve  impulses  from  the 
area  of  their  distribution  to  the  medulla  and  thence  through  cortical 
connections  to  the  sensor  cerebral  areas,  where  they  evoke  sensations. 
They  therefore  endow  all  parts  to  which  they  are  distributed  with 
sensibility. 

The  efferent  fibers  transmit  impulses  outward  which  excite  contrac- 
tion of  the  muscles  of  the  pharynx,  the  esophagus,  the  stomach,  the  small 
intestine,  and  the  gall-bladder,  and  the  muscles  of  the  bronchial  tubes; 
excite  secretion  from  the  glands  of  the  stomach,  pancreas,  and  kidney 
and  exert  an  inhibitor  influence  on  the  activity  of  the  heart.  The 
efferent  fibers  belong  to  the  autonomic  system  of  nerves  and  are  not 
connected  with  the  ganglia  of  the  vagus,  but  with  local  peripheral 
ganglia. 

The  afferent  fibers  also  assist  in  the  maintenance  of  certain  organic 
reflex  actions  which  are  highly  essential  to  the  life  of  the  individual, 
e.g.,  respiration,  the  heart  beat,  blood  pressure,  etc.,  all  of  which 
have  been  considered  in  foregoing  pages. 

ELEVENTH  PAIR.     THE  SPINAL  ACCESSORY. 

The  eleventh  cranial  nerve,  the  spinal  accessory,  consists  of  per- 
ipherally coursing  fibers  which  bring  the  nerve-cells  from  which  they 
arise  into  relation  with  separate  but  functionally  related  muscles. 
It  consists  of  two  portions,  the  medullary  or  bulbar  and  the  spinal. 

Origin. — The  axons  comprising  the  medullary  portion  arise  from 
a  group  of  nerve-cells  in  the  lower  part  of  the  nucleus  ambiguus. 
From  this  origin  the  axons  pass  forward  and  outward  to  emerge  from 
the  medulla  just  below  and  in  series  with  the  roots  of  the  vagus  nerve. 

The  axons  comprising  the  spinal  portion  have  their  origin  in 
nerve-cells  in  the  lateral  margin  of  the  anterior  horn  of  the  gray  matter 
in  the  cervical  portion  of  the  cord  as  far  down  as  the  fifth  cervical 
vertebra.  From  this  origin  the  fibers  pass  to  the  surface  of  the  cord 
to  emerge  between  the  ventral  and  dorsal  roots  in  from  six  to  eight 
filaments,  after  which  they  unite  from  below  upward  to  form  a  distinct 
nerve.  This  enters  the  cranial  cavity  through  the  foramen  magnum, 
where  it  joins  with  the  medullary  portion  to  form  the  common  trunk, 
which  then  passes  forward  to  emerge  from  the  cranium  through  the 
jugular  foramen.     (Fig.  283.) 


THE  CRANIAL  NERVES. 


613 


Distribution. — After  emerging  from  the  cranial  cavity  the  nerve 
soon  separates  into  two  branches: 

1.  iVn  internal  or  anastomotic  branch,  consisting  chiefly  of  filaments 

coming  from  the  medulla 
oblongata.  It  soon  enters 
the  trunk  of  the  vagus,  from 
which  fibers  pass  to  the 
muscles  of  the  pharynx,  to 
the  muscles  of  the  larynx 
through  the  inferior  laryn- 
geal nerve,  and  to  the  heart 
according  to  most  author- 
ities. 

2.  An  external  branch,  consisting 

chiefly    of     the     accessory 

fibers  from  the  spinal  cord. 
It    is  distributed  to  the  sterno- 

cleido-mastoid  and  trapezius 

muscles. 
Cortical  Connections. — 
The  nucleus  of  origin  of  the 
medullary  branch  at  least  is  in 
relation  with  nerve-cells  in  the 
lower  third  of  the  general  cerebral 
motor  area,  the  axons  of  which 
descend  in  the  pyramidal  tract. 

Properties.  —  Faradization 
of  the  medullary  portion  of  the 
nerve  near  its  origin  gives  rise  to 
contraction  of  the  muscles  to 
which  they  are  distributed.  De- 
struction of  the  medullary  root 
is  followed  by  impairment  of 
deglutition  and  a  loss  of  the 
power  of  producing  vocal  sounds 
on  account  of  paralysis  of  the 
constrictor  muscles  of  the  larynx. 
According  to  some  authorities, 
there  is  also  an  acceleration  of 
the  heart's  action  from  a  loss  of 
inhibitor  influences. 

Stimulation  of  the  external 
branch  gives  rise  to  contraction 
of  the  sterno-cleido-mastoid  and 
trapezius  muscles,  though  division  of  the  branch  does  not  give  rise  to 
complete  paralysis,  as  they  are  supplied  with  motor  fibers  also  from 
lhe  cervical  nerves.     In  consequence  of  division  of  the  external  branch 


Fig.  2S3. — Spinal  Accessory  Xerve. 
1.  Trunk  of  the  facial  nerve.  2.  2. 
Glossopharyngeal  nerve.  3,  3  Pneumo- 
gastric.  4,  4,  4.  Trunk  of  the  spinal  acces- 
sory. 5.  Sublingual  nerve.  6.  Superior 
cervical  ganglion.  7,  7.  Anastomosis  of 
the  first  two  cervical  nerves.  8.  Carotid 
branch  of  the  sympathetic.  9,  10,  11, 
12,  13.  Branches  of  the  glosso-pharyngeal. 
14,  15.  Branches  of  the  facial.  16.  Otic 
ganglion.  17.  Auricular  branch  of  the 
pneumogastric.  iS.  Anastomosing  branch 
from  the  spinal  accessory  to  the  pneu- 
mogastric. 10.  Anastomosis  of  the  first 
pair  of  cervical  nerves  with  the  sublingual. 
20.  Anastomosis  of  the  spinal  accessory 
with  the  second  pair  of  cervical  nerves.  2 1 . 
Pharyngeal  plexus.  22.  Superior  laryn- 
geal nerve.  23.  External  laryngeal  nerve. 
24.  Middle  cervical  ganglion. — {Hirsch- 
jeld.) 


6i4 


TEXT-BOOK  OF  PHYSIOLOGY. 


animals  experience  extreme  shortness  of  breath  during  exercise,  from  a 
want  of  coordination  of  the  muscles  of  the  fore-limbs  and  the  muscles 
of  respiration. 

Functions. — The  function  of  the  fibers  of  the  spinal  accessory 
nerve  is  the  transmission  of  nerve  impulses  from  the  cells  from  which 


Fig.  284. — Distribution  of  the  Hypoglossal  Nerve. — 1.  Root  of  the  fifth  nerve. 
2.  Ganglion  of  Gasser.  3,  4,  5,  6,  7,  9,  10,  12.  Branches  and  anastomoses  of  the  fifth 
nerve.  11.  Submaxillary  ganglion.  13.  Anterior  belly  of  the  digastric  muscle.  14. 
Section  of  the  mylo-hyoid  muscle.  15.  Glosso-pharyngeal  Nerve.  16.  Ganglion  of 
Andersch.  17,  18.  Branches  of  the  glosso-pharyngeal  nerve.  19,  19.  Pneumogastric. 
20,  21.  Ganglia  of  the  pneumogastric.  22,  22.  Superior  laryngeal  branch  of  the  pneu- 
mogastric. 23.  Spinal  accessory  nerve.  24.  Sublingual  nerve.  25.  Descendens  noni. 
6.  Thyro-hyoid  branch.  27.  Terminal  branches.  28.  Two  branches  one  to  the  genio- 
hyo-glossus  and  the  other  to  the  genio-hyoid  muscle. — (Sappey.) 


they  take  their  origin  to  the  muscles  to  which  they  are  distributed. 
They  therefore  excite  to  action  some  of  the  muscles  of  deglutition;  the 
muscles  which  regulate  the  tension  of  the  vocal  bands  during  phonal  ion 
and  the  muscles  which  control  the  respiratory  movements  associated 
with  sustained  or  prolonged  muscle  efforts.  The  fibers  also  convey 
nerve  impulses  which  exert  an  inhibitor  inllucncc  on  the  heart. 


THE  CRANIAL  NERVES.  615 

TWELFTH  PAIR.     THE  HYPOGLOSSAL. 

The  twelfth  cranial  nerve,  the  hypoglossal,  consists  of  peripherally 
coursing  nerve-fibers  which  serve  to  connect  the  nerve-cells  from 
which  they  arise  with  the  musculature  of  the  tongue. 

Origin. — The  axons  composing  the  hypoglossal  nerve  arise  from 
a  collection  of  nerve-cells  situated  beneath  the  floor  of  the  fourth 
ventricle.  This  nucleus  is  elongated  and  extends  from  the  medullary 
striae  downward  as  far  as  the  lower  border  of  the  olivary  body.  It-  is 
located  ventro-laterally  to  the  spinal  canal.  After  leaving  the  cells 
of  the  nucleus  the  axons  pass  forward  and  outward  toward  the  surface 
of  the  medulla,  from  which  they  emerge  in  ten  or  twelve  small  bundles 
or  filaments  in  the  groove  between  the  olivary  body  and  the  anterior 
pyramid.     Beyond  this  point  they  unite  to  form  a  common  trunk. 

Distribution. — The  common  trunk  thus  formed  passes  out  of 
the  cranial  cavity  through  the  anterior  condyloid  foramen.  In  its 
course  it  receives  filaments  from  the  first  and  second  cervical  nerves, 
the  sympathetic  and  vagus.  It  is  finally  distributed  to  the  intrinsic 
muscles  of  the  tongue  and  to  the  genio-hyo-glossus,  hyo-glossus,  and 
stylo-hyoid  muscles.  Branches  derived  from  the  cervical  plexus 
pass  to  muscles  which  elevate  and  depress  the  hyoid  bone.    (Fig.  284.) 

Cortical  Connections. — The  hypoglossal  nerve  nuclei  are  con- 
nected with  nerve-cells  in  the  lower  third  of  the  general  motor  area 
around  the  inferior  termination  of  the  fissure  of  Rolando  by  axons 
which  descend  in  the  pyramidal  tract. 

Properties. — Faradization  of  the  nerve  gives  rise  to  convulsive 
movements  of  the  muscles  to  which  it  is  distributed.  Division  of  the 
nerve  is  followed  by  a  loss  of  motion  and  an  interference  with  deglu- 
tition, mastication,  and  articulation,  especially  in  the  pronunciation 
of  the  consonantal  sounds.  In  hemiplegia,  complicated  with  paral- 
ysis of  the  tongue  from  injury  to  the  hypoglossal  tract,  the  opposite 
side  of  the  tongue  is  involved  in  the  paralysis.  On  protrusion  of 
the  tongue  the  tip  is  deviated  to  the  paralyzed  side,  due  to  the  un- 
opposed action  of  the  muscle  of  the  opposite  side. 

Function. — The  hypoglossal  nerve  transmits  nerve  impulses 
from  its  center  of  origin  to  the  intrinsic  and  extrinsic  muscles  of  the 
tongue,  endowing  them  with  motility.  The  coordinate  activity  of 
these  muscles  favorably  assists  mastication,  articulation,  and  deg- 
lutition. 


CHAPTER  XXIV. 

THE  SYMPATHETIC  NERVE  SYSTEM. 

•  The  sympathetic  nerve  system  consists  of  a  number  of  ganglia 
united  one  to  another  by  intervening  cords  of  nerve-fibers.  These 
ganglia  may  for  convenience  of  description  be  divided  into  three 
groups:  viz.,  the  vertebral  or  lateral,  the  pre- vertebral  or  collateral, 
and  the  peripheral  or  terminal. 

The  vertebral  ganglia  are  arranged  in  the  form  of  chains,  one  on 
each  side  of  the  vertebral  column.  The  number  of  ganglia  in  the 
chain  varies  in  animals  of  different  and  in  animals  of  the  same  species. 
In  man  the  number  varies  from  20  to  22.  Each  chain  may  be  divided 
into  a  cervical,  a  thoracic,  a  lumbar,  a  sacral,  and  a  coccygeal  portion. 
The  cervical  portion  is  usually  described  as  consisting  of  three  ganglia 
— a  superior,  a  middle,  and  an  inferior.  This  statement  is  open  to 
question,  however,  as  the  middle  one  is  frequently  absent  and  the 
inferior  one  is  regarded  by  some  anatomists  as  belonging  to  the  pre- 
vertebral series.  The  thoracic  portion  consists  of  ten  or  eleven  ganglia, 
the  lumbar  and  sacral  portions  of  four  each  and  the  coccygeal  portion 
of  one,  the  so-called  ganglion  impar. 

The  pre-vertebral  ganglia  are  also  united  in  the  form  of  a  chain 
situated  in  the  abdominal  cavity.  The  ganglia  constituting  this 
chain  are  known  as  the  semilunar,  the  renal,  the  superior  and  inferior 
mesenteric,  and  hypogastric. 

The  peripheral  ganglia  are  in  more  or  less  close  relation  with  the 
tissues  and  organs  in  different  parts  of  the  body.  As  members  of 
this  group  may  be  mentioned  the  ciliary  or  ophthalmic,  the  spheno- 
palatine, the  otic,  the  submaxillary  and  the  sublingual  ganglia;  the 
ganglia  in  walls  of  the  heart,  the  respiratory  organs,  the  intestines, 
the  bladder,  etc. 

The  general  arrangement  of  the  sympathetic  ganglia,  their  inter- 
connecting cords  and  branches,  is  shown  in  Figs.  285  and  286. 

Structure  of  the  Ganglia. — Each  ganglion  consists  of  a  capsule 
or  stroma  of  connective  tissue  in  which  are  contained  large  numbers 
of  nerve-cells,  nerve-fibers,  medullated  and  non-medullated,  and 
blood-vessels.  The  nerve-cells  give  origin  to  two  or  more  dendrites, 
which,  perforating  a  nucleated  capsule  by  which  each  cell  is  sur- 
rounded, branch  and  rebranch  and  interlace  to  form  a  pcricapsular 
plexus.  Each  cell  gives  origin  also  to  an  axon,  which  as  it  leaves 
the  cell  becomes  invested  with  a  sheath  continuous  with  the  capsule 
surrounding  the  cell-body.  Jt  is,  however,  wanting  in  a  medullary 
sheath,  and  hence  the  nerve  presents  a  gray  color.  Such  a  structure, 
in  its  entirety,  is  known  as  a  sympathetic  neuron. 

616 


THE  SYMPATHETIC  NERVE  SYSTEM. 


617 


Fig.  2S5. — Cervical  and  Thoracic  Portiox  of  the  Sympathetic,  i,  i,  i.  Right 
pneumogastric.  2.  Glossopharyngeal.  3.  Spinal  accessory.  4.  Divided  trunk,  of  the 
sublingual.  5,  5,  5.  Chain  of  ganglia  of  the  sympathetic.  6.  Superior  cervical  ganglion. 
7.  Branches  from  this  ganglion  to  the  carotid.  S.  Nerve  of  Jacobson.  9.  Two  filaments 
from  the  facial,  one  to  the  spheno-palatine  and  the  other  to  the  otic  ganglion.  10.  M<  it<  •; 
oculi  externus.  11.  Ophthalmic  ganglion,  receiving  a  motor  filament  from  the  motor 
oculi  communis  and  a  sensory  filament  from  the  nasal  branch  of  the  fifth.  12.  Spheno- 
palatine ganglion.     13.  Otic  ganglion.     14.  Lingual  branch  of  the  fifth  nerve.     15.  Sub- 


6i8  TEXT-BOOK  OF  PHYSIOLOGY. 

Structure  of  the  Interconnecting  Cords. — The  interconnecting 
cords  are  composed  of  non-medullated  and  medullated  nerve-fibers. 
The  former  are  the  axons  of  cells  found  in  the  ganglia  more  centrallv 
located;  the  latter,  as  will  be  stated  later,  are  derived  from  the  spinal 
nerves,  from  the  fibers  of  which,  however,  they  differ  in  character, 
being  much  smaller  and  finer.  The  fibers  of  the  interconnecting 
cords,  as  a  rule,  transmit  nerve  impulses  from  the  more  centrally  to 
the  more  peripherally  located  ganglia,  and  are  therefore  termed  rami 
ejferentes.  In  the  vertebral  chain  some  of  the  cords  transmit  nerve 
impulses  upward,  others  downward,  others  again  forward,  to  the  pre- 
vertebral and  peripheral  ganglia. 

Among  the  rami  efferentes,  interconnecting  cords,  there  are  some 
which  possess  special  interest  for  the  physiologist,  viz.: 
r.  The  cervical,  which  connects  the  thoracic  ganglia  with  the  superior 
cervical  ganglion.     It  is  composed  mainly  of  medullated  nerve- 
fibers  which  are  derived  originally  from  the  spinal  nerves. 

2.  The'  great  splanchnic  nerve,  formed  by  the  union  of  branches  from 

the  fifth  to  the  tenth  thoracic  ganglia.     It  connects  these  ganglia 
with  the  semilunar  ganglion. 

3.  The  small  splanchnic  nerve,  formed  by  the  union  of  branches  from 

the  ninth  and  tenth  thoracic  ganglia.     It  connects  these  ganglia 

with  the  solar  and  renal  plexuses. 
Distribution  of  the  Sympathetic  Fibers. — It  has  been  demon- 
strated by  histologic  and  physiologic  methods  of  investigation  that 
the  sympathetic  non-medullated  fibers  which  have  their  origin  in 
the  cells  of  the  sympathetic  ganglia,  vertebral,  pre-vertebral,  and 
peripheral,  are  distributed  ultimately  and  directly  to  but  two  struc- 
tures: viz.,  non-striated  muscle  and  secretor  epithelium.  Moreover, 
there  is  no  evidence  to  warrant  the  assumption  that  these  structures 
ever  receive  nerve  impulses  directly  from  the  spinal  or  cranial  nerves. 
All  nerve  impulses  which  influence  their  activities,  either  in  the  way 
of  augmentation  or  inhibition,  emanate  directly  though  not  originally 
from  the  sympathetic  ganglion  cells.  Since  non-striated  muscles 
are  found  in  the  walls  of  blood-vessels,  in  the  walls  of  hollow  viscera, 

maxillary  ganglion.  16,  17.  Superior  laryngeal  nerve.  18.  External  laryngeal  nerve. 
19,  20.  Recurrent  laryngeal  nerve.  21,  22,  23.  Anterior  branches  of  the  upper  four 
cervical  nerves,  sending  filaments  to  the  superior  cervical  sympathetic  ganglion.  24.  An- 
terior  branches  of  the  fifth  and  sixth  cervical  nerves,  sending  filaments  to  the  middle 
cervical  ganglion.  25,  26.  Anterior  branches  of  the  seventh  and  eighth  cervical  and  the 
first  dorsal  nerves,  sending  filaments  to  the  inferior  cervical  ganglion.  27.  Middle  cervical 
ganglion.  28.  Cord  connecting  the  two  ganglia.  29.  Inferior  cervical  ganglion.  30,31. 
Filaments  connecting  this  with  the  middle  ganglion.  32.  Superior  cardiac  nerve,  t,^. 
Middle  cardiac  nerve.  34.  Inferior  cardiac  nerve.  35,  35.  Cardiac  plexus.  36. 
fjanglioii  of  the  cardiac  plexus.  37.  Nerve  following  the  right  coronary  artery.  38,38. 
Intercostal  nerves,  with  their  two  filaments  of  communication  with  the  thoracic  ganglia. 
39,40,41.  Great  splanchnic  nerve.  42.  Lesser  splanchnic  nerve.  43,43.  Solar  plexus. 
II-  I.'-l'i  pneumogastric.  45.  Right  pneumogastric.  46.  Lower  end  of  the  phrenic 
nerve.  17.  Si ■<  tion  of  the  right  bronchus.  48.  Arch  of  the  aorta.  49.  Right  auricle. 
50.  Right  ventricle.  51,52.  Pulmonary  artery.  53.  Right  half  of  the  stomach.  54.  Sec- 
tion of  the  diaphragm.     (Sappey.) 


THE  SYMPATHETIC  NERVE  SYSTEM. 


619 


and  around  hair-follicles,  and  since  secretor  epithelium  is  found  in 
all  glands,  there  is  every  reason  to  believe  that  the  ganglia  in  some 


Fig.  286. — Lumbar  and  Sacral  Portions  of  the  Sympathetic,  i.  Section  of  the 
diaphragm.  2.  Lower  end  of  the  esophagus.  3.  Left  half  of  the  stomach.  4.  Small 
intestine.  5.  Sigmoid  flexure  of  the  colon.  6.  Rectum.  7.  Bladder.  S.  Prostate.  9. 
Lower  end  of  the  left  pneumogastric.  10.  Lower  end  of  the  right  pneumogastric.  n. 
Solar  plexus.  12.  Lower  end  of  the  great  splanchnic  nerve.  13.  Lower  end  of  the  lesser 
splanchnic  nerve.  14,  14.  Last  two  thoracic  ganglia.  15,  15.  The  four  lumbar  ganglia. 
16,  16,  17,  17.  Branches  from  the  lumbar  ganglia.  iS.  Superior  mesenteric  plexus.  19, 
21,22,23.  Aortic  lumbar  plexus.  20.  Inferior  mesenteric  plexus.  24,24.  Sacral  portion 
of  the  sympathetic.  25,  25,  26,  26,  27,  27.  Hypogastric  plexus.  28,  29,  30.  Tenth, 
eleventh,  and  twelfth  dorsal  nerves.  31,  32,  ^^,  34,  35,  36,  37,  ^S,  39.  Lumbar  and  sacral 
nerves. — (Sa  ppey.) 


62o  TEXT-BOOK  OF  PHYSIOLOGY. 

way  are  associated  with  vaso-augmentor  and  vaso-inhibitor,  viscero- 
motor and  viscero-inhibitor,  pilo-motor  and  secretor  phenomena. 

The  Anatomic  Relations  of  the  Sympathetic  and  Cerebro- 
spinal Systems. — The  sympathetic  ganglia  are  connected  with  the 
spinal  nerves  by  two  branches,  one  white,  the  other  gray  in  color, 
and  known  respectively  as  the  white  and  gray  rami  communicantes. 
These  two  rami  differ  somewhat  in  their  topographic  distribution. 
The  white  rami  are  found  passing  only  from  those  spinal  nerves 
included  between  the  first  thoracic  and  second  or  third  lumbar  and 
their  corresponding  ganglia.  The  gray  rami,  on  the  contrary,  are 
found  passing  from  the  ganglia  to  each  of  the  spinal  nerves.  In  the 
cervical  region,  where  the  ganglia  do  not  correspond  in  number  with 
the  cervical  nerves,  each  ganglion  gives  off  two  or  more  gray  rami. 
In  man  the  superior  cervical  ganglion  sends  gray  rami .  to  the  first 
four  cervical  nerves;  the  middle  and  inferior  ganglia  apparently  send 
gray  rami  to  the  fifth  and  sixth,  the  seventh  and  eighth  nerves  re- 
spectively. 

The  white  rami  are  composed  of  fine  medullated  nerve-fibers 
which  arise  from  nerve-cells  situated  in  the  lateral  portion  of  the  gray 
matter  in  the  thoracic  and  lumbar  regions  of  the  spinal  cord.  From 
this  origin  they  pass  forward  into  the  ventral  roots  of  the  spinal  nerves, 
in  which  they  are  contained  until  the  spinal  nerve  formed  by  the  union 
of  the  ventral  and  dorsal  roots  divides  into  its  anterior  and  posterior 
divisions.  At  this  point  the  fine  medullated  nerve-fibers  leave  the 
common  trunk  and  pass  forward  into  the  corresponding  vertebral 
ganglion,  around  the  cell-bodies  of  which  some  of  the  fibers  at  once 
arborize.  Other  fibers,  however,  pass  through  this  ganglion  and 
ascend  or  descend  the  cord  for  a  variable  distance,  and  arborize  around 
the  cells  of  more  or  less  distant  ganglia;  others  again  pass  forward 
into  the  pre-vertebral  and  even  the  peripheral  ganglia  before  they 
finally  terminate.  The  nerve-cells  in  the  spinal  cord  are  thus  brought 
into  relation  with  the  ganglia  of  all  three  chains,  though  for  each  cell 
there  is  but  one  ganglion  terminal,  one  cell  station,  between  the  spinal 
cord  and  the  tissues.  Though  innervated  by  the  spinal  cord,  these 
structures  receive  their  nerve  impulses,  as  previously  stated,  not 
directly  but  indirectly  through  the  ganglion  cells.  The  medullated 
nerve-fibers  coming  from  the  spinal  cord  are  known  as  pre- ganglionic 
-fibers;  the  non-medullated  fibers,  passing  from  the  ganglia,  as  post- 
ganglionic fibers. 

The  gray  rami  are  composed  of  non-mcdullated  nerve-fibers, 
axons  of  the  cells  in  the  vertebral  or  lateral  ganglia.  After  their 
emergence  from  the  ganglia  they  take  a  backward  direction  and  enter 
the  spinal  nerve-trunks,  in  company  with  which  they  pass  to  the 
periphery,  to  be  finally  distributed  to  structures  in  the  skin:  viz.,  non- 
striated  muscles  of  blood-vessels,  non-striated  muscles  of  the  hair- 
follicles  and  epithelium  of  glands.  They  may  therefore  be  regarded 
as  having  vaso-motor,  pilo-motor,  and  secretor  functions. 


THE  SYMPATHETIC  NERVE  SYSTEM.  621 

Afferent  Sympathetic  Fibers.— With  the  foregoing  groups  of 
efferent  fibers,  the  sympathetic  nerves,  in  the  thoracic  and  lumbar 
regions  more  especially,  contain  a  number  of  afferent  fibers  which 
when  stimulated  give  rise  to  sensations  of  pain  or  to  reflex  phenomena. 
The  routes  by  which  these  afferent  fibers  reach  the  spinal  cord  lead, 
on  the  one  hand,  into  and  through  the  gray  rami  to  the  ganglia  on 
the  posterior  roots,  where  they  have  their  cells  of  origin;  and,  on  the 
other  hand,  into  and  through  the  white  rami.  The  number  of  afferent 
fibers  in  any  trunk  in  comparison  with  the  efferent  is  quite  small. 

FUNCTIONS  OF  THE  SYMPATHETIC  SYSTEM. 

The  view  according  to  which  the  sympathetic  system  is  to  be 
regarded  as  an  independent  apparatus  endowed  with  functions  of  its 
own  and  in  nowise  directly  dependent  for  its  activities  on  the  spinal 
cord,  is  at  the  present  time  discarded.  Peripheral  structures  cease 
to  exhibit  their  characteristic  functions  after  division  of  the  spinal 
nerves  in  connection  with  their  related  ganglia.  This  does  not  exclude 
the  possibility  of  the  sympathetic  cell-body,  in  virtue  of  the  inter- 
changes between  it  and  the  blood  and  lymph  by  which  it  is  surrounded, 
maintaining  its  own  nutrition  and  exerting  a  favorable  influence  over 
the  nutrition  of  the  peripheral  tissues  to  which  its  efferent  branches 
are  distributed. 

The  nerve-tissue  in  its  entirety  may  be  regarded  as  a  single  system 
which  may  be  functionally  divided  into  a  nerve  system  of  animal  and 
a  nerve  system  of  vegetative  life,  according  as  the  nerve  energies 
originating  in  and  emanating  from  the  central  nervous  system  are 
transmitted  directly  to  the  skeletal  muscles  or  indirectly,  through 
the  intervention  of  a  sympathetic  neuron,  to  visceral  muscles  and 
glands.  In  the  former  system  but  one  neuron,  the  spino-periph- 
eric,  connects  the  spinal  cord  proper  with  the  muscle;  in  the  latter 
system  there  are  two,  the  spino-ganglionic  and  the  ganglio-peripheric. 

From  the  distribution  of  the  post-ganglionic  fibers  it  may  be  in- 
ferred that  the  activities  of  the  vascular  and  visceral  muscles,  either 
in  the  way  of  augmentation  or  inhibition,  the  activities  of  the  muscles 
of  the  hair-follicles,  and  of  the  epithelium  of  glands,  are  called  forth 
by  the  ganglia  in  consequence  of  the  arrival  of  nerve  impulses  coming 
from  the  spinal  cord  through  the  pre-ganglionic  fibers.  Experimental 
observations  show  this  to  be  true.  The  extent  to  which  these  different 
modes  of  activity  manifest  themselves  in  one  or  more  regions  of  the 
body  will  depend  to  some  extent  on  the  portion  of  the  sympathetic 
system  subjected  to  experimental  procedures. 

The  Functions  of  the  Cervical  Portion. — If  the  sympathetic 
cord  central  to  the  superior  cervical  ganglion  be  stimulated  with  the 
induced  electric  current,  among  the  resulting  phenomena  there  will  be 
observed  dilatation  of  the  pupil,  retraction  of  the  nictitating  mem- 
brane in  animals  possessing  it,   contraction  of  the  blood-vessels  of 


622  TEXT-BOOK  OF  PHYSIOLOGY. 

the  skin  and  mucous  membrane  in  different  parts  of  the  head  and 
face,  contraction  of  the  blood-vessels  of  the  salivary  glands,  increase 
of  secretion  from  the  submaxillary  gland,  the  perspiratory  and  mucous 
glands,  erection  of  hairs  in  different  localities  of  the  head  and  neck, 
and  in  the  dog  dilatation  of  the  blood-vessels  of  the  lips,  gums,  and 
hard  palate.  If  the  cervical  cord  be  divided,  opposite  effects  will  be 
observed:  viz.,  contraction  of  the  pupil,  dilatation  and  passive  conges- 
tion of  the  blood-vessels,  a  rise  in  temperature,  and  a  loss  of  the  power 
of  erecting  hairs.  Stimulation  of  the  peripheral  end  causes  a  dis- 
appearance of  the  latter  and  a  reappearance  of  the  former  phenomena. 
These  facts  indicate  that  the  cervical  portion  is  efferent  in  function. 
The  fibers  composing  it  are  medullated  nerve-fibers  derived  from  the 
thoracic  or  dorsal  nerves  from  the  first  to  the  fourth.  From  the 
several  sources  the  fibers  pass  via  the  white  rami  into  the  vertebral 
chain,  and  thence  without  interruption  to  the  superior  cervical  ganglion, 
in  and  around  the  cells  of  which  their  end-tufts  arborize  in  their  char- 
acteristic manner. 

That  the  superior  cervical  ganglion  is  the  cell  station  between  the 
spinal  cord  and  the  peripheral  organs  is  shown  by  the  fact  discovered 
and  applied  by  Langley  that  the  intravenous  injection  of  nicotin  or 
the  local  application  of  it  to  the  ganglion  itself,  impairs  the  conductivity 
of  the  terminals  of  pre-ganglionic  fibers,  after  which  their  stimulation 
has  no  effect  on  the  ganglion  cells,  though  the  latter  retain  their 
activity,  as  shown  on  direct  stimulation.  Of  the  nerve-centers  in  the 
spinal  cord  which  through  pre-ganglionic  fibers  influence  peripheral 
structures,  some  appear  to  be  in  a  state  of  constant  activity:  e.  g., 
the  vaso-constrictor  centers  and  the  pupillo-dilatator  centers.  In  how 
far  this  action  is  automatic  or  autochthonic,  or  reflex,  is  uncertain. 

The  Functions  of  the  Thoracic  Portion. — The  phenomena 
which  follow  stimulation  of  this  portion  of  the  sympathetic  system 
resemble  in  a  general  way  those  observed  in  the  head  when  the  cervical 
portion  is  stimulated.  The  situation  of  the  resulting  phenomena 
will  vary  in  accordance  with  the  part  the  subject  of  the  experiment. 
For  an  understanding  of  the  results  of  experiment  the  origin  and 
distribution  of  the  following  nerve-branches  must  be  kept  in  view: 

(a)  The  cardiac  nerves  which  take  their  origin  in  part  in  cells  in  the  first 

thoracic  or  stellate  ganglion,  and  in  part  in  cells  in  the  inferior 
cervical  ganglion.  From  this  origin  they  pass  do wn ward  and  for- 
ward and  reach  the  heart  by  way  of  the  cardiac  plexus.  Stimu- 
lation of  these  nerves  gives  rise  to  an  increased  frequency  and  an 
augmentation  in  the  force  of  the  heart-beat.  The  pre-ganglionic 
libers  by  which  these  cells  are  excited  to  activity  emerge  from  the 
cord  by  the  first  and  second  thoracic  nerves. 

(b)  The  splanchnic  nerves  the  roots  of  which  emerge  from  the  fourth 
to  the  tenth  or  eleventh  thoracic  ganglia.  The  fibers  composing 
these  nerves  arc  for  the  most  pari  pre-ganglionic  and  derived 
from  the  corresponding  spinal  nerves.     The  cell  stations  of  the 


THE  SYMPATHETIC  NERVE  SYSTEM.  623 

splanchnic  fibers  are  in  the  semilunar,  superior  mesenteric,  and 
renal  ganglia.  From  these  ganglia  non-medullated  post-gan- 
glionic  fibers  pass  peripherally  to  the  walls  of  the  intestines,  the 
blood-vessels  of  the  intestines,  liver,  kidneys,  spleen,  etc.  Stim- 
ulation of  the  great  splanchnic  produces  inhibition  of  the  in- 
testinal movements,  a  marked  primary  contraction  of  the  intes- 
tinal movements,  a  marked  primary  contraction  of  the  intestinal 
blood-vessels  and  other  viscera,  followed  by  dilatation,  coincidently 
with  which  there  is  a  primary  rise  succeeded  by  a  fall  of  blood- 
pressure  throughout  the  body.  Division  of  the  nerve  is  followed 
by  dilatation  of  the  intestinal  vessels  and  a  fall  of  blood-pressure. 
Stimulation  of  the  central  end  of  the  divided  nerve  excites  the 
activity  of  the  general  vaso-motor  center,  as  shown  by  the  rise  of 
the  general  blood-pressure.  Stimulation  of  the  smaller  splanchnics 
gives  rise  to  a  slight  primary  contraction  of  the  blood-vessels,  soon 
followed  by  a  marked  dilatation.  These  facts  indicate  that  the 
splanchnic  nerves  contain  visceral  nerves  which  inhibit  intestinal 
movements,  vaso-motor  fibers  both  augmentor  and  inhibitor. 
The  presence  of  secretor  nerves  for  the  intestinal  glands  is  dis- 
puted. 
(c)  The  cutaneous  nerves  for  the  trunk  leave  the  lateral  ganglia  by 
the  gray  rami,  enter  the  thoracic  spinal  nerves,  and  pass  in  com- 
pany with  them  to  their  terminations,  to  be  ultimately  distrib- 
uted to  the  walls  of  the  blood-vessels,  the  arrectores  pilorum 
muscles,  and  the  sweat-glands.  The  pre-ganglionic  fibers  come 
from  the  spinal  nerves  by  the  white  rami.  Their  functions  are 
vaso-motor,  pilo-motor,  and  secretor.  The  cutaneous  nerves 
for  the  fore-limbs  have  their  origin  from  cells  in  the  stellate  gan- 
glion (first  dorsal).  After  a  short  upward  course  they  enter 
the  trunks  of  the  nerves  composing  the  brachial  plexus.  The 
pre-ganglionic  fibers  come  from  the  white  rami  of  the  fourth  to  the 
ninth  thoracic  nerves.  After  entering  the  lateral  chain  they  take 
an  upward  direction  and  arborize  around  the  cells  of  the  stellate 
ganglion.  The  cutaneous  nerves  for  the  liind-limbs  are  derived 
from  the  lower  lumbar  and  the  upper  sacral  ganglia.  They  also 
enter  the  spinal  nerves  by  the  gray  rami  and  pass  to  the  blood- 
vessels and  glands  of  the  skin.  The  pre-ganglionic  fibers  come 
from  the  twelfth  thoracic  to  the  third  lumbar  nerves.  In  both 
the  brachial  and  sciatic  nerves  vaso-motor  fibers  (constrictors 
and  dilatators)  and  secretor  nerves  are  present,  as  shown  by 
experimental  methods  (see  page  374). 
The  Functions  of  the  Lumbo-sacral  Portion. — From  the 
ganglia  of  the  lumbar  and  sacral  regions  gray  rami  enter  the  lumbar 
and  sacral  nerves  and  accompany  them  to  their  distribution.  In 
the  lumbar  region  the  vertebral  chain  contains  a  number  of.  pre- 
ganglionic fibers  which  have  descended  from  the  thoracic  region  as 
well  as  fibers  which  have  come  into  the  chain  bv  the  white  rami  from 


624  TEXT-BOOK  OF  PHYSIOLOGY. 

the  lumbar  nerves  themselves.  Many  of  these  fibers  pass  to  the 
inferior  mesenteric  ganglion,  in  which  they  find  their  cell  station. 
Fibers  from  the  sacral  cord  pass  into  the  hypogastric  plexus.  The 
course  and  distribution  of  the  individual  nerves  is  complicated  and 
involved.  In  a  general  way  it  may  be  said  that  these  two  regions  of 
the  lateral  chain  send  viscero-motor  and  viscero-inhibitor,  vaso-con- 
strictor  and  dilator  nerves  to  the  pelvic  viscera  and  to  the  external 
organs  of  generation.  Their  function  therefore  is  to  regulate  the 
activities  of  the  viscera  as  well  as  the  blood-supply  in  accordance 
with  functional  needs. 

The  Functions  of  the  Cephalic  Ganglia. — The  ganglia  situated 
in  the  head  are  usually  described  in  connection  with  and  as  con- 
stituent parts  of  the  cranial  nerve  system.  They,  however,-  bear  the 
same  relation  to  the  cranial  nerves  that  the  ganglia  of  the  trunk  bear 
to  the  spinal  nerves.  They  consist  of  ganglion  cells  from  which  post- 
ganglionic fibers  pass  to  glands,  blood-vessels,  and  non-striated 
muscles,  and  to  which  pre-ganglionic  fibers  pass  from  the  cranial 
nerves.  Motor  and  sensor  nerves  pass  through  one  or  more  ganglia, 
though  they  have  no  anatomic  connection  with  them.  In  their  struc- 
ture, distribution,  and  functions  they  closely  resemble  the  collateral 
ganglia  of  the  abdominal  sympathetic: 

i.  The  ciliary  or  ophthalmic  ganglion  is  situated  in  the  orbital  cavity 
posterior  to  the  eyeball.  It  is  small  in  size,  gray  in  color,  and 
consists  of  a  connective-tissue  stroma  containing  nerve-cells. 
From  these  cells  post-ganglionic  fibers  emerge  which,  after  a 
short  course  forward,  penetrate  the  eyeball  and  terminate  in  the 
circular  fibers  of  the  iris  and  the  ciliary  muscle.  Pre-ganglionic 
fibers  of  small  size,  and  similar  in  their  anatomic  features  to 
the  fibers  of  the  white  rami  of  the  spinal  nerves,  leave  the  motor 
oculi  by  a  short  root  from  the  inferior  division  and  arborize 
around  the  ganglionic  cells.  Stimulation  of  the  pre-ganglionic 
fibers  gives  rise  to  contraction  of  the  circular  fibers  of  the  iris, 
with  a  diminution  in  the  size  of  the  pupil,  and  contraction  of  the 
ciliary  muscle  with  accommodation  of  the  eye  for  near  vision. 
Division  of  these  fibers  is  followed  by  the  opposite  results. 
Post-ganglionic  fibers  from  the  superior  cervical  ganglion  which 
come  through  the  cavernous  plexus  pass  through  the  ciliary 
ganglion  to  the  blood-vessels  of  the  iris  and  retina  which  are 
vaso-constrictor  in  function.  Sensor  fibers  from  the  peripheral 
division  of  the  fifth  nerve  pass  to  the  cornea  and  endow  it  with 
sensibility. 
2.  The  spheno- palatine  ganglion  is  situated  in  the  spheno-maxillary 
fossa.  Its  nerve-cells  send  non-medullated  post-ganglionic  fibers 
to  the  blood-vessels  and  glands  of  the  mucous  membrane  of  the 
nasal  and  oral  regions.  Stimulation  of  the  ganglion  gives  rise 
to  dilatation  of  the  blood-vessels  and  increase  of  secretion  in  this 
•  ntirc   region.     The    pre-ganglionic   fibers  are   derived  from   the 


THE  SYMPATHETIC  NERVE  SYSTEM.  625 

seventh  or  facial  nerve  by  way  of  the  great  petrosal.  Sensor 
fibers  from  the  superior  maxillary  division  of  the  fifth  nerve  pass 
through  the  ganglion  to  the  same  regions. 

3.  The   otic  ganglion   is   situated  just  below  the  foramen  ovale  and 

internal  to  the  third  division  of  the  fifth  nerve.  The  postgang- 
lionic fibers  pass  to  the  parotid  gland  by  way  of  the  auriculo- 
temporal division  of  the  fifth  nerve,  and  to  the  blood-vessels  of  the 
mucous  membrane  of  the  lower  lip,  cheek,  and  gums.  The  pre- 
ganglionic fibers  are  derived  from  the  efferent  fibers  in  the  glosso- 
pharyngeal or  ninth  nerve,  by  way  of  Jacobson's  nerve  and  the 
small  petrosal.  Stimulation  of  these  nerves  in  any  part  of  their 
course  gives  rise  to  vascular  dilatation  and  increase  of  secretion  in 
the  region  of  their  distribution.  Motor  fibers  from  the  small 
or  motor  root  of  the  fifth  nerve  pass  through  this  ganglion  to 
the  tensor  tympani  muscle. 

4.  The  submaxillary  and  sublingual  ganglia  are  situated  close  to  the 

corresponding  glands.     Their  post-ganglionic  fibers  pass  to  the 
blood-vessels    and    gland-cells.     The    pre-ganglionic    fibers    are 
derived   from  the   seventh  or  facial  nerve  through   the  chorda 
tympani  branch.     Stimulation  of  the  chorda  or  of  the  ganglia 
themselves  gives  rise  to  marked  dilatation  of  the  blood-vessels 
and   an   increased   flow   of   saliva.     It   therefore   contains   vaso- 
dilatator and  secretor  fibers  for  these  glands.     Yaso-constrictor 
and  a  few  secretor  nerves,  it  will  be  recalled,  come  to  these  glands 
from  the  superior  cervical  ganglion. 
Peripheral    Ganglia. — Among   the    peripheral    ganglia    may   be 
mentioned  those  in  the  heart  and  those  in  the  intestinal  walls.     The 
pre-ganglionic  fibers  are  contained  in  the  trunk  of  the  vagus  nerve. 
Stimulation  of  the  peripheral  end  of  the  divided  vagus  gives  rise  to 
inhibition  of  the  heart,  contraction  of  the  walls  of  the  stomach  and 
intestines,  secretion  from  the  gastric  and  perhaps  the  pancreatic  gland. 


40 


CHAPTER  XXV. 
PHONATION;  ARTICULATE  SPEECH. 

Phonation,  the  emission  of  vocal  sounds,  is  accomplished  by  the 
vibration  of  two  elastic  membranes  which  cross  the  lumen  of  the 
larynx  antero-posteriorly  and  which  are  thrown  into  vibration  by  a 
blast  of  air  from  the  lungs. 

Articulate  speech  is  a  modification  of  the  vocal  sounds  or  the  voice 
produced  by  the  teeth  and  the  muscles  of  the  lips  and  tongue  and  is 
employed  for  the  expression  of  ideas. 

The  larynx,  the  organ  of  the  voice,  is  situated  in  the  forepart  of 
the  neck,  occupying  the  space  between  the  hyoid  bone  and  the  upper 
extremity  of  the  trachea.  In  this  situation  it  communicates  with  the 
cavity  of  the  pharynx  above  and  the  cavity  of  the  trachea  below. 
From  its  anatomic  relations  and  its  internal  structure — the  interpola- 
tion of  the  elastic  membranes — the  larynx  subserves  the  two  widely 
different  yet  related  functions,  respiration  and  phonation. 

THE  ANATOMY  OF  THE  LARYNX. 

The  larynx  consists  primarily  of  a  series  of  cartilages  united  one 
with  another  in  such  a  manner  as  to  form  a  more  or  less  rigid  frame- 
work, yet  possessing  at  its  different  joints,  a  certain  amount  of  motion; 
and  secondarily,  of  muscles  and  nerves  which  conjointly  impart  to 
the  cartilages  the  degree  of  movement  necessary  to  the  performance 
of  the  laryngeal  functions.  It  is  covered  externally  by  fibrous  tissue 
and  lined  throughout  by  mucous  membrane  continuous  with  that 
lining,  the  pharynx  and  trachea. 

The  larynx  presents  a  superior  or  pharyngeal  and  an  inferior  or 
tracheal  opening.  The  pharyngeal  opening  is  triangular  in  shape, 
the  base  being  directed  forward,  the  apex  backward.  The  plane  of 
this  opening  in  the  living  subject  is  almost  vertical.  The  tracheal 
opening  is  circular  in  shape  and  corresponds  in  size  with  the  upper 
ring  of  the  trachea.  Viewed  from  above,  the  general  cavity  of  the 
larynx  is  seen  to  be  partially  subdivided  by  two  membranous  bands — 
the  vocal  bands  or  cords — which  run  from  before  backward  in  a  hori- 
zontal plane.  The  space  between  the  bands,  the  glottis,  varies  in 
size  and  shape  from  moment  to  moment  in  accordance  with  respira- 
tory and  phonatory  necessities.  The  average  width  of  the  glottis,  at 
its  widest  part,  during  quiet  respiration  is  about  13.5  mm.  in  men 
and  n. 5  mm.  in  women.  With  the  advent  of  phonation  the  vocal 
membranes  are  at  once  approximated,  and  to  such  an  extent  that  the 
glottic  opening  is  reduced  to  a  mere  slit.  It  is  then  spoken  of  as  the 
rima  glottidis,  or  chink  of  the  glottis. 

626 


PHONATION;  ARTICULATE  SPEECH. 


627 


The  space  above  the  vocal  bands,  the  supra-glottic  or  supra-rimal 
space,  is  triangular  in  shape  and  extends  from  the  pharyngeal  opening 
to  the  plane  of  the  vocal  bands.  The  mucous  membrane  lining  the 
walls  of  this  space,  presents  on  either  side,  just  above  the  vocal  bands, 
a  crescentic  fold  which  runs  from  be- 
fore backward,  and  is  known  as  the 
false  vocal  band  or  cord.  Between 
the  true  and  false  bands  there  is  a 
cavity  or  space  prolonged  upward  and 
outward  for  some  distance,  forming 
what  is  known  as  the  ventricle  of  the 
larynx.  The  space  below  the  vocal 
bands,  the  infra-glottic  or  infra-rimal 
space,  is  narrow  above  and  elongated 
from  before  backward,  but  wide  and 
circular  below,  corresponding  to  the 
lumen  of  the  trachea.  (Fig.  287.) 

The  Laryngeal  Cartilages,  Artic- 
ulations, and  Ligaments. — The 
cartilages  which  compose  the  frame- 
work of  the  larynx  are  nine  in  num- 
ber, three  of  which  are  single:  viz., 
the  cricoid,  the  thyroid,  and  the  epi- 
glottis, while  six  occur  in  pairs:  viz., 
the  arytenoids,  the  cornicula  laryngis, 
and  the  cuneiform.  (Figs.  288  and 
289.) 

The  cricoid  cartilage  is  the  founda- 
tion cartilage,  and  affords  support  to 
the  remaining  cartilages  and  the  struc- 
tures attached  to  them.  In  shape  it 
resembles  a  signet-ring,  the  broad 
quadrate  portion  of  which  is  directed 
backward,  while  the  narrow  circular 
portion  is  directed  forward.  It  rests 
upon  the  upper  ring  of  the  trachea,  to 
which  it  is  firmly  attached  by  fibrous 
tissue.  The  posterior  upper  border  of 
the  quadrate  portion  presents  on  either 
side  an  oval  convex  facet  for  articula- 
tion with  the  arytenoid  cartilage.  The 
long  axis  of  this  facet  is  directed  down- 
ward, outward,  and  forward. 

The  thyroid,  the  largest  of  the  laryngeal  cartilages,  is  composed 
of  two  flat  quadrilateral  plates,  united  anteriorly,  at  an  angle  of  about 
90  degrees.  Each  plate  is  directed  backward  and  outward  and  ter- 
minates in  a  free  border,  which  is  prolonged  upward  and  downward 


Fig.  2S7. — Longitudinal  Section 
of  the  Human  Larynx,  Showing 
the  Vocal  Bands,  i.  Ventricle  of 
the  larynx.  2.  Superior  vocal  cord 
3.  Inferior  vocal  cord.  4.  Arytenoid 
cartilage.  5.  Section  of  the  arytenoid 
muscle.  6,  6.  Inferior  portion  of  the 
cavity  of  the  larynx.  7.  Section  of 
the  posterior  portion  of  the  cricoid 
cartilage.  S.  Section  of  the  anterior 
portion  of  the  cricoid  cartilage.  9. 
Superior  border  of  the  cricoid  car- 
tilage. 10.  Section  of  the  thyroid 
cartilage.  11,  11.  Superior  portion 
of  the  cavity  of  the  larynx.  12,  13. 
Arytenoid  gland.  14,  16.  Epiglottis. 
15,  17.  Adipose  tissue.  iS.  Section 
of  the  hyoid  bone.  19,  19,  20. 
Trachea. — {Sap  fey.) 


628 


TEXT-BOOK  OF  PHYSIOLOGY. 


for  some  distance,  terminating  in  two  processes,  the  superior  and 
inferior  cornua.  The  upper  border  of  the  thyroid  is  deeply  notched 
in  front.     The  inferior  border  overlaps  laterally  the  cricoid. 

The  epiglottis  is  a  leaf-shaped  piece  of  cartilage  attached  to  the 
thyroid  at  the  median  notch.  It  is  firmly  united  by  membranes  and 
ligaments  to  the  thyroid  and  arytenoid  cartilages  and  to  the  base  of 
the  tongue. 

The   arytenoid  cartilages   are  two   in   number   and   symmetric  in 


Fig.  ,288. — Laryngeal  Cartilages 
and  Ligaments,  Anterior  Surface. 
1.  Hyoid  bone.  2,  2,  3,  3.  Greater  and 
lesser  cornua.  4.  Thyroid  cartilage.  5. 
Thyro-hyoid  membrane.  6.  Thyro-hyoid 
ligaments.  7.  Cartilaginous  nodule.  8. 
Cricoid  cartilage.  9.  The  crico-thyroid 
membrane.  10.  The  crico-thyroid  liga- 
ments.    11.  Trachea. — (Sappey.) 


Fig.  289. — Laryngeal  Cartilages 
and  Ligaments,  Posterior  Surface. 
1,  1.  Thyroid  cartilage.  2.  Cricoid  car- 
tilage. 3,  3.  Arytenoid  cartilages.  4,  4. 
Crico-arytenoid  articulations.  5,  5.  Crico- 
thyroid articulations.  6.  Union  of  the 
cricoid  cartilage  and  of  the  trachea.  7. 
Epiglottis.  8.  Ligament  uniting  it  to 
the  reentering  angle  of  the  thyroid  car- 
tilage.— {Sappey.) 


shape.  Each  cartilage  is  a  triangular  pyramid,  the  apex  of  which  is 
rei  urved,  and  directed  backward  and  inward.  The  base  presents  three 
angles — an  anterior,  an  external,  and  an  internal.  The  anterior  angle 
is  long  and  pointed  and  projects  forward  in  a  horizontal  plane.  It 
serves  for  the  attachment  of  the  vocal  membranes  and  is  therefore 
termed  the  vocal  process.  The  external  angle  is  short,  rounded,  and 
prominent,  and  serves  for  the  attachment  of  muscles.  The  internal 
angle  affords  a  poinl  of  insertion  for  a  ligament.  The  inferior  surface 
of  the  arytenoid  is  concave  for  articulation  with  the  convex  surface 
of  the  cricoid  facet.     Its  long  axis,  however,  is  directed  from  before 


PHONATION;  ARTICULATE  SPEECH.  629 

backward  and  almost  at  right  angles  to  the  long  axis  of  the  cricoid 
facet. 

The  cornicida  laryngis  and  the  cuneiform  cartilages  are  small 
nodules  of  yellow  elastic  cartilage  embedded  in  a  fold  of  membrane 
which  unites  the  arytenoid  and  the  epiglottis.  They  are  fragments 
of  a  ring  of  cartilage  which  in  some  animals — e.  g.,  anteater — extends 
between  these  two  cartilages. 

The  crico-thyroid  articulation  is  formed  by  the  opposition  of  the 
tip  of  the  inferior  cornu  of  the  thyroid  cartilage  and  an  articular  facet 
on  the  side  of  the  cricoid.  The  joint  is  provided  with  a  synovial 
membrane  and  enclosed  by  a  capsular  ligament.  The  movements 
permitted  at  this  joint  take  place  around  a  horizontal  axis  and  consist 
of  an  upward  and  downward  movement  of  both  the  thyroid  and 
cricoid,  combined  with  a  sliding  movement  of  the  latter  upward  and 
backward. 

The  crico-arytenoid  articulation  is  formed  by  the  apposition  of 
the  articulating  surfaces  of  the  cricoid  and  arytenoid  cartilages.  This 
joint  is  provided  with  a  synovial  membrane  and  enclosed  by  a  loose 
capsular  ligament  which  would  permit  of  an  extensive  sliding  of  the 
arytenoid  cartilage  downward  and  outward  were  it  not  prevented 
by  the  posterior  crico-arytenoid  ligament,  which  is  attached,  on  the 
one  hand,  to  the  cricoid,  and,  on  the  other,  to  the  inner  angle  of  the 
arytenoid.  The  movements  permitted  at  this  joint  are :  (1)  Rotation 
of  the  arytenoid  around  a  vertical  axis  which  lies  close  to  its  inner 
surface.  (2)  A  sliding  motion  inward  and  forward  with  inward 
rotation  of  the  vocal  process,  or  a  sliding  motion  outward  and  back- 
ward with  outward  rotation  of  the  vocal  process.  In  either  case  the 
process  describes  an  arc  of  a  circle.  (3)  A  sliding  movement  towards 
the  median  line  in  consequence  of  which  the  inner  surfaces  of  the 
arytenoids  are  brought  almost  in  contact. 

The  crico-thyroid  membrane  is  composed  mainly  of  elastic  tissue. 
It  may  be  divided  into  a  mesial  and  two  lateral  portions.  The  mesial 
portion  is  well  developed,  triangular  in  shape,  and  unites  the  con- 
tiguous borders  of  the  cricoid  and  thyroid  cartilages.  The  lateral 
portion  is  attached  below  to  the  superior  border  of  the  cricoid.  From 
this  attachment  it  passes  upward  and  inward  under  cover  of  the 
thyroid.  As  it  ascends  it  elongates  and  becomes  thinner,  and  is 
finally  attached  anteriorly  to  the  thyroid  near  the  median  line,  and 
posteriorly  to  the  vocal  process  of  the  arytenoid,  thus  constituting 
the  inferior  thyro-arytenoid  ligament.  It  is  covered  internally  by 
mucous  membrane  and  externally  by  the  internal  thyro-arytenoid 
muscle.  The  free  edge  of  this  ligament  forms  the  basis  of  the  true 
vocal  band.  A  superior  thyro-arytenoid  ligament  forms  the  basis 
of  the  false  vocal  band. 

The  thyro-hyoid  membrane,  composed  of  elastic  tissue,  unites  the 
superior  border  of  the  thyroid  to  the  hyoid  bone. 

The    mucous   membrane  lining  the   larynx   is   thin   and   pale.      As 


6.^o 


TEXT-BOOK  OF  PHYSIOLOGY. 


it  passes  downward  it  is  reflected  over  the  superior  thyro-arytenoid 
ligament,  and  assists  in  the  formation  of  the  false  vocal  band;  it  then 
passes  into  and  lines  the  ventricle,  after  which  it  is  reflected  inward 
over  the  superior  border  of  the  thyro-arytenoid  muscle  and  ligament, 
and  assists  in  the  formation  of  the  true  vocal  band;  it  then  returns 
upon  itself  and  passes  downward  over  the  lateral  portion  of  the  crico- 
thyroid membrane  into  the  trachea. 


'■;■'.■*$  'Mm 


~MJ 


Fig.  290. — Posterior  View  of  the 
Muscles  of  the  Larynx,  i.  Posterior 
crico-arytenoid  muscle.  2,  3,  4.  Differ- 
ent fasciculi  of  the  arytenoid  muscle. 
5.  Aryteno-epiglottidean  muscle. — (Sap- 
pey.) 

The  thin,  free,  reduplicated 
edge  of  the  mucous  membrane 
constitutes  the  true  vocal  band. 
The  surface  of  the  mucous  mem- 
brane is  covered  by  ciliated 
epithelium  except  in  the  im- 
mediate neighborhood  of  the 
vocal  bands. 

The  vocal  bands  are  attached  anteriorly  to  the  thyroid  cartilage 
near  the  receding  angle  and  posteriorly  to  the  vocal  processes  of  the 
arytenoid  cartilages.  They  vary  in  length  in  the  male  from  20  to  25 
mm.  and  in  the  female  from  15  to  20  mm. 

The  Muscles  of  the  Larynx.— The  muscles  which  have  a  direct 
action  on  the  cartilages  of  the  larynx  and  determine  the  position  of  the 
vocal  bands  both  for  respiratory  and  phonatory  purposes,  and  which 


Fig.  291. — Lateral  View  of  the 
Muscles  of  the  Larynx,  i.  Body  of 
the  hyoid  bone.  2.  Vertical  section  of 
the  thyroid  cartilage.  3.  Plorizontal  sec- 
tion of  the  thyroid  cartilage  turned  down- 
ward to  show  the  deep  attachment  of 
the  crico-thyroid  muscle.  4.  Facet  of 
articulation  of  the  small  cornu  of  the 
thyroid  cartilage  with  the  cricoid  cartilage. 
5.  Facet  on  the  cricoid  cartilage.  6. 
Superior  attachment  of  the  crico-thyroid 
muscle.  7.  Posterior  crico-arytenoid 
muscle.  8,  10.  Arytenoid  muscle.  9. 
Thyro-arytenoid  muscle,  n.  Aryteno- 
epiglottidean  muscle.  12.  Middle  thyro- 
hyoid ligament.  13.  Lateral  thyrohyoid 
ligament. — (Sappey.) 


PHONATION;  ARTICULATE  SPEECH. 


631 


regulate  their  tension  as  well,  are  nine  in  number  and  take  their  names 
from  their  points  of  origin  and  insertion:  viz.,  two  posterior  crico- 
arytenoids, two  lateral  crico-arytenoids,  two  thyro-arytenoids,  one 
arytenoid,  and  two  crico-thyroids  (Figs.  290  and  291). 

The  posterior  crico-arytenoid  muscle  lies  on  the  posterior  surface 
of  the  quadrate  plate  of  the  cricoid  cartilage,  on  either  side  of  the 
median  line,  from  which  it  takes  its  origin.  The  fibers  of  the  muscle 
pass  upward  and  outwrard  and  in  their  course  converge  to  be  inserted 
into  the  external  angle  of  the  arytenoid  cartilage.  The  superior  and 
more  horizontally  directed  fibers  rotate  the  arytenoid  around  its 
vertical  axis;  the  inferior  and  obliquely  directed  fibers  draw  the  cartilage 
downward  and  inward.  As  a  result  of  the  action  of  the  muscle  in  its 
entirety,  the  vocal  process  is  turned  upward  and  outwrard,  and  as  the 
vocal  band  is  carried  with  it  the  glottis  is  widened,  a  condition  nec- 


ttt$£&£ 


Fig.  292. — Glottis  Widely  Opened 
from  Simultaneous  Contraction  oe 
Both  Crico-arytenoid  Muscles,  b. 
Epiglottis,  rs.  False  vocal  band.  ri. 
True  vocal  band.  ar.  Arytenoid  car- 
tilages, a.  Space  between  the  arytenoids. 
c.  Cuneiform  cartilages,  ir.  Interarytenoid 
fold.  rap.  Aryepiglottic  fold.  cr.  Car- 
tilage rings. — {Mandl.) 


Fig.  293. — Position  of  the  Vocal 
Bands  Due  to  the  Simultaneous 
Contraction  of  Both  Lateral  Crico- 
arytenoid Muscles  and  Both  Thyro- 
arytenoid Muscles,  b.  Epiglottis,  rs. 
False  vocal  band.  ri.  True  vocal  band. 
or.  Space  between  the  arytenoid  cartil- 
ages, the  glottis  respiratoria.  ar.  Ary- 
tenoid cartilages,  c.  Cuneiform  carti- 
lages, rap.  Aryepiglottic  fold.  ir.  In- 
terarytenoid fold. — {Mandl.) 


essary  to  the  free  entrance  of  air  into  the  lungs  (Fig  292).  Since 
the  contraction  of  the  crico-arytenoid  has  this  result,  it  is  genera  11  v 
spoken  of  as  the  abductor  or  respiratory  muscle. 

The  lateral  crico-arytenoid  muscle  arises  from  the  side  of  the  cricoid 
cartilage.  From  this  point  its  fibers  are  directed  upward  and  back- 
ward to  be  inserted  into  the  external  process  of  the  arytenoid.  Its 
action  is  to  draw  the  arytenoid  cartilage  forward  and  inward,  thus 
approximating  and  relaxing  the  vocal  band. 

The  thyro-arytenoid  muscle  arises  from  the  inferior  two-thirds 
of  the  inner  surface  of  the  thyroid  cartilage  just  external  to  the  median 
line.  From  this  origin  the  fibers  pass  backward  and  outward,  to  be 
inserted  into  the  anterior  surface  and  external  angle  of  the  arytenoid 
cartilage.  The  inner  portion  of  the  muscle  lies  close  to  and  supports, 
if  it  does  not  constitute  a  part  of,  the  vocal  band.  The  action  of  the 
thyro-arytenoid  muscle  in  conjunction  with  the  lateral  crico-arvtenoid 
is  to  rotate  the  arytenoid  cartilage  around  the  vertical  axis  and  to 


632  TEXT-BOOK  OF  PHYSIOLOGY. 

draw  the  vocal  process  forward  and  inward,  thus  carrying  the  vocal 
cord  toward  the  median  line.  When  the  muscles  of  the  two  sides 
simultaneously  contract,  the  vocal  bands  are  closely  approximated 
and  the  space  between  them,  the  rima  vocalis,  reduced  to  a  mere  slit, 
one  of  the  conditions  essential  to  phonation  (Fig.  293). 

The  arytenoid  muscle  consists  (1)  of  transversely  arranged  fibers 
which  arise  from  and  are  inserted  into  the  outer  surface  of  the  oppo- 
site arytenoid  cartilages,  and  (2)  of  obliquely  directed  fibers  which 
arise  from  the  outer  angle  of  one  arytenoid  to  be  inserted  into  the 
apex  of  the  other.  In  their  course  they  decussate  in  the  median  line. 
The  action  of  this  muscle  is  to  approximate  the  arytenoid  cartilages 
and  thus  obliterate  that  portion  of  the  glottis  between  the  vocal  proc- 
esses, the  rima  respiratoria,  and  so  direct  the  expiratory  blast  of  air 
toward  and  through  the  rima  vocalis. 

The  collective  actions  of  the  three  foregoing  muscles  is  to  close  or 
constrict  the  glottis,  and  for  this  reason  they  are  spoken  of  as  the 
adductor  or  pJwnatory  muscles. 

The  crico-thyroid  muscle  arises  from  the  side  and  front  of  the 
cricoid  cartilage  and  is  inserted  above  into  the  lower  border  of  the 
thyroid  cartilage.  The  action  of  this  muscle  is  to  draw  up  the  an- 
terior part  of  the  cricoid  cartilage  toward  the  thyroid,  which  remains 
stationary,  and  to  swing  the  quadrate  plate  of  the  cricoid  and  the 
arytenoid  cartilages  downward  and  backward.  This  movement 
has  the  result  of  tensing  the  vocal  bands.  The  cricoid  is  at  the  same 
time  drawn  backward  by  the  action  of  the  more  longitudinally  dis- 
posed fibers. 

Nerves  of  the  Larynx. — The  nerves  which  innervate  the  muscles 
of  the  larynx  and  endow  the  mucous  membrane  with  sensibility  are 
derived  from  the  vagus  trunk.  The  superior  laryngeal  is  for  the 
most  part  sensor  and  distributed  to  the  mucous  membrane,  though 
it  contains  motor  fibers  for  the  crico-thyroid  muscle.  The  inferior 
laryngeal  is  purely  motor  and  is  distributed  to  all  the  muscles  with 
the  exception  of  the  crico-thyroid. 

THE  MECHANISM  OF  PHONATION. 

Phonation,  the  production  of  vocal  sounds  in  the  larynx,  is  the 
result  of  the  vibration  of  the  vocal  bands  caused  by  an  expiratory 
blast  of  air  from  the  lungs.  That  a  sound  may  arise  it  is  essential 
that  the  glottis  be  approximately  closed  and  the  vocal  bands  be  made 
more  or  less  tense. 

The  closure  of  the  glottis — the  approximation  of  the  vocal  proc- 
-  and  the  vocal  bands — is  accomplished,  it  will  be  recalled,  by 
the  contraction  of  the  lateral  crico-arytenoid,  the  arytenoid,  and  the 
thyro-arytenoid  muscles.  The  increase  in  tension  is  accomplished 
by  the  contraction  of  the  crico-thyroid  and  the  thyro-arytenoid  muscles, 
the  former  by  the  backward  displacement  of  the  cricoid  and  arytenoid 


PHOXATIOX;  ARTICULATE  SPEECH. 


633 


cartilages,  the  latter  by  converting  the  natural  concave  edge  of  the 
vocal  band  to  a  straight  line.  The  lengthening  and  tensing  of  the 
vocal  bands  by  the  crico-thyroid  muscle  is  regarded  by  some  investi- 
gators as  a  coarse  means,  the  approximation  of  the  free  edges  by  the 
thyro-arytenoid,  as  a  finer  means,  of  adjustment  for  the  production 
of  slight  changes  in  the  pitch  of  sounds.  The  extent  to  which  the 
glottis  is  closed  and  the  membranes  tensed  will  depend,  however, 
on  the  pitch  of  the  sound  to  be  emitted.  The  appearance  presented 
bv  the  glottis  just  previous  to  the  emission  of  a  note  of  medium  pitch, 
as  determined  by  laryngologic  examination,  is  shown  in  Fig.  294. 
When  the  foregoing  conditions  in  the  glottis  are  realized,  the  air  stored 
or  collected  in  the  lungs  is  forced  by  the  contraction  of  the  expiratory 
muscles,  through  the  narrow  space  between  the  bands.  As  a  result 
of  the  resistance  offered  by  this  narrow  outlet  and  the  force  of  the 
expiratory  muscles  the  air  within  the  lungs  and  trachea  is  subjected 
to  pressure,  and  as  soon  as  the  pressure  attains  a  certain  level  the  vocal 


Fig.  294. — Position  of  the  Vocal 
Bands  Previous  to  the  Emission  of  a 
Sound,  b.  Epiglottis,  rs.  False  vocal 
band.  ri.  True  vocal  band.  ar.  Ary- 
tenoid cartilages. — (Mandl.) 


Fig.  295. — Position  of  the  Vocal 
Bands  in  the  Production  of  Notes 
of  Low  Pitch.  /.  Epiglottis,  or.  Glottis. 
ns.  False  vocal  cord.  ni.  True  vocal  cord. 
ar.  Arytenoid  cartilages. — (Mandl.) 


bands  are  thrown  into  vibrations,  which  in  turn  impart  to  the  column 
of  air  in  the  upper  air-passages  a  corresponding  series  of  vibrations  by 
which  the  laryngeal  vibrations  are  reinforced.  The  degree  of  pressure 
to  which  the  air  in  the  lungs  and  trachea  is  subjected  was  determined 
by  Latour  to  vary  from  160  mm.  of  water  for  sounds  of  moderate, 
to  940  mm.  of  water  for  sounds  of  highest  intensity.  With  the  escape 
of  the  air  or  the  separation  of  the  vocal  bands  the  vibration  ceases 
and  the  sound  dies  away. 

The  Characteristics  of  Vocal  Sounds. — In  common  with  the 
sounds  produced  by  all  other  music  instruments,  all  vocal  sounds 
are  characterized  by  intensity,  pitch  and  quality,  tone  or  color. 

The  intensity  or  loudness  of  a  sound  depends  on  the  extent  or  am- 
plitude of  the  up-and-down  vibration  or  the  extent  of  the  excursion 
of  the  vocal  band  on  either  side  of  the  position  of  equilibrium  or  rest ; 
and  this  in  turn  depends  on  the  force  with  which  the  blast  of  air  strikes 
the  band.  The  more  forceful  the  blast  of  air,  the  larger,  other  things 
being  equal,  will  be  the  primary  vibrations  of  the  bands,  and  hence 
the  secondary  vibrations  of  the  air  in  the  upper  air-passages. 


634 


TEXT-BOOK  OF  PHYSIOLOGY. 


fejL 


The  pitch  of  the  voice  depends  on  the  number  of  vibrations  in  a 
unit  of  time,  a  second.  This  will  be  conditioned  by  the  length  of  the 
bands  in  vibration  or  the  length  and  width  of  the  aperture  through 
which  the  air  passes  and  the  degree  of  tension  to  which  the  bands  are 
subjected.  In  the  emission  of  sounds  of  highest  pitch  the  tension  of 
the  vocal  bands  and  the  narrowing  of  the  glottis  attain  their  maxi- 
mum. In  the  emission  of  sounds  of  lowest  pitch  the  reverse  conditions 
obtain.  In  passing  from  the  lowest  to  the  highest  pitched  sounds  in 
the  range  of  the  voice  peculiar  to  any  one  individual,  there  is  a  pro- 
gressive increase  in  both  the  tension  of  the  vocal  bands  and  the  narrow- 
ing of  the  glottic  aperture.     In  the  production  of  low-pitched  notes 

of  men,  those  due  to  vibrations  lying  be- 
tween 80  and  240  per  second,  the  tension 
is  regulated  by  the  crico-thyroid  muscle; 
the  aperture  of  the  glottis  during  this  time 
being  elliptic  in  shape  and  relatively  wide 
(Fig.  295).  In  the  production  of  notes  due 
to  vibrations  lying  between  240  and  512 
vibrations  per  second,  the  anterior  fibers  of 
the  cricothyroid  muscle  relax  and  the  thyro- 
arytenoid muscle  comes  into  play;  by  its 
action  the  vocal  bands  are  more  closely 
approximated  and  the  vocal  aperture  re- 
duced to  a  linear  slit.  In  the  high-pitched 
notes  emitted  by  soprano  singers  the  vocal 
bands  are  so  closely  applied  to  each  other 
that  only  a  very  small  portion  in  front, 
bounding  a  small  oval  aperture,  is  capable 
of  vibrating  (Fig.  296).  The  difference  in 
the  pitch  of  the  voice  in  men  and  women  is 
due  largely  to  the  greater  size  and  develop- 
ment of  the  vocal  bands  in  the  former  than  in  the  latter. 

The  quality  of  the  voice,  the  timbre  or  color,  depends  on  the  form 
combined  with  the  intensity  and  pitch  of  the  vibration.  As  with 
sounds  produced  by  music  instruments,  the  primary  or  fundamental 
vibration  of  the  vocal  band  is  complicated  by  the  superposition  of 
secondary  or  partial  vibrations  (overtones).  The  form  of  the  vibration 
will  therefore  be  a  resultant  of  the  blending  of  a  number  o'f  different 
vibrations.  The  quality  of  the  sound  produced  in  the  larynx  is, 
however,  modified  by  the  resonance  of  the  mouth  and  nasal  cavities; 
1  ertain  of  the  overtones  being  reinforced  by  changes  in  the  shape  of  the 
mouth  cavity  more  especially,  thus  giving  to  the  voice  a  somewhat 
different  quality. 

The  Varieties  of  Voice. — The  region  of  the  music  scale,  com- 
prising all  vibrations  between  32  and  2048  per  second,  with  which 
laryngeal  sounds  arc  in  accord  will  vary  in  the  two  sexes  and  in  different 
individuals  of  the  same  sex.     It  is  customary  to  classify  voices,  es- 


Fig.  296. — Glottis  Seen 
with  the  Laryngoscope  dur- 
ing the  Emission  of  High- 
pitched  Sounds,  i,  2.  Base 
of  the  tongue.  3,  4.  Epiglot- 
tis. 5,  6.  Pharynx.  7.  Ary- 
tenoid cartilages.  8.  Opening 
between  the  true  vocal  cords. 

9.  Aryteno-epiglottidean  folds. 

10.  Cartilage  of  Santorini.  11. 
Cuneiform  cartilage.  12.  Su- 
perior vocal  cords.  13.  In- 
ferior vocal  cords. — (Le  Bon.) 


PHONATIOX;  ARTICULATE  SPEECH.  635 

pecially  those  of  singers,  into  bass,  baritone,  tenor,  contralto,  mezzo- 
soprano,  and  soprano,  in  accordance  with  the  regions  of  the  music 
scale  with  which  they  correspond.  Thus  the  succession  of  notes 
characteristic  of  the  bass  voice  vary  in  pitch  from  F,  fa',  to  C,  do3, 
or  from  87  to  256  vibrations  per  second;  those  of  the  baritone  from 
A,  la,  to  F',  fa3,  or  from  106  to  341  vibrations  per  second;  those  of 
the  tenor  from  C,  do2,  to  a',  la3,  or  from  128  to  435  vibrations  per 
second;  those  of  the  contralto  from  e,  mi2,  to  C" ,  do4,  or  from  160  to 
512  vibrations  per  second;  those  of  the  mezzo-soprano  from  g,  sol,, 
to  e",  mi4,  or  from  192  to  640  vibrations  per  second;  those  of  the 
soprano  from  b,  si2,  to  g",  sol4,  or  from  240  to  768  vibrations  per  second. 
The  range  of  the  voice  is  thus  seen  to  embrace  from  one  and  three- 
quarters  to  two  octaves.  Some  few  individual  singers  have  far  ex- 
ceeded this  range,  but  they  are  exceptional. 

Speech  is  the  expression  of  ideas  by  means  of  articulate  sound?. 
These  sounds  may  be  divided  into  vowel  and  consonant  sounds. 

The  vowel  sounds,  a,  e,  i,  0,  u,  are  laryngeal  sounds  modified  by 
the  superposition  and  reinforcement  of  certain  overtones  developed 
in  the  mouth  and  pharynx  by  changes  in  their  shapes.  The  number 
of  vibrations  underlying  the  production  of  each  vowel  sound  is  a 
matter  of  dispute.  According  to  Konig,  the  sound  of  a  is  the  result 
of  940  vibrations;  of  e,  1880  vibrations;  of  i,  3760  vibrations;  of  o, 
470  vibrations;  of  ou,  235  vibrations. 

Consonant  sounds  are  produced  by  the  more  or  less  complete  in- 
terruption of  the  vowel  sounds  during  their  passage  through  the  organs 
of  speech.     These  may  be  divided  into: 

1.  Labials,  p,  b,  m. 

2.  Labio-dentals,  /,  v.   . 

3.  Linguo-dentals,  s,  z. 

4.  Anterior  linguo-palatals,  t,  d,  1,  n. 

5.  Posterior  linguo-palatals,  k,  g,  h,  y,  r. 

The  names  of  these  different  groups  of  consonants  indicate  the 
region  of  the  mouth  in  which  they. are  produced  and  the  means  by 
which  the  air  blast  is  interrupted. 

THE  NERVE  MECHANISM  OF  THE  LARYNX. 

The  nerve  mechanism  by  which  the  musculature  of  the  larynx 
is  excited  to  action  and  coordinated  so  as  to  subserve  both  respiration 
and  phonation  involves  the  fibers  contained  in  the  superior  and  inferior 
laryngeal  nerves  (both  branches  of  the  vagus)  and  their  related  nerve- 
centers  in  the  central  nerve  system. 

For  respiratory  purposes  it  is  essential  that  the  lumen  of  the  glottis 
shall  be  sufficiently  large  to  permit  the  entrance  and  exit  of  air  without 
hindrance.  Laryngoscopic  examination  of  the  larynx  in  the  human 
being  shows  that  during  quiet  respiration  the  vocal  bands  are  widely 
separated  and  almost  stationary,   moving  but  slightly  during  either 


636  TEXT-BOOK  OF  PHYSIOLOGY. 

inspiration  or  expiration.  At  this  time,  according  to  the  investigations 
of  Semon,  the  area  of  the  glottis  is  approximately  160  sq.  mm.,  some- 
what less  than  the  area  of  either  the  supraglottic  or  infraglottic  regions, 
which  is  about  200  sq.  mm.  This  condition  of  the  glottis  is  maintained 
bv  the  steady  continuous  contraction  of  the  posterior  crico-arytenoid 
muscles,  the  abductors  of  the  vocal  bands. 

For  phonatory  purposes  it  is  essential  that  the  respiratory  function 
be  temporarily  suspended  and  the  vocal  bands  closely  approximated. 
This  is  accomplished  by  the  contraction  of  the  remaining  muscles  of 
the  larynx,  with  the  exception  of  the  crico-thyroid,  which  are  collectively 
known  as  the  adductors  of  the  vocal  bands.  During  phonation  the 
adductor  muscles  overcome  the  activity  of  the  abductors.  With  the 
cessation  of  phonation  the  abductors  immediately  restore  the  vocal 
bands  to  their  former  respiratory  position. 

The  activities  of  these  two  antagonistic  groups  of  muscles  are  under 
the  control  of  the  central  nerve  system.  The  only  pathway  for  the 
excitatory  nerve  impulses  is  through  the  fibers  of  the  inferior  or  re- 
current laryngeal  nerve.  The  relation  of  these  nerve-fibers  both 
centrally  and  peripherally,  as  well  as  their  physiologic  action,  has  been 
the  subject  of  much  experimentation.  The  results  have  not  always 
been  in  accord,  owing  to  the  choice  of  animal,  the  use  of  anesthetics, 
strength  of  stimulus,  etc. 

As  the  outcome  of  many  investigations  it  is  believed  that  each 
muscle  group  is  innervated  by  its  own  bundle  of  nerve-fibers,  both 
of  which  are  contained  in  the  inferior  laryngeal,  though  coming  from 
two  separate  centers  in  the  medulla  oblongata.  Russell  succeeded 
in  separating  the  fibers  for  the  abductors  from  the  fibers  for  the  ad- 
ductors in  the  inferior  laryngeal,  and  in  tracing  them  to  their  termi- 
nations. So  completely  was  this  done  that  it  became  possible  to 
produce  at  will,  through  stimulation,  either  abduction  or  adduction, 
without  contraction  of  the  muscle  of  opposite  function. 

The  laryngeal  respiratory  center  was  located  by  Semon  and  Horsley, 
in  the  cat,  in  the  upper  part  of  the  floor  of  the  fourth  ventricle.  Stim- 
ulation of  this  area  during  etherization  was  followed  by  abduction 
of  the  vocal  bands.  The  efferent  fibers  of  this  center  are  believed  by 
some  investigators  to  leave  the  central  nerve  system  in  the  spinal 
accessory  nerve,  by  others  in 'the  lower  roots  of  the  vagus. 

From  the  continuous  activity  of  the  abductor  muscle,  and  the 
stationary  position  of  the  vocal  bands,  it  is  probable  that  the  medullary 
center  is  in  a  state  of  continuous  activity  or  tonus,  the  result  probably 
of  reflex  influences. 

A  cortical  representation  for  laryngeal  respiratory  movements 
has  been  determined  by  Semon  and  Horsley  in  different  classes  of 
animals.  In  the  cat  especially,  stimulation  of  the  border  of  the 
olfactory  sulcus  gives  rise  to  complete  abduction  of  the  vocal  bands 
on  both  sides.     The  representation  is  therefore  bilateral. 

The   phonatory  center  was  located   by  the  same  investigators  in 


PHOXATION;  ARTICULATE  SPEECH.  637 

the  medulla  near  the  ala  cinerea  and  the  upper  border  of  the  calamus 
scriptorius.  Stimulation  of  this  area  was  invariably  followed  by 
bilateral  adduction  of  the  vocal  bands  and  closure  of  the  glottis. 

A  cortical  representation  for  phonatory  movements  also  was 
located  in  the  lower  portion  of  the  precentral  convolution,  near  the 
anterior  border.  Stimulation  of  this  area  gives  rise  to  marked  ad- 
duction of  both  vocal  bands,  indicating  that  the  representation  is 
also  bilateral. 

Faradic  stimulation  of  the  inferior  laryngeal  nerve  during  slight 
ether  anesthetization  gives  rise  to  closure  of  the  glottis;  the  same 
stimulation,  however,  during  deeper  anesthetization  gives  rise  to 
opening  or  dilatation  of  the  glottis,  a  fact  indicating  that  either  the 
adductor  muscles  or  their  nerve  terminals  are  depressed  by  the  action 
of  the  ether  before  the  muscles  and  nerves  of  opposite  function.  The 
superior  laryngeal  nerves  contain  motor  fibers  for  the  crico-thyroid 
muscles.  Stimulation  of  the  nerve  gives  rise  to  contraction  of  the 
muscle  and  increased  tension  of  the  vocal  bands.  It  is  believed  that 
these  fibers  are  derived  originally  from  the  efferent  fibers  of  the  glosso- 
pharyngeal nerve.  The  remaining  fibers  of  the  superior  laryngeal 
endow  the  upper  portion  of  the  larynx  wTith  extreme  sensibility  which 
to  a  certain  extent  protects  the  air-passages  against  the  entrance  of 
foreign  bodies.  Irritation  of  the  terminal  filaments  of  this  nerve  by 
particles  of  food,  solid  or  liquid,  gives  rise  to  marked  reflex  spasm 
of  the  adductor  muscles  and  closure  of  the  glottis,  followed  by  a  strong 
expiration  blast  of  air  from  the  lungs  by  which  the  offending  particles 
are  removed.  Division  of  this  nerve  on  both  sides  is  followed  by  a 
paralysis  of  the  crico-thyroid  muscles,  a  lowering  of  the  tension  of  the 
vocal  bands,  and  a  loss  of  sensibility  of  the  laryngeal  mucous  mem- 
brane. 


CHAPTER  XXVI. 
THE  SPECIAL  SENSES. 

It  is  one  of  the  functions  of  the  nerve  system  to  bring  the  individual 
into  conscious  relation  with  the  external  world.  This  is  accomplished 
in  part  through  the  intermediation  of  afferent  nerves,  connected  per- 
ipherally, with  highly  specialized  terminal  organs,  and  centrally,  with 
specialized  areas  in  the  cerebral  cortex. 

Excitation  of  the  terminal  organs  by  material  changes  in  the  en- 
vironment develops  nerve  impulses  which,  transmitted  to  the  cortical 
areas,  evoke  sensations.  These  sensations,  differing  in  character 
from  those  vague  ill-defined  sensations — e.  g.,  fatigue,  well-being, 
discomfort,  etc. — caused  by  material  changes  occurring  within  the 
body,  are  termed  special  sensations — e.  g.,  touch;  pressure;  pain; 
temperature;  taste;  smell;  light  and  its  varying  qualities,  intensity, 
hue,  and  tint;  sound  and  its  varying  qualities,  intensity,  pitch,  and 
timbre. 

The  terminal  organs  which  receive  the  impress  of  the  external 
world  are  the  skin,  tongue,  nose,  eye,  and  ear,  and  collectively  con- 
stitute the  special  sense-organs.  The  physiologic  mechanisms  which 
underlie  and  develop  these  special  sensations  are  known  respectively 
as  the  tactile,  gustatory,  olfactory,  optic,  and  auditory.  Each  mechan- 
ism responds  to  but  a  single  form  of  stimulus  and  to  no  other.  Thus, 
the  stimulus  for  the  skin  is  mechanic  pressure;  for  the  tongue,  soluble 
organic  and  inorganic  matter;  for  the  nose,  volatile  or  gaseous  matter; 
for  the  eye,  ether  vibrations;  for  the  ear,  atmospheric  undulations. 
These  stimuli  alone  are  adequate  to  the  physiologic  excitation  of  the 
different  mechanisms. 

The  factors  involved  in  the  production  of  the  sensations  include 
(i)  a  special  physical  stimulus;  (2)  a  specialized  terminal  organ;  (3) 
an  afferent  nerve  pathway,  and  (4)  a  specialized  receptive  sensor  cell 
in  the  cerebral  cortex. 

Though  the  resulting  sensations  in  each  instance  differ  widely  in 
their  characteristics,  it  is  difficult  to  present  a  satisfactory  explanation 
for  these  differences.  If  it  be  assumed  that  the  nerve  impulses  which 
ascend  the  different  nerves  of  special  sense  are  alike  in  quality,  then 
it  must  be  admitted  that  the  character  of  the  sensation  is  the  expression 
of  a  specialization  and  organization  of  the  cortical  area.  If,  on  the 
other  hand,  specialization  of  the  cortex  is  denied,  then  there  must  be 
admitted  a  specialization  of  the  peripheral  organ — with  a  resulting 
difference  in  quality  or  rapidity  of  the  nerve  impulses  which  would 
impress  or  excite  the  non-specialized  cortex  in  such  a  way  as  to  call 
forth  the  characteristic  sensation,     it  is  possible,  however,  that  neither 

638 


THE  SENSE  OF  TOUCH.  639 

supposition  is  wholly  correct,  and  that  the  character  of  the  sensation 
depends  on  the  construction  and  adaptation  of  the  entire  sense  appara- 
tus to  the  character  of  the  stimulus. 

Whatever  the  conditions  for  their  origin  and  whatever  their  char- 
acteristics, sensations  in  themselves  do  not  constitute  knowledge; 
they  are  but  elementary  states  of  consciousness,  raw  materials  out 
of  which  the  mind  elaborates  conceptions  and  forms  judgments  as 
to  the  character  of  any  given  object  in  comparison  with  former  ex- 
periences. 

THE  SENSE  OF  TOUCH. 

The  physiologic  mechanism  involved  in  the  sense  of  touch  in- 
cludes the  skin  and  the  mucous  membrane  lining  the  mouth,  the 
afferent  nerves,  their  cortical  connections,  and  nerve-cells  in  the  cortex 
of  the  parietal  lobe. 

Peripheral  excitation  of  this  mechanism  develops  nerve  impulses 
which,  transmitted  to  the  cortex,  evoke  sensations  of  touch  and 
temperature.  To  the  skin,  therefore,  is  ascribed  a  touch  sense  and 
a  temperature  sense.  Of  the  touch  sensations  two  kinds  may  be  dis- 
tinguished: viz.,  pressure  sensations  and  place  sensations.  With  the 
contact  of  an  external  body  there  arises  the  perception  not  only  of  the 
pressure,  but  also  the  perception  of  the  place  or  locality  of  the  contact. 
In  accordance  with  this,  it  is  customary  to  attribute  to  the  skin  a  pres- 
sure sense  and  a  location  sense. 

The  specific  physiologic  stimuli  to  the  terminal  organs  in  the  skin 
and  oral  mucous  membrane  are  mechanic  pressure  and  thermic 
vibrations. 

The  Skin. — The  skin,  which  constitutes  the  basis  for  the  sense 
of  touch,  covers  and  closely  invests  the  entire  body.  It  varies  in 
thickness  and  delicacy  in  different  regions,  though  its  structure  is 
everywhere  essentially  the  same.  As  the  physiologic  anatomy  of  the 
skin  has  elsewhere  been  detailed  (page  486),  it  is  only  necessary  to 
state  here  that  it  is  divided  into  a  deep  and  a  superficial  layer.  The 
former,  known  as  the  derma,  consists  of  an  inner  layer  of  rather  loose 
connective  tissue  and  an  outer  layer  of  condensed  connective  tissue. 
The  latter,  known  as  the  epidermis,  consists  of  an  inner  layer  of  pig- 
ment cells  and  a  thick  outer  layer  of  epithelial  cells.  The  derma  is 
characterized  by  the  presence  of  elevations  (papilla?)  which  are  every- 
where extremely  abundant.  Throughout  the  derma  ramify  blood- 
vessels and  nerves. 

The  Peripheral  or  Terminal  Organs. — Between  the  contact 
surface  and  the  afferent  nerves  specialized  structures  are  found  which 
serve  as  intermediates  between  the  stimulus,  on  the  one  hand,  and  the 
afferent  nerves,  on  the  other  hand.  By  virtue  of  their  structure  they 
are  far  more  irritable  than  the  nerve-fibers  and  hence  respond  more 
quickly  to  the  physiologic  stimulus  than  the  nerve-fiber  itself.     To 


640 


TEXT-BOOK  OF  PHYSIOLOGY. 


these  specialized  organs,  found  not  only  in  the  skin  but  in  other  sense- 
organs  as  well,  the  term  peripheral  or  terminal  organ  is  given.  It  is 
these  structures  that  are  primarily  excited  to  activity  by  the  physiologic 
stimulus,  and  that  in  turn  arouse  the  nerve  to  activity.  Peripheral 
organs  are  to  be  regarded  as  special  modes  of  termination  of  afferent 
nerves  and  adapted  for  the  impress  of  a  specific  stimulus.  The  periph- 
eral organs  of  afferent  nerves  found  in  the  skin  and  oral  mucous  mem- 
brane present  a  variety  of  forms,  some  of  which  are  as  follows: 

1.  Free  Endings. — These   are  pointed  or  club-shaped  processes,  the 

ultimate  terminations  of  afferent  nerve-fibrils,  found  in  and 
among  epidermic  cells. 

2.  Tactile    Cells. — These    are    oval   nucleated   bodies   found   in   the 

deeper  layers  of  the  epidermis.  They  rest  upon  or  are  embraced 
by  a  crescentic  shaped  body,  the  tactile  meniscus,  which  in  turn  is 

directly  connected  with  the  nerve- 
fibril  and  probably  a  modification 
of  it  (Fig.  297). 
The  Corpuscles    of  Meissner    and 
Wagner. — In  the  papillae  of  the 
derma,  especially  in  the  palm  of 
the  hand  and  in  the  finger-tips, 
are   found   elliptical  bodies  con- 
sisting of  a  connective-tissue  cap- 
sule    containing    a    number    of 
tactile     discs     with     which     the 
nerve-fibrils    are    connected.     If 
the  afferent  nerve  is  traced  to  the 
capsule,  it  is  found  to  lose  both 
its  neurilemma  and  medulla,  after 
which  the  naked  fibril  penetrates  the  capsule,  breaks  up  into  a 
number  of  branches,  and  after  pursuing  a  more  or  less  spiral 
course  becomes  connected  with  the  tactile  discs  (Fig.  298). 
Hair  Wreaths. — Just  below  the  openings  of  the  sebaceous  gland 
the    hair-follicles   are  surrounded  by  naked  axis-cylinder  fibrils 
in  the  form  of  a  wreath,  which  in   all   probability  terminate  in 
the  cells  of  the  external  root-sheath.     These,  too,  are  to  be  re- 
garded as  part  of  the  touch  apparatus. 

Corpuscles  of  Vater  or  Pacini. — These  are  oval-shaped  structures 
found  along  the  nerves  distributed  to  the  palms  of  the  hands  and 
the  soles  of  the  feet,  on  the  nerves  distributed  to  the  external 
genital  organs,  to  joints  and  other  structures.  They  consist  of 
a  thick  capsule  of  lamellated  connective  tissue  in  the  interior 
of  which  is  a  bulb  resembling  granular  protoplasm.  The  axis- 
cylinder  of  the  nerve-fiber  enters  the  capsule  and  becomes  con- 
nected with  the  bulb  (Fig.  299). 

Other  forms  of  peripheral  organs  are  found  in  special  regions  of 
the  skin  as  well  as  in  different  animals. 


Fig.  297. — Tactile  Cells  from  Snout 
OF  Pig.  a.  Tactile  cell.  m.  Tactile  disc. 
n.  Xerve-fiber. — (Stirling.) 


5- 


THE  SENSE  OF  TOUCH. 


641 


Touch  Sense. — The  area,  stimulation  of  which  evokes  sensations 
of  touch,  is  coextensive  with  the  skin  and  that  limited  portion  of  the 
mucous  membrane  lining  the  mouth.  Careful  stimulation  of  the 
skin  by  means  of  a  fine  stiff  bristle  has  revealed  the  fact,  however,  that 
the  touch  area  is  not  continuous,  but  discrete,  presenting  itself  under 
the  form  of  small  areas  or  spots,  separated  by  relatively  large  areas 
insensitive  to  the  same  agent.  Stimulation  of  these  spots  always  calls 
forth  a  sensation  of  touch.  For  this  reason  they  are  known  as  "touch 
spots."  The  number  of  such  spots  in  any  given  area  of  skin  varies 
considerably.  Thus,  in  the  skin  of  the  calf  fifteen  such  spots  have 
been  counted  in  a  square  centimeter.  In  the 
palm  of  the  hand  from  forty  to  fifty  have  been 
counted  in  an  area  of  the  same  extent.  They 
are  also  especially  abundant  in  the  immediate 
neighborhood  of  the  hair-follicles. 

The  peripheral  end-organ  associated  with 
the  touch  spots  in  the  neighborhood  of  a  hair- 
follicle  is  in  all  probability  the  wreath  of  nerve- 
fibrils  surrounding  the  follicle.  In  regions 
devoid  of  hairs  the  end-organ  is  the  Meissner 
corpuscle,  for  in  the  palmar  surface  of  the  last 
phalanx  of  the  index-finger,  where  the  touch 
sense  is  quite  acute,  about  20  corpuscles  are 
present  in  each  square  millimeter  of  surface,  f 
The  specific  stimulus  necessary  to  evoke  the 
sensation  of  touch  is  a  deformation  of  the  skin; 
and  the  greater  this  is  within  physiologic  limits, 
the  more  pronounced  is  the  sensation. 

Pressure  Sense. — The  contact  of  an  ex- 
ternal body  is  attended  by  a  certain  amount 
of  pressure,  which,  however,  must  attain  a  cer- 
tain degree  before  the  sensation  can  be  evoked. 
This  is  known  as  the  threshold  value,  or  the 
degree  of  liminal  intensity.  Since  the  sensa- 
tions are  the  result  of  pressure,  they  are  termed 
pressure  sensations,  and  their  intensity  may  be 
expressed  in  terms  of  pressure. 

The  sensitivity  of  the  skin  as  determined 
by  the  pressure  sense  varies  in  different  re- 
gions of  the  body  and  in  accordance  with  the 

size  of  the  area  pressed.  Thus,  the  liminal  intensity  of  a  stimulus 
for  an  area  of  nine  square  millimeters  for  the  skin  of  the  forehead  is 
0.002  gram;  for  the  flexor  aspect  of  the  forearm,  0.003  gram;  and  for 
the  hips,  thigh,  and  abdomen,  0.005  gram5  f°r  the  palmar  surface  of 
the  finger,  0.019  gram;  for  the  heel,  1  gram.  The  delicacy  of  the  sense 
of  touch  is  measured  by  the  slight  increase  or  decrease  in  the  intensity 
of  the  stimulus  that  will  produce  an  appreciable  change  in  the  intensity 
41 


Fig.  298. — Touch-cor- 
puscle of  Meissxer  axd 
Wagner,  b.  Papilla  of 
cutis.  d .  Nerve-fiber  of 
touch-corpuscle,  e,  /. 
Nerve-fiber  in  touch-cor- 
puscles, g.  Cells  of  Mal- 
pighian  layer. — (From 
Stirling.) 


642 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  the  sensation.  Not  all  changes  in  the  stimulus,  however,  are 
attended  by  a  change  in  the  sensation.  It  has  been  determined  that 
the  latter  will  change  only  when  the  former  changes  in  a  definite 
ratio,  which  for  the  volar  surface  of  the  third  phalanx  of  the  index- 
finger  is  as  29  is  to  30.  Thus,  other  things  being  equal,  a  sensation 
caused  by  a  given  weight  will  only  change  with  moderate  stimulation 
when  one-thirtieth  of  the  weight  is  either  added  or  subtracted.  The 
ratio  of  change,  however,  varies  in  different  regions  of  the  body:  thus, 
for  the  back  of  the  hand  the  ratio  varies  from  one-tenth  to  one-twentieth; 
for  the  tongue,  one-thirtieth  to  one-fortieth.  The  difference  of  stimulus 
necessary  to  evoke  a  sensation  is  known  as  the  threshold  difference. 
It  seems  to  be  a  law  not  only  for  the  skin,  but  for  other  senses  as  well, 
that  a  change  in  the  intensity  of  a  sensation, 
to  an  appreciable  extent,  will  occur  only  when 
the  objective  stimulus  changes  in  a  definite 
ratio.  This  ratio,  however,  will  vary  not  only 
in  different  regions  of  the  skin,  in  different 
individuals,  but  with  the  sense-organ  investi- 
gated. 

Place  Sense. — The  sensation  evoked  by 
stimulation  of  the  skin  is  always,  under  normal 
conditions,  referred  to  the  place  stimulated. 
This  holds  true  not  only  for  two  or  more  points 
near  or  widely  separated  on  the  same  side,  but 
also  for  corresponding  points  on  opposite  sides 
of  the  body,  even  when  the  stimuli  have  the 
same  intensity  and  are  simultaneously  applied. 
The  cause  for  this  localizing  power  is  to  be 
found  in  a  difference  in  the  quality  of  the  sen- 
sation related  in  some  way  to  the  part  stimu- 
lated. Each  cutaneous  area  is  supposed  to 
give  to  the  tactile  sensation  a  quality  or  local 
sign  by  virtue  of  which  the  mind  is  enabled  to 
localize  the  point  of  contact. 
Each  cutaneous  area  which  has  a  local  sign  of  its  own  is  known 
as  a  sensory  circle,  for  the  reason  that  the  mind  does  not  refer  the 
sensation  to  a  point,  but  to  an  area  more  or  less  circular  in  outline. 
The  skin  may  therefore  be  regarded  as  composed  of  myriads  of  such 
circles  varying  in  size  in  different  regions  of  the  body. 

The  delicacy  of  the  localizing  power  in  any  part  of  the  skin  is 
determined  by  testing  the  power  which  the  part  possesses,  of  dis- 
tinguishing the  sensations  produced  by  the  contact  of  the- points  of  a 
pair  of  compasses  placed  close  together.  The  distance  to  which  the 
points  must  be  separated  in  order  to  evoke  two  separate  recognizable 
sensations  is  a  measure  of  the  diameter  of  the  sensory  circle.  Within 
this  circle  the  two  sensations  become  fused  into  one  sensation.  The 
discriminative  sensibility  of  different  regions  as  determined  by  com- 


Fig.  299. — Pacinian 
Corpuscles,  c.  Capsules. 
d.  Endothelial  lining  sepa- 
rating the  latter,  n.  Nerve. 
/.  Funicular  sheath  of  nerve. 
m.  Central  mass.  n'.  Ter- 
minal fiber;  and  a.  Where 
it  splits  up  into  finer  fibrils. 
— (Stirling.) 


THE  SENSE  OF  TOUCH.  643 

pass  points  is  shown  in  the  following  table;  the  numbers  represent 
the  distances  at  which  two  sensations  are  recognized: 

MM. 

Tip  of  tongue, 1.1 

Palmar  surface  of  third  phalanx  of  index  finger, 2.2 

Red  surface  of  lips, 4.5 

Palmar  surface  of  first  phalanx  of  finger, 5.5 

Tip  of  nose, 6.8 

Palm  of  hand, 8.9 

Lower  part  of  forehead, 22.6 

Dorsum  of  hand, 31.6 

Dorsum  of    foot, 40.6 

Middle  of  the  back, 67.7 

The  discriminative  sensibility  of  any  portion  of  the  body  is  a 
function  of  its  mobility.  This  is  shown  by  the  fact  that  it  increases 
rapidly  from  the  shoulders  to  the  fingers  and  from  the  hips  to  the  toes. 

The  Temperature  Sense. — The  sensations  of  heat  and  cold  which 
are  experienced  from  time  to  time  are  caused  by  changes  in  the  tem- 
perature of  the  skin  produced  in  a  variety  of  ways.  As  these  sensations 
are  specifically  different  from  those  of  touch,  as  well  as  different  from 
each  other,  it  is  highly  probable  that  for  each  sensation  there  are 
special  nerve-endings  distributed  throughout  the  skin.  Investigations 
have  shown  that  all  over  the  skin  there  are  innumerable  spots  of 
varying  size  which  if  stimulated  evoke  sensations  of  heat  or  cold. 
Such  points  are  termed  heat  and  cold  spots.  Each  responds  to  but 
one  kind  of  stimulus.  A  warm  object  applied  to  a  heat  spot  will 
evoke  a  sensation  of  warmth.  It  will  have  no  effect  on  the  cold  spot. 
The  reverse  is  also  true.  Between  the  cold  and  heat  spots  there  are 
areas  that  are  neutral,  insensitive  to  either  heat  or  cold.  The  cold 
spots  are  more  numerous  than  the  heat  spots  in  almost  all  regions  of 
the  body.     (See  Fig.  300.) 

The  sensitivity  of  the  skin  to  temperature  changes  is  very  acute, 
as  shown  by  the  fact  that  even  0.050  C.  is  readily  appreciable.  This 
holds  true,  however,  only  when  the  temperature  of  the  object  lies 
between  270  and  330  C.  This  capability  varies  in  different  regions 
of  the  skin,  and  depends  on  the  number  of  heat  and  cold  spots  present, 
the  thickness  of  the  epidermis,  the  thermal  conductivity  of  the  object 
touching  it,  and  the  extent  to  which  it  is  habitually  exposed  or  protected. 

The  physiologic  stimulus  to  the  thermic  end-organs  is  the  passage 
of  heat  through  the  skin  from  the  interior  of  the  body  to  the  sur- 
rounding air.  If  the  radiation  is  continuous  and  uniform,  the  end- 
organs  soon  adapt  themselves  to  the  temperature  of  the  surrounding 
air  and  the  sensation  of  heat,  under  physiologic  conditions,  is  not 
evoked.  If  there  is  a  sudden  rise  in  the  external  temperature  caused 
by  natural  or  artificial  means,  which  diminishes  the  radiation,  the 
temperature  of  the  skin  will  at  once  rise,  the  end-organs  will  be  stim- 
ulated, and  a  sensation  of  warmth  developed.  If,  on  the  other  hand, 
there  is  a  sudden  fall  in  temperature  and  an  increased  radiation, 
the  temperature  of  the  skin  will  fall,  the  end-organs  will  be  stimulated, 


644 


TEXT-BOOK  OF  PHYSIOLOGY. 


and  a  sensation  of  cold  developed.  Experiment  also  teaches  that  the 
intensity  of  a  warm  or  cold  sensation  will  depend  on  the  existing  tem- 
perature of  the  skin,  and  not  upon  the  absolute  temperature  of  the 
object.  Thus,  water  as  200  C.  will  evoke  a  sensation  of  heat  or  cold 
according  as  the  skin  has  previously  been  cooled  below  or  warmed 
above  this  temperature. 

The  Muscle  Sense. — As  a  result  of  the  activities  of  the  muscula- 
ture of  the  body  or  even  of  its  individual  parts,  there  arises  in  con- 
sciousness a  series  of  sensations,  which  are  termed  muscle  sensations. 
These  sensations  give  rise  to  the  perception — 
1.   Of  the  direction  and  duration  of   both   passive    (due  to  external 


K3 

aifc  illllllll  It" 
*:llff 

Fig.  300. — Cold  and  Hot  Spots  from  the  Anterior  Surface  of  the  Forearm. 
a.  Cold  spots,  b.  Hot  spots.  The  dark  parts  are  the  most  sensitive,  the  hatched  the 
medium,  the  dotted  the  fe^'v  and  the  vacant  spaces  the  non-sensitive. — (Landois  and 
Stirling.) 


causes)  and  active  movements  (due  to  internal,  volitional  efforts) 
which  take  place  without  hindrance; 

2.  Of  the  resistance  offered  to  movements  by  external  bodies;  and — 

3.  Of  the  posture  of  the  body  or  of  its  individual  parts. 

As  to  the  seat  of  the  physiologic  processes  which  precede  and 
underlie  the  development  of  the  sensations  two  views,  at  least,  may 
be  advanced,  viz.: 

1.  That  the  processes  arc  central  in  origin  and  partake  of  the  nature 

of  a  discharge  of  nerve  impulses  from  the  nerve-cells  through 
the  motor  nerves  to  the  muscles,  the  entire  process  being  accom- 
panied by  sensation.     This  is  known  as  the  innervation  theory. 

2.  That  the  processes  are  peripheral  in  origin,  initiated  by  stimulation 

of  specialized  end-organs  in  the  muscles  and  tendons  which  are 


THE  SENSE  OF  TOUCH. 


645 


connected  through  the  intermediation  of  afferent  nerves  with 
nerve-cells  in  the  cerebral  cortex. 

The  physiologic  mechanism  subserving  the  muscle  sense,  accord- 
ing to  the  second  theory,  now  held  by  many  physiologists,  thus  in- 
volves peripheral  end-organs,  afferent  nerves,  their  cortical  connec- 
tions and  nerve-cells  in  the  cerebral  cortex  at  or  near  the  junction 
of  the  superior  and  inferior  parietal  convolutions. 

The  End-organs. — These  are  small  fusiform  structures  found  in 
and  among  the  muscle  bundles  of  all  the  muscles  of  the  bodv  with 
the  exception  of  the  diaphragm  and  eye  muscles.  In  the  muscles  of 
the  arm  and  in  the  small  muscles  of  the  hand  they  are  especially  abund- 
ant. From  their  shape  they  are  known  as  muscle  spindles.  Thev 
vary  in  length  from  2  to  12  mm.  and  in  breadth  from  0.15  to  0.4  mm. 
Each  spindle  (Fig.  301)  consists  of  a  connective-tissue  capsule  con- 
taining from  two  to  ten  longitudinally  arranged  striated  muscle  fibers 
of  fine  diameter.  In  the  middle  or  equatorial  region  of  these  intra-fusal 
fibers  there  is  frequently  found  a  quantity  of  non-striated  protoplasmic 
matter.     The  spindle  is  supplied  with  both  sensor  and  motor  nerves. 


Fig.  301. — A  Neuro-muscle  Spixdle  of  a  Cat.  (Ruffini.)  c.  Capsule,  pr.  e. 
Primary  ending.  5.  e.  Secondary  ending.  i>l.  e.  Plate  ending.  (All  these  are  probably 
sensor  in  function.) — (Starling's  "Physiology.") 


The  sensor  fiber  loses  its  external  investments  as  it  approaches  the 
capsule.  The  naked  axis-cylinder  then  penetrates  the  capsule,  and 
after  dividing  several  times  terminates  in  a  ribbon-like  or  spiral  manner 
around  the  intra-fusal  muscle  fiber.  This  ending  was  described  by 
and  is  known  as  Ruffini's  "annulo-spiral  ribbon."  The  motor  nerve 
also  penetrates  the  capsule  and  terminates  in  the  polar  extremities 
of  the  intra-fusal  fiber.  Sensor  end-organs  supposed  to  be  connected 
with  the  muscle  sense  are  also  found  in  the  tendons  of  muscles. 

Afferent  Nerves. — That  muscles  are  abundantly  supplied  with 
afferent  nerves  has  been  proved  by  different  methods  of  investigation. 
With  histologic  methods  Sherrington  has  traced  afferent  fibers  from 
the  muscle  spindles  directly  into  the  spinal  nerve  ganglia.  The  con- 
tractions of  muscles  from  electric  stimulation  as  well  as  the  con- 
tractions known  as  muscle  cramp,  due  to  unknown  agents,  give  rise 
to  sensations  of  pain,  a  fact  which  indicates  the  presence  in  muscles 
of  afferent  or  sensor  nerves. 

Cortical  Area. — Pathologic  findings  have  shown  that  an  im- 
pairment or  a  loss  of  the  muscle  sense  is  associated  with  destructive 


646 


TEXT-BOOK  OF  PHYSIOLOGY. 


lesions  of  perhaps  the  super  and  sub-parietal  convolutions  (Figs. 
252,  253).  In  a  case  reported  by  Starr  the  removal  of  a  small  tumor 
in  the  pia  mater  situated  over  the  junction  of  the  superior  and  inferior 
parietal  lobules  was  followed  by  a  loss  of  the  muscle  sense  and  marked 
ataxia  in  the  right  hand  for  a  period  of  six  weeks,  after  which  recovery 
took  place.  These  symptoms  were  attributed  to  injury  of  the  cortex 
from  unavoidable  surgical  procedures. 

The  muscle  sensations,  as  stated  in  foregoing  paragraphs,  form  the 
basis  of  the  perception  not  only  of  the  direction  and  the  duration  of  a 
body  movement  and  the  resistance  experienced,  but  also  of  the  position 
and  the  tension  of  the  muscle  groups.  The  latter  fact  more  especially 
makes  it  possible  for  the  mind  to  direct  the  muscles  and  to  graduate 
the  energy  necessary  to  the  accomplishment  of  a  definite  purpose. 

Active  Touch. — Active  touch  or  the  application  of  the  fingers 
to  the  surfaces  of  external  objects  implies  the  cooperation  of  the  skin 
and  the  muscles.  The  sensations  which  are  evoked  are  combina- 
tions of  contact  and  muscle  sensations.  The  union  of  these  sensa- 
tions forms  the  basis  of  the  perception  of  hardness,  softness,  smooth- 
ness, and  roughness  of  bodies. 


THE  SENSE  OF  TASTE. 

The  physiologic  mechanism  involved  in  the  sense  of  taste  includes 
the  tongue,  the  gustatory  nerves  (the  chorda  tympani  and  the  glosso- 
pharyngeal), their  cortical  connections  and 
nerve-cells  in  the  gray  matter  of  the  fourth 
temporal  convolutions.  The  peripheral  ex- 
citation of  this  apparatus  gives  rise  to  nerve 
impulses  which  transmitted  to  the  brain  evoke 
the  sensations  of  taste.  The  specific  physio- 
logic stimulus  is  matter,  organic  and  inorganic, 
in  a  state  of  solution. 

The  Tongue. — The  tongue  consists  of 
both  intrinsic  and  extrinsic  muscles,  in  virtue 
of  which  it  is  susceptible  of  both  a  change  in 
shape  and  position.  The  movements  of  the 
tongue,  though  not  essential  to  taste,  are  made 
use  of  in  the  finer  discrimination  of  tastes. 

The  tongue  is  covered  over  by  mucous 
membrane  continuous  with  that  lining  the  oral 
cavity.  The  dorsum  of  the  tongue  presents  a 
series  of  papillae  richly  supplied  with  blood- 
vessels and  nerves.  Of  these  there  are  three 
varieties,  the  filiform,  the  fungiform,  and  the 
circumvallate  (Fig.  302). 

1.  The  filiform  papilla,  the  most  numerous,  cover  the  anterior  two- 
thirds  of  the  tongue;  they  are  conical  or  filiform  in  shape  and 


Fig.  ,302. — The  Tongue. 

1.  Papilla;      circumvallatac. 

2.  Papillae  fungiformes. 


THE  SENSE  OF  TASTE. 


647 


covered  with  horny  epithelium  which  is  often  prolonged  into 
filamentous  tufts. 

2.  The  fungiform  papilla,  found  chiefly  at  the  tip  and  sides  of  the 

tongue,  are  less  numerous  but  larger  than  the  preceding  and  of  a 
deep  red  color. 

3.  The    circumvallate    papilla:,   from    eight    to   ten   in   number,    are 

situated  at  the  base  of  the  tongue  arranged  in  the  form  of  the 
letter  A  They  consist  of  a  central  projection  surrounded  by 
a  wall  or  circumvallation  from  which  they  take  their  name. 

The  Peripheral  End-organs.  The  Taste-buds. — Embedded 
in  the  epithelium  covering  the  mucous  membrane  not  only  of  the 
tongue  but  of  the  palate  and  posterior  surface  of 
the  epiglottis  are  small  ovoid  bodies  which  from 
their  relation  to  the  gustatory  nerves  are  regarded 
as  their  peripheral  end-organs  and  known  as  taste- 
buds  or  taste-beakers.  Each  bud  is  ovoid  in  shape 
(Fig.  303).  Its  base  rests  on  the  tunica  propria;  its 
apex  comes  up  to  the  epithelium,  where  it  presents 
a  narrow  funnel-shaped  opening,  the  taste-pore. 
The  wrall  of  the  bud  is  composed  of  elongated  curved 
epithelium.  The  interior  contains  narrow  spindle- 
shaped  neuro-epithelial  cells  provided  at  their  outer 
extremity  wTith  stiff  hair-like  filaments  wdiich  pro- 
ject into  the  taste-pore. 

The  neuro-epithelial  cells  are  in  physiologic  re- 
lation with  the  nerves  of  taste.  The  terminal 
branches,  after  entering  the  bud  at  its  base,  develop 
fine  tufts  which  come  into  contact  with  the  cells. 
That  the  taste-buds  are  connected  writh  the  nerves 
of  taste  is  rendered  probable  from  the  fact  of  their 
degeneration  after  division  of  the  nerves. 

The  Taste  Area. — -The  taste  area,  though  con- 
fined for  the  most  part  to  the  tongue,  extends  itself 
in  different  individuals  to  the  mucous  membrane  of 
the  hard  palate,  to  the  anterior  surface  of  the  soft  palate,  to  the  uvula, 
the  anterior  and  posterior  half  arches,  the  tonsils,  the  posterior  wall 
of  the  pharynx,  and  the  epiglottis. 

The  Taste  Sensations. — The  sensations  which  arise  in  conse- 
quence of  impressions  made  by  different  substances  on  the  peripheral 
apparatus  of  this  area  are  in  so  many  instances  combinations  of  taste, 
touch,  temperature,  and  smell  that  they  are  extremely  difficult  of 
classification.  Nevertheless  four  primary  tastes  can  be  recognized: 
viz.,  bitter,  sweet,  acid  or  sour,  salt  or  saline.  Though  the  contact 
of  any  bitter,  sweet,  acid,  or  salt  substance  with  any  part  of  the  tongue 
will,  if  the  substance  be  present  in  sufficient  quantity  or  concentration, 
develop  a  corresponding  sensation,  some  regions  of  the  tongue  are 
more    sensitive    and    responsive    than    others.     Thus,    the    posterior 


Fig.  303. — Taste- 
bud  from  Circum- 
yallate  p.apilla  of 
A  Child.  The  oval 
structure  is  limited  to 
the  epithelium  (e) 
lining  the  furrow, 
encroaching  slightly 
upon  the  adjacent 
connective  tissue  (/); 
0,  taste-pore  through 
which  the  taste-cells 
communicate  with 
the  mucous  surface. 
— (Ajter  Piersol.) 


648  TEXT-BOOK  OF  PHYSIOLOGY. 

portion  is  more  sensitive  to  bitter  substances  than  the  anterior;  the 
reverse  is  true  for  sweet  substances  and  perhaps  for  acids  and  salines. 
The  intensity  of  the  resulting  sensation  in  any  given  instance 
will  depend  on  the  degree  of  concentration  of  the  substance,  while 
its  massiveness  will  depend  on  the  area  affected. 

THE  SENSE  OF  SMELL. 

The  physiologic  mechanism  involved  in  the  sense  of  smell  in- 
cludes the  nasal  fossae,  the  olfactory  nerves,  the  olfactory  tracts^  and 
nerve-cells  in  those  areas  of  the  cortex  known  as  the  uncinate  con- 
volution and  anterior  part  of  the  gyrus  fornicatus.  Peripheral  stimu- 
lation of  this  mechanism  develops  nerve  impulses  which,  transmitted 
to  the  cortex,  evoke  the  sensations  of  odor.  The  specific  physiologic 
stimulus  is  matter  in  the  gaseous  or  volatile  state. 

The  Nasal  Fossae. — The  nasal  fossa?  are  irregularly  shaped 
cavities  separated  by  a  vertical  septum  formed  by  the  perpendicular 
plate  of  the  ethmoid  bone,  the  vomer,  and  the  triangular  cartilage. 
The  outer  wall  presents  three  recesses  separated  by  the  projection 
inward  of  the  turbinated  bones.  Each  fossa  opens  anteriorly  and 
posteriorly  by  the  anterior  and  posterior  nares,  the  latter  communicat- 
ing with  the  pharynx.  Both  fossae  are  lined  throughout  by  mucous 
membrane.  The  upper  part  of  the  fossa  is  known  as  the  olfactory, 
the  lower  portion  as  the  respiratory  region.  In  the  former,  the  mucous 
membrane  over  the  septum  and  superior  turbinated  bone  is  somewhat 
thicker  than  elsewhere  and  covered  with  a  neuro-epithelium  which 
constitutes — 

The  Peripheral  End-organ. — This  consists  of  a  basement  mem- 
brane supporting  two  kinds  of  cells,  the  olfactory  and  the  sustentacular. 
The  olfactory  cells  are  bipolar  nerve-cells,  the  center  of  which  contains 
a  large  spheric  nucleus.  The  peripheral  pole  is  cylindric  or  conic 
in  shape  and  provided  at  its  extremity  with  several  hair-like  processes. 
The  central  pole  becomes  the  axon  process  and  passes  directly  to  the 
olfactory  bulb. 

The  sustentacular  cells  are  epithelial  in  character  and,  as  their 
name  implies,  support  or  sustain  the  olfactory  cells. 

For  the  appreciation  of  odorous  particles  the  air  must  be  drawn 
through  the  nasal  fossae  with  a  certain  degree  of  velocity.  If  the 
particles  are  widely  diffused  in  the  air,  they  must  be  drawn  not 
only  more  quickly  but  more  forcibly  into  contact  with  the  olfactory 
hairs,  as  in  the  act  of  sniffing,  the  result  of  short  energetic  inspira- 
tions. To  many  substances  the  olfactory  apparatus  is  extremely 
sensitive.  Thus,  it  has  been  shown  that  a  particle  of  mercaptan 
the  actual  weight  of  which  was  calculated  to  be  Too'irFfrinTo'  °f  a  milli- 
gram gave  rise  to  a  distinct  sensation. 

The  Olfactory  Sensations. — The  sensations  which  arise  in  conse- 
quence of  ilif  excitation  of  the  olfactory  apparatus  are  very  numerous 


THE  SENSE  OF  SMELL.  649 

and  their  classification  is  extremely  difficult.  For  this  reason  it  is 
customary  to  divide  them  into  two  groups:  viz.,  agreeable  and  dis- 
agreeable, in  accordance  with  the  feelings  they  excite  in  the  individual. 
As  the  olfactory  sensations  give  rise  to  feelings  rather  than  ideas, 
this  sense  plays  in  man  a  subordinate  part  in  the  acquisition  of  knowl- 
edge. In  lower  animals  this  sense  is  employed  for  the  purpose  of 
discovering  and  securing  food,  for  detecting  enemies  and  friends, 
and  for  sexual  purposes.  In  land  animals  the  entire  olfactory  appa- 
ratus is  well  developed  and  the  sense  keen;  in  some  aquatic  animals,  as 
the  dolphin,  whale,  and  seal,  the  apparatus  is  poorly  developed  and 
the  sense  dull. 


CHAPTER  XXVII. 
THE  SENSE  OF  SIGHT. 

The  physiologic  mechanism  involved  in  the  sense  of  sight  includes 
the  eyeball,  the  optic  nerve,  the  optic  tracts,  their  cortical  connections, 
and  nerve-cells  in  the  cuneus  and  adjacent  gray  matter.  Peripheral 
stimulation  of  this  mechanism  develops  nerve  impulses  which  trans- 
mitted to  the  cortex  evoke  (i)  the  sensation  of  light  and  its  different 
qualities — colors;  (2)  the  perception  of  light  and  color  under  the  form 
of  pictures  of  external  objects;  and  (3)  in  connection  with  the  ocular 
muscles,  the  production  of  muscle  sensations  by  which  the  size,  dis- 
tance, and  direction  of  objects  may  be  judged. 

The  specific  physiologic  stimulus  to  the  terminal  end-organ, 
the  retina,  is  the  impact  of  ether  vibrations.  In  general,  it  may  be 
said  that,  at  least  for  the  same  color,  the  intensity  of  the  objective 
vibration  determines  the  intensity  of  the  sensation. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EYEBALL. 

The  eyeball  is  situated  at  the  fore  part  of  the  orbit  cavity,  and 
in  such  a  position  as  to  permit  of  an  extensive  range  of  vision.  It  is 
loosely  held  in  position  by  a  fibrous  membrane,  the  capsule  of  Tenon, 
which  is  attached,  on  the  one  hand,  to  the  eyeball  itself,  and,  on  the 
other,  to  the  walls  of  the  orbit  cavity.  Thus  suspended,  the  eyeball 
is  susceptible  of  being  moved  in  any  direction  by  the  contraction  of 
the  muscles  attached  to  it. 

The  ball  is  spheroid  in  shape,  measuring  about  24  millimeters 
in  its  anterio-posterior  diameter  and  a  little  less  in  its  transverse  and 
vertical  diameters.  When  viewed  in  profile,  it  is  seen  to  consist  of 
the  segments  of  two  spheres,  of  which  the  posterior  is  the  larger, 
occupying  five-sixths,  and  the  anterior  is  the  smaller,  occupying  one- 
sixth  of  the  ball.  It  is  composed  of  several  concentrically  arranged 
membranes  enclosing  various  refracting  media  essential  to  vision. 

The  membranes,  enumerating  them  from  without  inward,  are 
as  follows:  the  sclera  and  cornea,  the  chorioid  and  iris,  and  the  retina. 
The  refracting  media  are  the  aqueous  humor,  the  crystalline  lens, 
an  el  the  vitreous  humor. 

The  Sclera  and  Cornea. — The  sclera  is  the  thick  opaque  mem- 
brane covering  the  posterior  five-sixths  of  the  ball.  It  is  composed 
of  layers  of  connective  tissue  which  are  arranged  transversely  and 
longitudinally.     It   is  pierced   posteriorly  by  the  optic  nerve   about 

650 


THE  SENSE  OF  SIGHT. 


651 


15  13  1G 


3  or  4  millimeters  internal  to  the  optic  axis.  By  virtue  of  its  firmness 
and  density  the  sclera  gives  form  to  the  eyeball,  protects  delicate 
structures  enclosed  by  it,  and  serves  for  the  attachment  of  the  muscles 
by  which  the  ball  is  moved  (Figs.  304,  309.)  The  cornea  is  the 
transparent  membrane  forming  the  anterior  one-sixth  of  the  ball.  It 
is  nearly  circular  in  shape,  measuring  in  its  horizontal  meridian  12  mm., 
in  its  vertical  meridian  11  mm.  The  curvature  is  therefore  sharper 
in  the  latter  than  in  the  former.  The  radius  of  curvature  of  the 
anterior  surface  at  that  central  portion  ordinarily  used  in  vision  is 
7.829  mm.;  that  of  the  posterior  surface  about  6  mm. 

The  substance  of  the  cornea  is  made  up  of  thin  layers  of  delicate 
transparent  fibrils  of  connective  tissue  continuous  with  those  found  in 
the  sclera.  Lymph- 
spaces  are  present 
throughout  the  cor- 
nea, in  which  are  to 
be  found  lymph-cor- 
puscles. The  anterior 
surface  of  the  cornea 
is  covered  with  several 
layers  of  nucleated 
epithelium  supported 
by  a  structureless 
membrane,  the  an- 
terior elastic  lamina. 
The  posterior  surface 
also  is  covered  by  a 
layer  of  epithelium 
supported  by  a  similar 
membrane,  the  mem- 
brane of  Descemet, 
which  at  its  periphery 
becomes  continuous 
with  the  iris.  At  the 
junction  of  the  cornea 
and  sclera  there  is  a 
circular  groove, 
known  as  the  canal 
of  Schlemm. 

The  Chorioid,  Iris,  Ciliary  Muscle,  and  Ciliary  Processes.— 
The  chorioid  is  the  dark  brown  membrane  which  extends  forward 
nearly  to  the  cornea,  where  it  terminates  in  a  series  of  folds,  the  ciliary 
processes.  Posteriorly,  it  is  pierced  by  the  optic  nerve.  It  is  com- 
posed largely  of  blood-vessels,  arteries,  capillaries,  and  veins,  sup- 
ported by  connective  tissue.  Externally  it  is  loosely  connected  to  the 
sclera;  internally  it  is  lined  by  a  layer  of  hexagonal  cells  containing 
black  pigment  which,  though  usually  described  as  a  part  of  the  chorioid. 


Fig.  304. — Chorioid  Coat  of  the  Eye.  i.  Optic 
nerve.  2,  2,  2,  2,  3,  3,  3,  4.  Sclerotic  coat  divided  and 
turned  back  to  show  the  chorioid.  5,  5,  5,  5.  The 
cornea,  divided  into  four  portions  and  turned  back. 
6,  6.  Canal  of  Schlemm.  7.  External  surface  of  the 
chorioid,  traversed  by  the  ciliary  nerves  and  one  of  the 
long  ciliary  arteries.  8.  Central  vessel  into  which 
open  the  vasa  vorticosa.  9,  9,  10,  10.  Chorioid  zone. 
11,  11.  Ciliary  nerves.  12.  Long  ciliary  artery.  13, 
13,  13,  13.  Anterior  ciliary  arteries.  14.  Iris.  15,  15. 
Vascular  circle  of  the  iris.     16.  Pupil. — {Sappey.) 


652  TEXT-BOOK  OF  PHYSIOLOGY. 

is  now  known  to  belong,  embryologicly  and  physiologicly,  to  the 
retina.  Lying  within  the  outer  layer  of  arteries  and  veins  there  is  a 
thick  layer  of  small  arterioles  and  capillaries,  known  as  the  chorio- 
capillaris.  The  chorioid  with  its  contained  blood-vessels  bears  an 
important  relation  to  the  nutrition  and  function  of  the  eye.  It  provides 
a  free  supply  of  lymph  and  presents  a  uniform  temperature  to  the 
retina  in  contact  with  it. 

The  iris  is  the  circular,  variously  colored  membrane  in  the  anterior 
part  of  the  eye  just  behind  the  cornea.  It  presents  a  little  to  the 
nasal  side  of  the  center  a  circular  opening,  the  pupil.  The  outer  or 
circumferential  border  is  united  by  connective  tissue  to  the  cornea, 
sclera,  and  ciliary  muscle;  the  inner  border  forms  the  boundary  of 
the  pupil.  The  iris  consists  of  a  framework  of  connective  tissue  sup- 
porting blood-vessels,  muscle-fibers,  and  pigmented  connective-tissue 
cells.  The  anterior  surface  is  covered  by  a  layer  of  cells  continuous 
with  those  covering  the  posterior  surface  of  the  cornea.  The  pos- 
terior surface  is  formed  by  a  thin  structureless  membrane  supporting 
a  layer  of  pigment  cells  continuous  with  those  lining  the  chorioid. 
The  color  which  the  iris  presents  in  different  individuals  depends  on 
the  relative  amount  of  pigment  in  the  connective-tissue  corpuscles. 
In  blue  eyes  the  pigment  is  wanting.  In  gray,  brown,  and  black 
eyes  the  pigment  is  present  in  progressively  increasing  amounts.  The 
blood-vessels  are  connected  with  those  of  the  chorioid  coat. 

The  muscle-fibers  are  of  the  non-striated  variety  and  arranged  in 
two  sets,  one  circularly,  the  other  radially,  disposed. 

The  circular  fibers  are  found  close  to  the  pupil  near  the  posterior 
surface  of  the  iris.  Contraction  of  this  band  of  fibers  diminishes, 
relaxation  increases,  the  size  of  the  pupil.  This  muscle  is  known 
as  the  sphincter  pupillce  or  sphincter  iridis. 

The  radial  fibers  form  a  more  or  less  continuous  layer  in  the 
posterior  part  of  the  iris,  extending  from  the  margin  of  the  pupil, 
where  they  blend  with  the  circular  fibers,  to  the  outer  border.  Con- 
traction of  the  fibers  enlarges  the  size  of  the  pupil.  The  muscle  is 
known  as  the  dilatator  pupillce. 

The  nerves  exciting  the  sphincter  pupillce  to  action  are  the  ciliary 
nerves,  axons  of  nerve-cells  located  in  the  ciliary,  or  ophthalmic  gang- 
lion. Stimulation  of  these  fibers  gives  rise  to  contraction  of  the 
sphincter  and  diminution  in  the  size  of  the  pupil.  The  nerves  excit- 
ing the  dilatator  pupillce  to  action  are  axons  of  nerve-cells  located  in 
the  superior  cervical  ganglion.  They  reach  the  iris  by  way  of  the 
cervical  sympathetic,  the  ophthalmic  division  of  the  fifth,  and  the  long 
ciliary  nerve.  Stimulation  of  these  nerves  is  followed  by  contraction 
of  the  dilatator  and  enlargement  in  the  size  of  the  pupil.  Both  the 
ciliary  and  superior  cervical  ganglia  are  in  relation  with  pre-ganglionic 
fibers  coming  from  the  central  nerve  system.,    (See  page  589.) 

The  ciliary  muscle  is  a  gray  circular  band  about  two  millimeters 
in  width,  consisting  of  non-striated   muscle-fibers.     The  majority  of 


THE  SENSE  OF  SIGHT. 


653 


these  fibers  pursue  a  radial  or  meridional  direction.  Taking  their 
origin  from  the  junction  of  the  sclera,  cornea,  and  iris,  they  pass 
backward  to  be  inserted  into  the  chorioid  coat  opposite  the  ciliary 
processes.  The  inner  portion  of  the  muscle  is  interrupted  by  bundles 
of  fibers  which  pursue  a  circular  direction.  (Fig.  305.)  They  collec- 
tively constitute  the  annular  or  ring  muscle  of  Miiller.  The  ciliary 
muscle  in  common  with  the  circular  fibers  of  the  iris  receives  its  nerve- 
supply  direct  from  the  nerve-cells  in  the  ciliary  ganglion.  Contraction 
of  the  ciliary  muscle  tenses  the  chorioid  coat,  and  for  this  reason  it 
is  frequently  termed  the  tensor  chorioidece. 

The  Retina. — The  retina  is  the  internal  coat  of  the  eye,  extending 
forward  almost  to  the  ciliary  processes,  where  it  terminates  in  an 
indented  border,  known  as  the  ora  serrata.  In  the  living  condition 
it  is  clear,  transparent,  and  pink  in  color.     After  death  it  becomes 


Fig.  305. — Section  through  the  Ciliary  Region  of  the  Human  Eye.  a.  Radi- 
ating bundles  of  the  ciliary  muscle,  b.  Deeper  bundles,  c.  Circular  network,  d. 
Annular  muscle  of  Miiller.  e.  Tendon  of  ciliary  muscle.  /.  Muscle-fibers  on  posterior 
side  of  the  iris.  g.  Muscles  on  the  ciliary  border  of  the  same.  h.  Ligamentum  pectin- 
atum. — {After  Iwanoff.) 


opaque.  The  retina  is  abundantly  supplied  with  blood-vessels,  de- 
rived from  the  arteria  centralis  retina,  a  branch  of  the  ophthalmic, 
which  pierces  the  optic  nerve  near  the  sclera,  runs  forward  in  its 
center,  to  the  retina,  in  which  its  terminal  branches  are  distributed. 
The  veins  arising  from  the  capillary  plexus  leave  the  retina  by  the 
same  route. 

In  the  posterior  portion  of  the  retina,  at  a  point  corresponding 
with  the  axis  of  vision,  there  is  a  small  oval  area  bout  2  mm.  in  its 
transverse  and  about  0.8  mm.  in  its  vertical  diameter.  From  the  fact 
that  it  presents  a  yellow  appearance,  it  is  known  as  the  macula  lu/ea. 
This  area  presents  in  its  center  a  depression  with  sloping  sides,  known 
as  the  fovea  centralis.  About  3.5  mm.  to  the  nasal  side  of  the  macula 
is  the  point  of  entrance  of  the  optic  nerve. 

The  retina  is  remarkably  complex  in  structure,  presenting  an 
appearance  when  viewed  microscopically,  something  like  that  repre- 
sented in  Fig.  306,  indicating  that  it  is  composed  of  different  cellular 


654 


TEXT-BOOK  OF  PHYSIOLOGY. 


Uril    A( 


elements  arranged  in  layers.     These  have  been  named,  from  behind 
forward,  as  folloAvs: 

i.  The  layer  of  pigment  cells. 
2.  The  layer  of  rods  and  cones,  or  Jacobson's  layer. 
The  external  limiting  membrane. 
The  outer  nuclear  or  granular  layer. 
The  outer  molecular  or  reticular  layer. 
The  inner  nuclear  or  granular  layer. 
The  inner  molecular  or  reticular  layer. 
The  layer  of  ganglion  cells. 
9.  The  layer  of  nerve-fibers. 
Modern   histologic   methods   of   research   have   made   it   possible   to 
reduce  the  retina,  exclusive  of  the  pigment  cells,  to  three  successive 
layers   of   nerve-cells,    supported   by   a   highly   developed   neuroglia, 
forming  what  has  been  termed  the  fibers  of  Miiller.     These  nerve- 
cells  are  as  follows: 

1.  The  visual  cells. 

2.  The  bipolar  cells. 

3.  The  ganglion  cells. 

The  relation  of  these  nerve-cells  one  to  another  and  to  the  supporting 
neuroglia  tissue  and  the  manner  in  which  they  unite  to  form  the 

above-mentioned  layers 
are  schematicly  shown 
in  Fig.  307. 

The  pigment  layer  is 

3.  External  limiting  membrane,      composed    of     hexagonal 

cells.  Though  formerly 
described  as  forming  a 
part  (the  inner  layer)  of 
the  chorioid,  these  cells 
belong  embryologicly  to 
the  retina.  From  their 
retinal  surface  delicate 
pigmented  processes  ex- 
tend into  and  between 
the  rods  and  cones.  On 
exposure  to  light  these 
processes  elongate  and 
push  themselves  between 
the  rods.  In  the  dark 
they  retract  and  with- 
draw into  the  cell-body. 
The  visual  cells  which 

form  the  layer  of  rods  and  cones  are  of  two  varieties,  the  rod-shaped 

and  the  cone-shaped. 

The  rod-shaped  visual  cells  consist  of  a  straight  elongated  cylinder 

extending  through  the  entire  thickness  of  Jacobson's  membrane  and 


1 
If 


Fig.  306.- 


Pigment-layer  (not  shown). 


Layer  of  rods  and  cones. 


Outer  nuclear  layer. 


5.  Outer  molecular  layer. 

6.  Inner  nuclear  layer. 

7.  Inner  molecular  layer. 

8.  Layer  of  ganglion  cells. 

9.  Layer  of  nerve-fibers. 


-Vertical  Section  of  Human  Retina. 
— (Schaper.) 


THE  SENSE  OF  SIGHT. 


655 


a  fine  fiber  containing  a  nucleus,  which,  after  piercing  the  external 
limiting  membrane,  passes  into  the  outer  molecular  layer,  where  it 
terminates  in  a  spheric  enlargement.  The  outer  portion  of  the  rod 
is  clear  and  homogeneous,  though  containing  a  pigment  known  as 
visual  purple  or  rhodopsin;  the  inner  portion  of  the  rod  is  slightly 
granular. 

The  cone-shaped  visual  cells  also  consist  of  two  portions,  a  conic 
portion  situated  in  Jacobson's  membrane  between  the  rods,  and  a 
fine  fiber,  containing  a  nucleus,  which,  after  piercing  the  external 
limiting  membrane,  passes  into 
the  outer  molecular  layer,  where 
it  terminates  in  a  fine  tuft.  The 
inner  portion  of  the  cone  is 
thicker  than  the  rod  and  rests  on 
the  limiting  membrane;  the  outer 
portion  tapers  to  a  fine  point  and 
is  known  as  the  cone-style.  The 
cones,  as  a  rule,  are  shorter  than 
the  rods.  The  proportion  of 
rods  to  cones  varies  in  different 
parts  of  the  retina,  though  there 
are  on  the  average  about  four- 
teen rods  to  one  cone.  In  the 
macula  the  rods  are  almost  en- 


The  layer  of  visual  cells  to- 
gether with  the  neuroglia  consti- 
tute the  first  three  layers  of  the 
retina  proper.  The  external 
limiting  membrane  is  formed  by 
the  blending  of  the  ends  of  neu- 
roglia cells. 

The  bipolar  cells  consist  of  a 
central  portion,  found  in  the 
inner  nuclear  layer,  from  which 
are  given  off  twro  processes  which 
pass  in  opposite  directions,  one 
toward  the  visual  cells,  the  other  toward  the  ganglion  cells.  The 
former  terminate  in  tufts  which  arborize  around  the  tufts  and  spheric 
enlargements  of  the  visual  cells,  and  assist  in  the  formation  of  the 
outer  molecular  layer;  the  latter  terminate  in  similar  tufts  in  the  inner 
molecular  layer. 

The  ganglion  cells  are  arranged  in  a  single  layer,  as  a  rule.  They 
are  large  and  nucleated.  From  the  inner  side  of  each  cell  there  is 
given  off  a  single  axon  which  passes  toward  the  center  of  the  retina 
(forming  the  nerve-fiber  layer),  where  it  enters  and  assists  in  forming 


Fig.  307. — Cross-section  of  the  Retixa 
from  a  Mammal.  A.  Layer  of  rods  and 
cones.  B.  Visual  cells  (outer  granules). 
C.  Outer  molecular  layer.  E.  Bipolar  cells 
(inner  granules).  F.  Inner  molecular  layer. 
G.  Ganglion  cells.  H.  Layer  of  nerve- 
fibers,  a.  Rods.  b.  Cones.  e.  Bipolar 
rod.  f.  Bipolar  cone.  r.  Lower  ramifica- 
tion of  a  bipolar  rod.  f.  Lower  ramification 
of  a  bipolar  cone,  g,  h,  i,  j,  k.  Ganglion 
cells  in  various  stages,  branching  from  F. 
x,  z.  Bipolar  contact  of  rods  and  cones,  t. 
Muller's  supporting  fibers.  S.  Centrifugal 
nerve-fibers. — {After  Ramon  y  Cajal.) 


6;6 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  optic  nerve.  From  the  outer  side  of  the  ganglion  cell  dendrites 
pass  into  and  assist  in  forming  the  inner  molecular  layer.  These 
dendrites  come  into  physiologic  relation  with  those  of  the  inner  proc- 
esses of  the  bipolar  cells. 

Horizontally  disposed  nerve-cells  are  also  present  in  the  outer 
molecular  layer  in  relation  with  the  visual  cells.  Spongioblasts  or 
amacrine  cells  are  also  present  at  the  border  of  and  in  the  inner  molec- 
ular layer. 

From  the  relation  of  the  ganglion  cells,  from  which  the  optic  nerve- 
fibers  take  their  origin,  to  the  visual  cells  and  the  bipolar  cells,  the 
latter  may  be  regarded  as  the  terminal  visual  organ,  the  intermediate 


imtiiiuiiii 


Fig.  308. — Horizontal  Section  through  the  Macula  and  Fovea  of  a  Man  Sixty 
Years  Old.  The  section  is  not  through  the  exact  center  of  the  fovea,  for  there  are  only 
cone  visual  cells  and  no  remnants  of  the  confluence  of  the  inner  granule  and  ganglion  cell 
layers  are  present.  1.  Cones.  2.  External  limiting  membrane.  3.  Outer  nuclear 
layer.  4.  Henle's  fiber  layer.  5.  Outer  molecular  or  reticular  layer.  6.  Inner  nuclear 
layer.  7.  Inner  molecular  or  reticular  layer.  8.  Layer  of  ganglion  cells.  9.  Nerve- 
fiber  layer. — {After  Schaper,  Stohr's  "Histology") 


between  the  ether  vibrations  and  the  ganglion  cell.  The  visual  cells 
are  directed  toward  the  chorioid,  away  from  the  entering  light,  dip- 
ping into  the  pigment  cells.  They,  with  the  pigment  layer,  are  the 
elements  by  which  the  ether  vibrations  are  transformed  into  nerve 
energy. 

In  the  fovea  most  of  the  retinal  elements  are  wanting  or  are  re- 
duced in  thickness.  The  cones  alone  are  present.  The  cone-fibers 
with  their  nuclei  are  directed  'obliquely  upward  and  outward  along 
the  slope  of  the  fovea,  to  end  in  tufts  which  come  into  physiologic 
relation  with  the  dendrites  of  the  ganglion  cells  which  at  the  top  of 
the  fovea  are  generally  increased  in  number  (Fig.  308). 

It  is  estimated  that  the  optic  nerve  contains  about  500,000  nerve- 
fibers,  and  that  for  each  fiber  there  are  about  7  cones,  noo  rods,  and 


THE  SENSE  OF  SIGHT. 


657 


7  pigment  cells.  In  accordance  with  this  estimate  there  would  be 
about  3,500,000  cones,  50,000,000  rods,  and  3,500,000  pigment  cells. 
The  distance  between  the  centers  of  two  adjacent  cones  in  the  fovea 
is  4  micromillimeters. 

The  vitreous  humor  is  the  largest  of  the  refracting  media  and 
occupies  by  far  the  largest  portion  of  the  interior  of  the  eyeball.  From 
its  position  it  gives  support  to  the  retina.  Anteriorly  it  presents  a 
concavity,  in  which  the  crystalline  lens  is  lodged.  The  vitreous 
humor  consists  of  water  (97  per  cent.),  organic  matter  and  salts,  en- 
closed in  a  transparent  membrane,  the  tunica  hyaloidea.  The  mass  of 
the  vitreous  humor  is  penetrated  by  a  species  of  connective  tissue. 


Fig.  309. — Horizontal  Section  of  the  Eyeball,  i.  Sclera.  2.  Cornea.  3. 
Chorioid.  4.  Iris.  5.  Ciliary  muscle.  6.  Retina.  7.  Lens.  8.  Suspensory  ligament. 
9.  Canal  of  Schlemm.     10.  Canal  of  Petit,     n.  Optic  nerve. — (Deaver.) 


The  aqueous  humor  is  small  in  amount  in  comparison  with  tte 
vitreous  and  is  found  in  the  space  bounded  by  the  cornea,  the  ciliary 
body,  the  suspensory  ligament,  and  the  lens.  The  projection  of 
the  iris  into  this  space  partially  divides  into  an  anterior  and  posterior 
portion  or  chamber.  The  aqueous  humor  is  a  clear,  watery,  alkaline 
fluid  derived  from  or  secreted  by  the  capillary  blood-vessels  of  the 
ciliary  body.  From  this  origin  it  passes  through  the  pupil  into  the 
anterior  chamber.  It  serves  to  keep  the  cornea  tense  and  smooth. 
The  ocular  tension  partly  depends  on  the  presence  of  this  fluid  in 
t  he  eyeball.  There  is  every  reason  for  believing  that  there  is  a  constant 
stream  of  fluid  from  the  blood-vessels  into  the  eye  and  from  the  ey< 
through  the  spaces  of  Fontana  at  the  base  of  the  iris  into  the  canal  of 


658  TEXT-BOOK  OF  PHYSIOLOGY. 

Schlemm,  and  so  into  the  blood.  Any  interference  with  the  exit  of 
this  fluid  rapidly  increases  the  ocular  tension. 

The  lens  is  the  transparent  biconvex  body  situated  just  behind 
the  iris,  in  the  concavity  of  the  vitreous.  The  thickness  of  the  lens 
is  3.6  mm. j  the  diameter  about  9  mm.  It  consists  of  a  transparent 
capsule  containing  elongated  hexagonal  fibers  which,  having  their 
origin  near  the  anterior  central  portion  of  the  lens,  pass  out  toward 
the  margin,  where  they  bend  around  to  terminate  in  a  triradiate 
figure  on  the  opposite  side.  Chemicly  the  lens  consists  of  water, 
a  globulin  body  (crystallin) ,  and  salts. 

The  Suspensory  Ligament. — The  lens  is  held  in  position  by  the 
suspensory  ligament,  formed  in  part  by  the  hyaloid  membrane  and 
in  part  by  fibers  derived  from  the  ciliary  processes.  The  former  be- 
comes attached  to  the  posterior  surface,  the  latter  to  the  anterior 
surface  of  the  lens  near  the  equator.  The  space  between  the  two 
layers  of  the  ligament  is  the  canal  of  Petit.  The  anterior  surface  of 
the  ligament  presents  a  series  of  plications  conforming  to  correspond- 
ing plications  on  the  surface  of  the  ciliary  processes. 

The  relations  of  all  the  parts  entering  into  the  structure  of  the  eye 
are  shown  in  Fig.  309. 

THE  PHYSIOLOGY  OF  VISION. 

The  Retinal  Image. — The  general  function  of  the  eye  is  the 
formation  of  images  of  external  objects  on  the  free  ends  of  the  per- 
cipient elements  of  the  retina,  the  rods  and  cones.  The  existence  of 
an  image  on  the  retina  can  be  readily  seen  in  the  excised  eye  of  an 
albino  rabbit,  when  placed  between  a  lighted  candle  and  the  eye  of 
an  observer.  Its  presence  in  the  human  eye  can  be  demonstrated 
with  the  ophthalmoscope.  It  is  this  image,  composed  of  focal  points 
of  luminous  rays,  which  stimulates  the  rods  and  cones,  which  is  the 
basis  of  our  sight-perceptions,  and  out  of  which  the  mind  constructs 
space  relations  of  external  objects.  In  only  two  essential  respects 
does  the  image  on  the  retina  differ  from  the  object,  aside  from  the 
fact  that  the  object  has  usually  three,  the  image  only  two,  dimensions — 
viz.,  in  size  and  relative  arrangement  of  its  parts.  Whatever  the  dis- 
tance, the  image  is  generally  smaller  than  the  object;  it  is  also  reversed, 
the  upper  part  of  the  object  becoming  the  lower  part  of  the  image, 
and  the  right  side  of  the  object  the  left  of  the  image. 

The  Dioptric  Apparatus. — The  formation  of  an  image  is  made 
possible  by  the  introduction  of  a  complex  refracting  apparatus  con- 
sisting of  the  cornea,  aqueous  humor,  lens,  and  vitreous  humor. 
Without  these  agencies  the  ether  vibrations  would  give  rise  only  to  a 
sensation  of  diffused  luminosity.  Rays  of  light  emanating  from 
any  one  point — that  is,  homocentric  rays — arriving  at  the  eye  must 
traverse  successively  the  different  refracting  media.  In  their  passage 
from  one  to  the  other,  they  undergo  at  the  surfaces  changes  in  direc- 
tor] before  they  are  finally  converged  to  a  focal  point.     In  order  to 


THE  SENSE  OF  SIGHT.  659 

mathematically  follow  the  rays  in  all  their  deviations  through  the 
media,  to  determine  their  focal  points  and  to  construct  an  image,  a 
knowledge  of  the  form  of  the  refracting  surfaces,  the  refractive  indices 
of  the  different  media,  and  the  distances  of  the  surfaces  from  one 
another  must  be  known. 

The  following  constants  are  now  accepted:  The  radius  of  curva- 
ture of  that  portion  of  each  refracting  surface  used  for  distinct  vision  is 
for  the  cornea  7.829  mm.,  for  the  anterior  and  posterior  surfaces  of 
the  lens  10  and  6  mm.,  respectively.  The  indices  of  refraction  of  the 
different  media  are  as  follows:  cornea  and  aqueous  humor,  1.3365; 
lens,  1. 4371;  vitreous  body,  1.3365.  The  distance  from  the  vertex  of 
the  cornea  to  the  lens  is  3.6  mm.;  the  thickness  of  the  lens,  3.6  mm.; 
the  distance  from  the  posterior  surface  of  the  lens  to  the  retina,  15  mm. 
As  the  two  surfaces  of  the  cornea  are  practically  parallel,  and  as  the 
index  of  refraction  of  the  aqueous  humor  is  the  same  as  that  of  the 
cornea,  they  may  be  re- 
garded as  but  one  medium. 
The  refracting  surfaces  may 
therefore  be  reduced  to  the 
anterior  surface  of  the 
cornea,  the  anterior  surface 
of  the  lens,  and  the  posterior 
surface  of  the  lens.* 

Parallel    ravs    of   light 
,      •         ,1  "  p  Fig.     310. — Refraction     of     Homocentric 

entering  tne  eye  pass  irom  rays  and  the  Formation  of  an  Image. 
air,  with  an  index  of  re- 
fraction of  1.00025,  into  the  cornea,  with  an  index  of  refraction 
of  1.3365.  In  passing  from  the  rarer  into  the  denser  medium  they 
undergo  refraction  in  accordance  with  the  laws  of  optics  and  are 
rendered  somewhat  convergent.  The  extent  of  this  first  refraction 
and  convergence  is  sufficiently  great  to  bring  parallel  rays,  if  con- 
tinued, to  a  focus  about  10  mm.  behind  the  retina.  This  would  be 
the  condition  in  aphakia  whether  the  lens  is  congenitally  absent  or 
has  been  removed  by  surgical  procedures.  Perfect  vision,  however, 
requires  that  the  convergence  of  the  light  must  be  great  enough  to 
bring  the  focal  point,  the  image,  on  the  retina.  This  is  accomplished 
by  the  introduction  of  an  additional  refracting  body,  the  lens.  On 
entering  the  lens  the  rays  are  for  the  same  reason — i.  e.,  the  passage 
from  a  rarer  into  a  denser  medium — again  refracted  and  converged, 
and  if  continued  would  come  to  a  focus  about  6.5  mm.  behind  the 
retina.  On  passing  from  the  lens  into  the  vitreous — i.  e.,  from  a 
denser  into  a  rarer  medium — the  rays  are  once  more  converged  and  to 
an  extent  sufficient  to  focalize  them  on  the  retina  (Fig.  310). 

*  Strictly  speaking,  the  posterior  surface  of  the  cornea  is  not  parallel  to  the  anterior 
surface,  and  the  index  of  refraction  of  the  cornea  is  a  trifle  greater  than  that  of  the 
aqueous  humor,  viz.,  1.377.  But  as  the  increase  in  the  corneal  refraction  due  to  a  higher 
index  is  almost  exactly  counteracted  by  a  decrease  in  refraction,  due  to  the  higher  curva- 
ture of  the  posterior  corneal  surface,  the  usual  assumptions  furnish  quite  accurate  result?. 


660  TEXT-BOOK  OF  PHYSIOLOGY. 

While  it  is  thus  possible  to  geometricly  follow  the  rays  through 
these  media  by  means  of  the  above-mentioned  factors,  the  procedure 
is  attended  with  many  difficulties.  Moreover,  as  the  relations  all 
change  when  rays  enter  the  eye  from  objects  situated  progressively 
nearer  the  eye,  a  separate  calculation  is  necessitated  for  each  distance 
for  the  determination  of  the  size  of  the  image. 

A  method  by  which  these  difficulties  are  much  reduced  was  sug- 
gested by  Gauss  and  developed  by  Listing.  It  was  demonstrated  by 
Gauss  that  in  every  complicated  system  of  refracting  media  separated 
by  centered  spheric  surfaces  there  may  be  assumed  certain  ideal 
or  cardinal  points,  to  which  the  system  may  be  reduced,  and  which, 
if  their  relative  position  and  properties  be  known,  permit  of  the  de- 
termination, either  by  calculation  or  geometric  construction,  of  the 
path  of  the  refracted  ray,  and  the  position  and  size  of  the  image  in 
the  last  medium,  of  the  object  in  the  first. 

Every  dioptric  system  can  be  replaced,  as  Gauss  showed,  by  a 
single  system  composed  of  six  cardinal  points  and  six  planes  per- 
pendicular to  the  common  axis — e.  g.,  two  focal  points,  two  principal 


jr, 


prV 


j> 


Fig.  311. — Diagram  showing  the  Position  and  Relation  of  the  Cardinal  Points. 

points,  two  nodal  points,  two  focal  planes,  two  principal  planes,  and 
two  nodal  planes. 

Properties  of  the  Cardinal  Points. — The  first  focal  point,  F„  in 
Fig.  311,  has  the  property  that  every  ray  which  before  refraction  passes 
through  it,  after  refraction  is  parallel  to  the  axis. 

The  second  focal  point,  F2,  has  the  property  that  every  ray  which 
before  refraction  is  parallel  to  the  axis,  passes  after  refraction 
through  it. 

The  second  principal  point,  H2,  is  the  image  of  the  first,  FLt',  that 
is,  rays  in  the  first  medium  which  go  through  the  first  principal  point 
pass  after  the  last  refraction  through  the  second.  Planes  at  right 
angles  to  the  axis  at  these  points  are  principal  planes.  The  second 
principal  plane  is  the  image  of  the  first.  Every  point  in  the  first 
principal  plane  has  its  image  after  refraction  at  a  corresponding  point 
in  the  second  principal  plane  at  the  same  distance  from  the  axis  and 
on  the  same  side. 

The  second  nodal  point,  N2,  is  the  image  of  the  first,  NT:  a  ray 
whi(  h  in  the  first  medium  is  directed  to  the  first  nodal  point  passes 
after  refraction  through  the  second  nodal  point,  and  the  directions  of 
the  rays  before  and   after  refraction  are  parallel  to  each  other.     In 


THE  SENSE  OF  SIGHT. 


66 1 


Fig.  311  let  A  B  represent  the  axis.  The  distance  of  the  first  focal 
point,  FJ}  from  the  first  principal  plane,  Hx,  is  the  anterior  focal 
distance.  The  distance  of  the  posterior  focal  point,  F2,  from  the 
second  principal  plane,  H2,  is  the  posterior  focal  distance.  The  dis- 
tance of  the  first  nodal  point,  N1}  from  the  first  focal  point  is  equal 
to  the  second  focal  distance.  The  distance  of  the  second  nodal 
point,  N2,  from  the  posterior  focal  point  is  equal  to  the  anterior  focal 
distance.     It  is  evident,  therefore,  that  the  distance  of  the  correspond- 


Fig.  312. — Diagram  to  Find  the  Image  in  Last  Medium  of  a  Luminous  Point  in 

the  First. 

ing  principal  and  nodal  points  from  each  other  is  equal  to  the  differ- 
ences between  the  two  focal  distances.  Also  the  distance  of  the  two 
principal  points  from  each  other  is  equal  to  the  distance  of  the  two 
nodal  points  from  each  other.  Finally,  the  focal  distances  are  pro- 
portional to  the  refractive  indices  of  the  first  and  last  media.  Planes 
passing  through  the  focal  points  vertically  to  the  axis  are  known  as 
focal  planes. 

From  these  properties  of  the  cardinal  points  the  position  of  an 
image  in  the  last  medium  of  a  luminous  point  in  the  first  may  be 


Yic.  313. — Diagram  to  Find  the  Refracted  Ray  in  the  Last  Medium  of  a  Given 
Ray  in  the  First  Medium. 


determined,  and  the  course  of  a  refracted  ray  in  the  last  medium  be 
constructed  if  its  direction  in  the  first  be  given  according  to  the  fol- 
lowing rules: 

1.  To  find  the  image  in  the  last  medium  of  a  luminous  point  in  the 
first:  Let  A  (Fig.  312)  be  this  given  point.  Draw  A  B  parallel  to 
the  axis  until  it  meets  the  second  principal  plane  in  B;  then  BF2 
will  be  this  ray  after  refraction.  Draw  a  second  ray  from  A  to 
the   first    nodal   point;   then   draw   another   ray,    D  E,   from   the 


662 


TEXT-BOOK  OF  PHYSIOLOGY. 


second  nodal  point  parallel  to  .4  C.  This  will  be  the  refracted 
ray  in  the  last  medium.  Where  the  two  refracted  rays,  BF2  and 
D  E,  intersect,  the  image  of  A  will  be  Az* 

To  find  the  refracted  ray  in  the  last  medium  of  a  given  ray  in  the 
first  medium:  Let  A  B  (Fig.  313)  be  the  given  ray.  Continue 
this  ray  until  it  meets  the  first  principal  plane  in  C.  Draw  C  D 
parallel  to.  the  axis.  Now  assume  any  point,  such  as  E,  in  the 
given  ray,  and  find  its  image  Ex  by  the  Rule  1.  Then  D  ET 
becomes  the  course  of  the  refracted  ray. 

The  Schematic  Eye. — Accepting  the  system  of  cardinal  points, 


Fig.  314. — Diagram  showing  the  Position  of  the  Cardinal  Points  in  the  "Sche- 
matic Eye."  The  continuous  lines  in  the  upper  half  of  the  figure  show  their  position 
in  the  passive  emmetropic  eye.  The  dotted  lines  indicate  the  change  in  their  position 
in  an  eye  accommodated  for  the  object  A  at  the  distance  a  from  the  cornea,  or  152  mm. 
The  lower  half  of  the  figure  shows  the  formation  of  a  distinct  image  on  the  retina  of  an  eye 
accommodated  for  the  object  A  at  the  distance  a  from  the  cornea. 


Listing,   Donders,  and  v.   Helmholtz  have  constructed   "schematic" 
eyes  to  be  substituted  for  the  refracting  system  of  the  natural  eye. 

For  this  purpose  it  is  necessary  to  make  use  of  the  various  esti- 
mates of  the  indices  of  refraction  of  the  different  media,  of  the  radii 
of  curvatures  of  the  different  refracting  surfaces,  and  of  the  distances 
separating  them,  to  deduce  an  average  eye  as  a  basis  for  calculation. 
The  most  widely  accepted  attempt  is  that  of  v.  Helmholtz.  The  data 
he  assumed  are  as  follows:  The  refractive  index  of  air  =  1;  of  the 
cornea  and  aqueous  humor,  1.3365;  of  the  lens,  1.4371;  of  the  vitreous 

*If  the  point  A  is  infinitely  far  from  the  eye,  all  the  rays  striking  the  eye  will  be  paral- 
lel to  each  other.  The  nodal  ray  must  therefore  be  drawn,  and  the  point  where  this 
nodal  ray  meets  the  second  focal  plane  will  be  the  image  of  A  =A^  where  all  rays  parallel 
to  the  nodal  ray  will  meet. 


THE  SENSE  OF  SIGHT.  663 

humor,  1.3365;  the  radius  of  curvature  of  the  cornea,  7.829  mm.; 
of  the  anterior  surface  of  the  lens,  10  mm.;  of  the  posterior  surface, 
6  mm. ;  the  distance  from  the  apex  of  the  cornea  to  the  anterior  surface 
of  the  lens,  3.6  mm.;  thickness  of  lens,  3.6  mm.  From  the  above- 
mentioned  data  v.  Helmholtz  calculated  the  position  of  the  cardinal 
points  for  the  eye  as  follows  (see  Fig.  314):  The  first  focal  point  is 
situated  13.745  mm.  before  the  anterior  surface  of  the  cornea;  the 
posterior  focal  point  is  situated  15.619  mm.  behind  the  posterior 
surface  of  the  lens;  the  first  principal  point,  1.753  mm-  behind  the 
cornea;  the  second  principal  point,  2.106  mm.  behind  the  cornea; 
the  first  and  second  nodal  points,  6.968  and  7.321  mm.  behind  the 
apex  of  the  cornea,  respectively.  The  anterior  focal  distance  of  this 
schematic  eye,  the  distance  between  FT  and  HT,  therefore  amounts 
to  15.498  mm.,  and  the  posterior  focal  distance,  H2  to  F2,  to  20.713 
mm. 

When  the  eye,  however,  is  accommodated  for  near  vision,  the 
relations  of  the  cardinal  points  are  changed  and  will  be  as  follows, 

if  the  point  accommodated  for,  lies  152  

mm.    from   the   cornea:      Anterior  focal  » A  ^\ 

distance,  is-QQO  mm.;  posterior  focal  dis-  IfCsk  \ 

>      o  yy  '    r  isms  mm       \^II±"" I 

tance,  18.689  rnm. ;  distance  from  cornea    F' y- V   "  207^m        ¥• 

of  the  first  and  second  principal  points,  \  J 

1.858    and   2.257   mm.  respectively;    dis-  ^**=B=a^^ 

tance   of   the   posterior  focus,  20.955  mm-      Fig.  315.— The  Reduced  Eye. 
from  cornea.      Given  this  schematic  eye 

in  the  accommodated  state,  the  course  of  the  rays  and  the  determina- 
tion of  the  position  of  an  image  in  the  last  medium  of  a  luminous 
point  in  the  first  can  easily  be  determined  by  the  rules  already  given. 

The  Reduced  Eye.— As  suggested  by  Listing,  this  schematic,  eye 
may  be  yet  further  simplified  or  reduced  to  a  single  refracting  surface 
bounded  anteriorly  by  air  and  posteriorly  only  by  aqueous  or  vitreous 
humor.  Without  introducing  any  noticeable  error  in  the  determination 
of  the  size  of  the  retinal  image,  the  anterior  principal  and  the  anterior 
nodal  points  may  be  disregarded,  owing  to  the  minuteness  of  the 
distances  (0.39  mm.)  separating  the  two  systems  of  points.  There 
is  thus  obtained  one  principal  point  and  one  nodal  point,  which  latter 
becomes  the  center  of  curvature  of  the  single  refracting  surface.  The 
dimensions  of  this  "reduced"  eye  are  as  follows  (see  Fig.  315).  From 
the  anterior  surface  of  the  cornea,  corresponding  to  the  principal 
plane  H,  to  the  nodal  point  N,  5.215  mm.,  from  the  anterior  focal 
point  FI}  to  the  principal  plane  H,  i.  e.,  the  anterior  focal  distance  /', 
15.498  mm.;  from  the  principal  plane  H  to  the  posterior  focal  point 
F2,  i.e.,  the  posterior  focal  distance  /",  20.713  mm.;  the  index  of 
refraction  is  1.3365.  There  is  thus  substituted  for  the  natural  eye  a 
single  refracting  surface  with  a  radius  of  curvature,  r,  of  5.215  mm. 
In  such  an  eye  luminous  rays  emanating  from  the  anterior  focal 
point  are  parallel  to  the  axis  after  refraction  in  the  interior  of  the  eve. 


664  TEXT-BOOK  OF  PHYSIOLOGY. 

Also  rays  parallel  to  the  axis  before  refraction  unite  at  the  posterior 
focal  point. 

By  means  of  this  reduced  eye  the  construction  of  the  refracted  ray, 
the  various  calculations  as  to  the  size  of  the  image,  the  size  of  diffusion 
circles,  etc.,  are  greatly  facilitated:  e.  g., 

In  Fig.  316  let  A  B  represent  an  object.  From  A  homocentric 
rays  fall  on  the  single  refracting  surface.     One  of  the  rays,  the  nodal 

A_  ray,  falling  on  the  surface 

/r         ^v  perpendicularly,    passes 

^^\^~~--~^^^  ^_^\p        unrefracted    through    the 

- - - ■"-'^^d(g^  single  nodal  point,  N,  to 

-- —" — "~"      ""\ ^jr  trie  Posterior  focal  plane. 

-=-"""  ^w—^^^  The  remaining  rays,  par- 

tially   represented  in  the 
Fig.  316.-THE  Formation  of  an  Image  in  the         figure    fallinfir  on  this  sur- 
Reduced  Eye.  .  °  .      °  .  . 

face  under  varying  de- 
grees of  incidence,  undergo  corresponding  degrees  of  refraction,  by 
which  they  form  a  converging  cone  of  rays  which  unite  at  a  point 
situated  on  the  nodal  ray.  These  two  points,  A,  a,  are  known  as 
conjugate  foci.  The  same  holds  true  for  homocentric  rays  emanating 
from  B  or  any  other  point  of  the  object. 

The  Size  of  the  Retinal  Image. — The  size  of  the  retinal  image, 
I  (in  Fig.  316  a  b),  may  now  be  easily  calculated,  when  the  size  of  the 
object,  O  (in  Fig.  316  A  B),  and  its  distance,  D,  from  the  refracting 
surface  with  radius  of  curvature,  r,  are  known,  by  the  following 
formula: 

O:  I  =  D  +  r:f"— r. 

For,  as  the  triangles  A  N  B  and  a  N  b  are  similar,  we  have 

A  B:  ab  =  /  N:  N  g,  or  a  b  =  A  B '  *N  8. 

Independent  of  the  foregoing  method,  the  size  of  the  retinal  image 
may  be  calculated  if  it  is  remembered  that  the  eye,  like  any  optic 
system,  has  a  point  of  such  a  quality  that  a  ray  of  light  which  before 
entering  the  eye  was  directed  toward  it,  after  refraction  continues  as 
if  it  came  from  this  point.  In  other  words,  there  is  in  the  eye  a  point 
which  allows  a  ray  of  light  to  pass  unrefracted.  This  point,  termed 
the  nodal  point  of  the  eye,  determines  the  size  of  the  image;  for  if  a 
line  be  drawn  from  both  the  upper  and  lower  ends  of  an  object  through 
this  nodal  point,  it  is  clear  that  the  images  of  the  respective  points 
must  lie  on  these  two  rays  where  they  intersect  the  retina.  The  dis- 
tance of  this  nodal  point  from  the  retina  is  15.498  mm.  It  is  clear, 
therefore,  that  the  size  of  the  object  is  to  the  size  of  the  image,  as  the 
distance  of  the  object  from  the  nodal  point  is  to  the  distance  of  the 
nodal  point  from  the  retina;  or,  in  other  words,  to  find  the  size  of  the 
retinal  image:  multiply  the  size  of  the  object  by  15.5  mm.  and  divide 
by  the  distance  of  the  object  from  the  eye. 

The  visual  angle  is  defined  as  the  angle  formed  by  the  intersection 


THE  SENSE  OF  SIGHT. 


665 


of  two  lines  drawn  from  the  extremities  of  an  object  to  the  nodal  point 
of  the  eye.  Beyond  the  nodal  point,  however,  the  lines  again  diverge 
and  form  an  inverted  or  reversed  image  of  the  object  on  the  retina. 
The  size  of  the  visual  angle  increases  with  the  nearness  and  decreases 
with  the  remoteness  of  the  object;  the  retinal  image  correspondingly 

increases  and  decreases  in  size. 
A 


These  facts  will  become  ap- 
parent from  an    examination 


6' 


of  Fig.  317.      As  the  size  of 


Fig.    317. 


-Drawing  Designed  to  show 
how  the  Visual  Angle  and  Size  of  Retinal 
Image  Varies  with  the  Distance  of  an 
Object  of  Given  Size.  For  the  distant  position 
of  A-B  the  visual  angle  is  a;  for  the  near 
position  (dotted  lines)  /3.     (From  Stewart.) 


the  retinal  image  diminishes 
as  the  visual  angle  diminishes 
either  as  a  result  of  the  re- 
moval of  a  given  object  from 
the  eye,  or  of  a  diminution  of 
the  size  of  the  object,  there 
comes  a  limit  in  the  size  of  the 
visual  angle,  beyond  which  it 
is  impossible  to  see  the  two  end  points  (A  and  B)  of  the  object 
separatelv.  When  this  limit  is  reached  the  size  of  the  angle  expressed 
in  degrees  of  the  circle,  may  be  determined  if  the  distance  between  the 
two  points  and  their  distance  from  the  eye  be  known.  Thus  it  has  been 
experimentally  determined  that  at  a  distance  of  5  meters,  the  smallest 
object  or  the  smallest  interval  between  two  points  which  permits  the 
eye  to  distinguish  them  as  such,  is  about  1.454  mm.  Lines  drawn  from 
the  extremities  of  such  an  object  or  interval,  to  the  nodal  point,  subtend 
an  angle  of  60  seconds.*  Beyond  this  the  two  points  are  indistinguish- 
able. In  other  words  the  emmetropic  eye  possesses  the  power  of  distin- 
guishing the  correspondingly  small  interval  between  the  two  images  on 
the  retina  of  the  two  objective  points.  The  size  of  the  image  or  the  inter- 
val between  the  two  retinal  points,  determined  from  the  foregoing  factors 
by  the  formulas  on  page  664  is  0.004  mm.,  and  which  would  correspond 

*The  size  of  the  visual  angle,  under  which  an  object  of  this  size  and  situated  at  a 
distance  of  5  meters  is  distinctly  seen,  can  be  determined  from  the  following  Fig.  318, 
in  which  A  B,  represent 
the  size  of  the  object 
1.454  mm.:  Ar,  thenodal 
point;  CN,  the  line 
which  bisects  the  object, 
represent  the  distance  of 
the  object  from  the  nodal 
point;  a,  the  visual  angle 
subtended  and  whose 
value  it  is  desired  to 
know,  and  b  one-half  of 
the  angle  (7.  By  trigo- 
nometry the  size  of  the 
angle  </  can  be  deter- 
mined in  the  following  way:  one-half  the  size  of  the  object  A  B,  is  divided  by  its  distance 
from  the  nodal  point;  the  quotient  is  the  tangent  of  half  the  angle.  Thus  0.727  -r-  5000  = 
0.0001454.  By  reference  to  logarithmic  tables  it  will  be  found  that  the  angle  or  fraction 
of  the  circle  corresponding  to  this  tangent  is  30  seconds,  and  that  therefore  the  whole  angle 
is  60  seconds. 


Fig.  318. — Figure  showing  the  Method  of  Obtaining 
the  Visual  Angle  Enpressed  in  Degrees  or  Fraction 
of  a  Degree  of  the  Circle. 


666  TEXT-BOOK  OF  PHYSIOLOGY. 

to  a  visual  angle  of  60  seconds.  If  the  retinal  distance  is  less  than  this 
the  two  sensations  fuse  into  one.  The  reason  assigned  for  this  is, 
that  the  distance  between  the  centers  of  two  adjoining  cones  in  the 
macula  is  0.004  mm.  With  a  visual  angle  not  less  than  60  seconds, 
the  two  foci  fall  on  separate  cones.  With  a  smaller  visual  angle  the 
two  foci  fall  on,  and  excite  but  a  single  cone  and  hence  there  arises 
the  sensation  of  but  a  single  point.  The  acuteness  of  vision,  therefore, 
of  the  emmetropic  eye  depends  on  its  power  of  distinguishing  the 
smallest  retinal  image  or  the  smallest  interval  between  two  points  on 
the  retina  corresponding  to  a  visual  angle  of  60  seconds. 

In  ophthalmic  practice  it  is  customary  in  testing  the  acuteness  of 
vision  to  employ  test  letters  of  specific  sizes  for  specific  distances.  The 
letters  are  so  proportioned  that  when  they  are  placed  at  the  specified 
distances,  the  extremities  of  the  letters  subtend  an  angle  of  5  minutes. 
The  letters  have  been  constructed  on  the  following  basis:  Since  to 
an  angle  of  60  seconds  there  corresponds  an  object  of  1.454  mm.  at 
the  distance  of  5  meters  as  shown  before  and  as  the  object  decreases 
in  proportion  to  the  distance  (for  the  same  visual  angle)  it  is  evident 


E    T 


E  L 


B. 
Fig.  319. — Standard  Test  Letters. 

that  the  object  would  have  to  be  one-fifth  of  1.454  mm.  or  0.2908  mm. 
in  order  to  subtend  an  angle  of  60  seconds  at  one  meter.  From  this 
the  size  for  any  other  distance  in  meters  is  found  simply  by  multiplying 
0.2908  mm.  with  the  distance. 

The  standard  letters  are  so  constructed  that  each  meridian  is 
composed  of  five  squares,  each  of  which  at  a  specified  distance,  sub- 
tends an  angle  of  1  minute,  and  hence  the  entire  letter  an  angle  of  5 
minutes.  The  letter  that  could  be  distinctly  seen  at  a  distance  of  5 
meters,  would  have,  therefore,  a  vertical  and  a  horizontal  meridian 
of  5  times  1.454  mm.  or  7.27  mm.  (Fig.  319  A),  and  at  10  meters 
corresponding  meridians  of  14.54  mm.,  etc.     (Fig.  319  B.) 

If  with  the  accommodation  suspended,  the  emmetropic  eye  could 
clearly  distinguish  a  letter  7.27  mm.  in  size  at  a  distance  of  5  meters 
and  which  would,  therefore,  subtend  an  angle  of  5  minutes,  the  acuity 
of  1  he  vision  would  be  normal  and  could  be  expressed  as  follows: 
V=|  or  V  =  i.  If  on  the  contrary  at  this  distance  the  smallest 
letter  that  could  be  clearly  seen  is  one  that  subtends  an  angle  of  5 
minutes  at  a  distance  of  10  meters  then  the  visual  acuity  would  be  only 
one  half  l  he  normal  and  could  be  expressed  as  follows  V=-yV  or  V=|, 
etc.  The  acuity  of  vision  is  expressed,  therefore,  by  a  fraction  the 
enumerator  of  which  is  the  distance  at  which  the  test  is  made,  and 


THE  SENSE  OF  SIGHT. 


667 


whose  denominator  is  the  distance  at  which  the  smallest  letters  dis- 
tinguished by  the  patient,  subtend  an  angle  of  5  minutes,  but  which 
in  the  instance  cited  above  subtends  an  angle  of  10  minutes;  or  in 
other  words  the  distance  at  which  the  patient  reads  divided  bv  the 
distance  at  which  he  ought  to  read  the  smallest  letters  seen  by  him  were 
his  visual  acuity  normal. 

Accommodation. — Accommodation  may  be  defined  as  the  power 
which  the  eye  possesses  of  adjusting  itself  to  vision  at  different  dis- 
tances; or  in  other  words,  the  power  of  focusing  rays  of  light  on  the 
retina,  which  come  from  different  distances  at  different  times.  That 
such  a  power  is  a  necessity  is  apparent  from  the  fact  that  it  can  not 
focus  rays  coming  from  a  distant  and  a  near  object  at  the  same  time. 
Thus,  if  an  object  is  held  before  the  eye  at  a  distance  of  22  centimeters, 
for  example,  and  the 
vision  is  directed  to  a 
distant  object  it  is  evi- 
dent that  the  near  ob- 
ject is  indistinctly  seen; 
but  if  the  vision  is  then 
directed  to  the  near  ob- 
ject, it  in  turn  becomes 
clear  and  distinct,  while 
the  distant  object  be- 
comes blurred  and  indis- 
tinct. It  is  evident, 
therefore,  that  rays  of 
light  coming  from  a  dis- 
tant and  a  near  object 
can  not  be  simultane- 
ously, but  only  alter- 
nately, focused  on  the 
retina.  The  observer  at 
the  same  time  becomes 

conscious,  as  the  vision  is  directed  from  the  distant  to  the  near  object, 
of  a  change  in  the  eye  itself,  a  change  that  involves  time  and  effort. 
The  reasons  for  these  facts  will  become  apparent  from  a  consideration 
of  the  following  facts: 

In  a  normal  or  emmetropic  eye,  homocentric  parallel  rays  of  light 
(Fig.  320,  a,  b)  after  passing  through  the  optic  media  are  converged 
and  brought  to  a  focus  on  the  retina,  /.  Rays,  however,  which  come 
from  a  luminous  point  situated  near  the  eye,  P,  and  are  therefore 
divergent,  passing  through  the  optic  media  at  the  same  time,  are  inter- 
cepted by  the  retina  before  they  are  focused,  and  give  rise  to  the  for- 
mation of  difjusion-circles  and  indistinctness  of  vision.  The  reverse 
is  also  true.  When  the  eye  is  adjusted  for  the  refraction  and  focusing 
of  divergent  rays  (Fig.  320,  P)  parallel  rays  will  be  brought  to  a  focus 
before  reaching  the  retina,  and,  again  diverging,  will  form  ditlusion- 


Fig.  320. — The  Refraction  of  Parallel  and 
Divergent  Rays  in  the  Emmetropic  Eye  in  the 
Passive  and  in  the  Active  or  Accommodated 
Condition. 


668 


TEXT-BOOK  OF  PHYSIOLOGY. 


Distance  of  the  Focal 
oint  behind  the  Posterior 
Surface  of  the  Retina. 

Diameter  of  the  Diffusion-circle 

o.o     mm. 

0.0       mm. 

0.005    " 

O.OOII     " 

0.012    " 

0.0027    " 

0.025    " 

0. 0*056    " 

0.050    " 

O.OII2      " 

O.IOO     " 

0.0222    " 

0.20      " 
0.40      " 
o.So      " 

0.0443    " 
0.0825    " 
0.1616    " 

1.60      " 

0.3122    " 

3.20      " 
3-42      " 

0.5768    " 
0.6484    " 

circles.  It  is  evident,  therefore,  that  it  is  impossible  to  simultaneously 
focus  both  parallel  and  divergent  rays,  and  to  see  distinctly  at  the  same 
time,  two  objects  which  are  situated  at  different  distances.  The  eye 
must  be  alternately  adjusted  first  to  one  object  and  then  to  another. 
To  this  adjustment  the  term  accommodation  has  been  given. 

The  following  table  of  Listing  shows  the  size  of  the  diffusion- 
circles  formed  of  objects  situated  at  different  distances  when  the 
accommodative  power  is  suspended  in  an  emmetropic  eye: 


Distance  of  Luminous  Point. 


6 

3 
1.500 

0.750 

0-375 
0.188 
0.094 
0.088 

The  normal  eye  when  adjusted  for  distant  vision  is  in  a  passive 
condition,  and  hence  vision  of  distant  objects  is  unattended  with 
fatigue.  In  the  act  of  adjustment,  however,  for  near  vision  the  eye 
passes  into  an  active  state,  the  result  of  a  muscle  effort,  the  energy  of 
which  is  proportional  to  the  nearness  of  the  object  toward  which  the 
eye  is  directed. 

From  the  foregoing  table  it  is  evident  that  between  infinity  and 
65  meters,  the  diffusion-circles  are  so  slight  that  no  perceptible  ac- 
commodative effort  is  required  to  eliminate  them.  From  65  meters 
to  6  meters  the  diffusion-circles  gradually  become  larger,  though  they 
are  yet  so  faint  as  to  require  for  their  correction  an  accommodative 
effort  which  is  scarcely  measurable.  From  6  meters  up  to  6  centi- 
meters, however,  a  progressive  increase  in  accommodative  power  is 
demanded  for  distinct  vision. 

Mechanism  of  Accommodation. — Inasmuch  as  neither  the 
corneal  curvature  nor  the  shape  of  the  eyeball  undergoes  any  change 
during  accommodation,  the  necessary  change,  whatever  it  may  be, 
is  to  be  sought  for  in  the  lens.  As  to  the  character  of  the  changes 
in  this  body,  two  views  are  held,  based  largely  on  the  fact  and  its 
interpretation,  that  images  of  a  luminous  point  reflected  from  the 
anterior  surface  of  the  cornea,  the  anterior  and  posterior  surfaces 
of  the  lens,  change  their  relative  positions  during  accommodation. 

Thus,  if  in  a  darkened  room  a  lighted  candle  be  placed  in  front 
of  and  to  the  side  of  an  individual  whose  eye  is  directed  to  a  distant 
object,  an  observer  placed  in  the  same  relative  position  as  the  candle 
will  observe  three  images  in  the  eye,  one  at  the  surface  of  the  cornea, 
two  at  the  pupillary  margin   (Fig.  321;.     Of  the  two  latter,  one  is 


THE  SENSE  OF  SIGHT.  669 

quite  large  and  situated  apparently  in  front  of  the  third,  which  is  faint, 
small,  and  inverted.  The  middle  image  is  reflected  from  the  convex 
surface  of  the  lens,  the  last  from  the  concave  surface.  These  images 
of  reflection  are  known  as  catoptric  images.  If  now  the  individual 
be  directed  to  fix  the  gaze  on  a  near  object,  the  second  image  changes 
its  position,  advances  toward  the  corneal  image  and  at  the  same  time 
becomes  smaller,  a  change  which,  in  accordance  with  the  laws  of 
optics,  could  only  be  due  to  an  increase  in  the  convexity  of  the  anterior 
surface  of  the  lens.  A  slight  displacement  of  the  third  image  some- 
times observed  indicates  a  possible  increase  in  the  convexity  of  the 
posterior  surface  lens. 

According  to  Helmholtz,  during  accommodation  the  entire  anterior 
surface  of  the  lens  becomes  more  convex,  while  at  the  same  time  it 
slightly  advances,  possibly  as  much  as  0.4  mm.  in 
extreme  efforts.  This  change  is  represented  in 
Fig.  322.  According  to  Tscherning,  the  increase 
in  convexity  of  the  anterior  surface  is  confined  to 
the  central  portion,  the  remainder  of  the  surface 
becoming  somewhat  flattened.  There  is,  moreover, 
no  evidence  that  there  is  any  advance  of  the  surface  a      b    c 

or  any  increase  in  the  thickness  of  the  lens.  A  FlG  ,2i— Catop- 
series  of  new  and  ingenious  experiments  lend  sup-  tric  Images  in  the 
port  to  Tscherning's  view.     The  radius  of  curvature      ?YE-      ?■    J-Pn.gbt 

i        .  ,  °      .  ,  „  image    of    reflection, 

in  either  case  approximates  6  mm.  in  extreme  efforts  fror£  tne  cornea,  b. 
of  accommodation.  The  increase  in  convexity  Upright  image  from 
naturally  increases  the  refracting  power.  the  anterior  surface 

TTT1    -  .  .  .°f  of   the    lens.     c.  In- 

Whichever  view  is  accepted,  the  nearer  the  ob-  verted  image,  from 
ject— that  is,  the  greater  the  degree  of  divergence  the  posterior  surface 
of  the  light  rays — the  more  pronounced  must  be  the  holtzA 
increase  in  convexity  in  order  that  they  may  be 
sufficiently  converged  and  focalized  on  the  retinal  surface.  Changes  in 
the  convexity  of  the  lens,  either  of  increase  or  decrease,  are  attended 
by  changes  in  the  distinctness  of  images.  Coincident  with  the  lens 
change,  the  pupillary  margin  advances  and  the  pupil  itself  becomes 
smaller.  By  this  means  an  indistinctness  of  the  image  is  prevented 
by  cutting  off  the  rays  which  would  give  rise,  owing  to  the  angle  at 
which  they  fall  on  the  surface,  to  diffusion-circles,  from  spheric  aber- 
ration. 

The  Function  of  the  Ciliary  Muscle.— Though  it  is  generally 
admitted  that  the  increase  in  the  convexity  of  the  lens  is  caused  by 
the  contraction  of  the  ciliary  muscle,  the  exact  manner  in  which  this 
is  accomplished  is  not  clearly  understood.  According  to  Helmholtz, 
when  the  eye  is  in  repose  and  directed  to  a  distant  object  the  lens  is 
somewhat  flattened  from  a  traction  exerted  by  the  suspensory  liga- 
ment. When  the  eye  is  directed  to  a  near  object,  the  ciliary  muscle 
contracts,  thereby  relaxing  the  ligament,  as  a  result  of  which  the  lens, 
by  virtue  of  an  inherent  elasticity,  bulges  forward  and  becomes  more 


6jo 


TEXT-BOOK  OF  PHYSIOLOGY. 


convex.  In  consequence  of  this  latter  fact  the  refracting  power  is 
proportionally  increased.  In  extreme  efforts  of  accommodation  it  is 
believed  by  some  observers  that  the  circularly  arranged  fibers,  the 
so-called  annular  muscle,  contract  and  exert  a  pressure  on  the  periphery 
of  the  lens  and  thus  aid  other  mechanisms  in  relaxing  the  ligament 
and  in  increasing  the  convexity.  This  view  appears  to  be  supported 
by  the  fact  that  in  hypermetropia,  where  a  constant  effort  is  required 
to  obtain  a  distinct  image  of  even  distant  objects,  the  annular  muscle 
becomes  very  much  hypertrophied,  thus  reinforcing  the  meridional 
fibers.  In  myopia,  on  the  contrary,  where  the  accommodative  effort 
is  at  a  minimum,  the  entire  muscle  possesses  less  than  its  average  size 
and  development. 

According  to  Tscherning,  a  different  explanation  of  the  action  of 
the  ciliary  muscle  must  be  given.  Thus,  when  it  contracts,  the  antero- 
internal  angle,  that  portion  in  close  relation  with  the  suspensory 
ligament,  recedes  and  exerts  on  the  ligament  a  pressure  which  in  turn 
exerts  a  traction  on  the  peripheral  portions  of  the  anterior  surface  of 


UptfAeZtum. 
■_fion>mansJfem&r£tn>6 
Gyrrtea  proper 
-Itejeemef  'Jfeminxne 
-EndptheZiusrv 
SpOzcterJridis 
CbuiaZ  <5c?tfemSri£L 
%*&&aryJfajcZe 
■ivV  •fcZera,   ■ 


IProc&sraj  CUiarit 


Fig.  322. — The  Left  Half  Represents  the  Eye  in  a  State  of  Rest.     The  Right 
Half  in  State  of  Accommodation. 


the  lens,  which  produces  the  deformation  observed.  At  the  same 
time  the  postero-external  portion  of  the  muscle  exerts  traction  on  the 
chorioid,  thus  sustaining  the  vitreous  and  indirectly  the  lens. 

The  reason  for  the  flattening  of  the  periphery  of  the  lens  from 
zonular  compression  and  the  sharpening  of  the  central  convexity  is 
to  be  found  in  the  fact  that  the  convexity  of  the  more  solid  central 
portion,  the  nucleus,  is  greater  than  that  of  the  lens  itself.  Hence  it 
is  easily  understood  why  a  zonular  traction  would  give  rise  to  peripheral 
flattening. 

There  is,  however,  one  point  which  seems  difficult  to  harmonize 
with  Tschcrning's  view;  that  is,  the  fact  that  during  accommodation 
the  lens  appears  to  be  slightly  tremulous,  thus  showing  relaxation, 
and  not  increased  tension,  of  the  suspensory  ligament. 

Range  of  Accommodation.— It  has  been  stated  that  rays  of 
light  coming  from  a  luminous  point  situated  at  any  distance  beyond 
65  meters  are  so  nearly  parallel  that  no  accommodative  effort  is  re- 
quired for  their  focalization.  So  long  as  the  luminous  point  remains 
between  infinity  and  65  meters,  the  eye,  directed  toward  it,  remains 


THE  SENSE  OF  SIGHT.  671 

completely  relaxed.  The  point  at  which  the  object  can  be  distinctly 
seen  without  accommodation  is  termed  the  far  point  or  the  punctum 
remotum.  This  for  the  normal  eye  is  at  a  distance  of  65  meters  or 
beyond.*  If  the  luminous  point  gradually  approaches  the  eye  from 
a  point  65  meters  distant,  the  accommodative  power  comes  into  play 
and  gradually  increases  until  it  attains  its  maximum.  The  nearest 
point  up  to  which  the  eye  is  able  to  form  distinct  images  of  objects 
is  called  its  near  point  or  punctum  proximum.  This  near  point 
in  a  healthy  boy  of  twelve  years  will  lie  at  2§  inches  from  the  eye, 
while  the  same  point  lies  only  at  8  inches  or  20  cm.  in  a  man  of  forty 
years.  Of  objects  which  lie  nearer  than  the  punctum  proximum  the 
eye  cannot  form  distinct  images.  The  distance  between  the  punctum 
remotum  and  the  punctum  proximum  is  termed  the  range  of  accommo- 
dation. 

Force  of  Accommodation. — The  increase  in  curvature  of  the 
lens  necessary  to  focalize  rays  wdien  the  eye  is  directed  from  the  far 
to  the  near  point  necessitates  the  expenditure  of  energy  on  the  part 
of  the  ciliary  muscle.  The  energy  expended  in  the  act  of  accommo- 
dation may  be  measured  by  a  lens,  the  refracting  power  of  which  is 
such  as  to  enable  it  to  produce  the  same  result — that  is,  to  give  the 
diverging  rays  coming  from  the  near  point,  e.  g.,  20  cm.,  a  parallel 
direction.  A  lens,  therefore,  which  has  for  a  near  point  a  focal  dis- 
tance of  20  cm.  would  be  a  measure  of  the  force  expended,  for  such  a 
lens  placed  in  front  of  the  crystalline  lens,  when  in  a  state  of  repose, 
would,  with  the  assistance  of  the  latter,  bring  diverging  rays  coming 
from  the  near  point  to  a  focus  on  the  retina.  A  lens  of  this  character 
is  said  to  have  a  refracting  power  of  5  dioptries. 

Since  lenses  of  the  same  curvature  made  from  different  materials 
have  different  refracting  powers,  it  becomes  necessary  to  have,  for 
purposes  of  comparison,  some  unit  of  measurement.  The  unit  now 
accepted  is  the  refracting  power  of  a  glass  lens  which  is  sufficient 
to  focalize  parallel  rays  at  a  distance  of  100  cm.  or  1  meter.  This 
amount  of  refracting  power  is  termed  a  dioptry.  Lenses  which 
would  focalize  parallel  rays  at  a  distance  of  50,  20,  or  10  cm.  are 
said  to  have  a  refractive  power  of  2,  5,  or  10  dioptries,  respectively, 
obtained  by  dividing  into  100  cm.  the  focal  distance.  The  refracting 
power  of  a  biconcave  lens  is  determined  by  prolonging  backward  in 
the  direction  the  parallel  rays  have  come,  the  rays  which  have  been 
rendered  divergent  by  the  lens. 

The  refracting  media  of  the  human  eye  in  repose  have  collectively 
a  refracting  power  of  about  64  dioptries,  the  reciprocal  of  its  focal 
length.  The  refracting  power  of  the  corneal  surface  alone  is  equiva- 
lent to  42  dioptries.  The  crystalline  lens  could  in  the  schematic  eye 
be  replaced  by  a  lens  of  about  13  dioptries  in  front  of  the  eye,  as  is 
done  after  the  extraction  of  a  cataract.     But  owing  to  its  position  in  a 

*In  practical  ophthalmic  work  a  point  six  meters  distant  is  taken  as  the  far  point  for 
the  reason  that  the  rays  at  this  distance  are  practically  parallel. 


672  TEXT-BOOK  OF  PHYSIOLOGY. 

medium  denser  than  air,  it  has  been  calculated  that  its  refracting 
power  is  about  20  dioptries. 

The  capability  of  the  lens  to  increase  its  refraction  during  accom- 
modative efforts  beyond  the  20  dioptries  varies  considerably  at  differ- 
ent periods  of  life.  At  ten  years  the  increase  is  14  dioptries,  as  the 
near  point  is  7  cm.;  at  thirty  years  the  increase  is  but  7  dioptries,  as 
the  near  point  is  14  cm.;  at  sixty  the  increase  is  but  1  dioptry  and 
the  near  point  100  cm.;  at  seventy  it  is  zero.  From  youth  to  old 
age,  the  elasticity  of  the  lens  steadily  declines,  and  the  range  of  accom- 
modation diminishes  from  the  recession  of  the  near  point. 

Convergence  of  the  Eyes  during  Accommodation. — In  binocu- 
lar vision  of  near  objects  the  eyes  are  turned  inward  and  the  optic 
axis  of  each — a  line  passing  through  the  center  of  the  cornea  and 
the  center  of  the  eye — turned  toward  the  median  line  during  accom- 
modation. So  long  as  the  eyes  are  directed  toward  the  far  point,  65 
meters  or  beyond,  the  optic  axes  are  parallel.  When  the  eyes  are 
directed  to  any  point  within  65  meters  the  optic  axes  are  converged, 
the  convergence  increasing  steadily  as  the  near  point  is  approached. 
In  this  way  the  fovea  of  each  eye  is  directed  to  the  same  point  and 
single  vision  made  possible.  Were  this  not  the  case,  double  vision 
would  result. 

Functions  of  the  Iris.— For  purposes  of  distinct  vision  it  is  essen- 
tial that  the  quantity  of  light  entering  the  interior  of  the  eye  shall 
be  so  adjusted  that  the  formation  and  subsequent  perception  of  the 
image  shall  be  sharp  and  distinct.  This  is  accomplished  by  the  .iris, 
the  circular  fibers  of  which  alternately  contract  and  relax  with  in- 
creasing and  decreasing  intensities  of  the  light.  The  size  of  the  pupil, 
therefore,  through  which  the  light  passes,  will  vary  from  moment 
to  moment  and  in  accordance  with  variation  in  the  light  intensity. 
The  quantity  of  light  necessary  to  distinct  vision  is  thus  regulated. 

In  the  total  absence  of  light  the  sphincter  pupillse  muscle  is  relaxed 
and  the  pupil  widely  dilated.  With  the  appearance  of  light  and  an 
increase  in  its  intensity  the  muscle  again  contracts  and  the  pupil 
progressively  narrows.  With  a  given  intensity  in  the  light,  the  sphinc- 
ter contraction  is  greater  when  the  light  falls  directly  into  the  fovea. 
Contraction  of  this  muscle  also  occurs  as  an  associated  movement  in 
the  convergence  of  the  eyes  during  accommodation  and  in  consensus 
with  the  other  eye. 

In  addition  to  this  function  of  the  iris,  it  constitutes,  by  virtue  of 
the  sphincter  muscle  contraction,  an  important  corrective  apparatus. 
Being  non-transparent,  it  serves  as  a  diaphragm  intercepting  those 
rays  which  would  otherwise  pass  through  the  peripheral  portions  of 
the  lens  and  by  spheric  aberration  give  rise  to  indistinctness  of  the 
image.  The  movements  of  the  iris  by  which  the  size  of  the  pupil  is 
determined  are  caused  by  the  contractions  and  relaxations  of  the 
sphincter  piipiUai  and  dilatator  pupillcR  muscles.  The  contraction  of 
the  sphincter  is  entirely  reflex  and  involves  those  structures  necessary 


THE  SENSE  OF  SIGHT.  673 

to  the  performance  of  any  reflex  act,  viz.:  a  receptive  surface,  the 
retina;  afferent  nerves,  the  pupillary  fibers  of  the  optic  nerve;  a  central 
emissive  center  situated  in  the  gray  matter  beneath  the  aqueduct  of 
Sylvius;  and  efferent  nerves,  the  motor  oculi  and  the  ciliary  nerves. 
The  stimulus  requisite  to  the  excitation  of  this  mechanism  is  the  impact 
of  light  waves  or  ether  vibrations  on  the  rods  and  cones.  According 
to  the  intensity  of  these  vibrations  will  be  the  resulting  contraction  of 
the  muscle.  The  contraction  of  the  dilatator  pupillae  muscle  is  deter- 
mined by  the  activity  of  a  continuously  active  nerve-center  in  the 
medulla  oblongata  which  transmits  its  nerve  impulses  through  the 
spinal  cord,  along  the*  first  and  second  dorsal  nerves  to  the  superior 
cervical  ganglion,  and  thence  to  the  iris  by  way  of  the  fifth  nerve. 
(See  Fig.  272,  page  589.)  These  two  muscles  appear  to  bear  an  an- 
tagonistic relation  to  each  other,  for  section  of  the  motor  oculi  is  fol- 
lowed by  relaxation  of  the  sphincter  muscle  and  dilatation  of  the  pupil. 
Stimulation  of  the  sympathetic  is  followed  by  a  more  pronounced 
dilatation.  The  size  of  the  pupil  is  the  resultant  of  a  balancing  of  these 
two  forces. 

OPTIC  DEFECTS. 

Presbyopia. — This  is  a  condition  of  the  eye  characterized  by  a 
defective  or  diminished  accommodative  power.  As  age  advances  the 
lens  loses  its  elasticity  and  the  power  to  increase  its  refraction,  and 
vision  at  the  normal  reading  distance  becomes  impossible.  The  near 
point,  therefore,  advances  toward  the  far  point,  or  recedes  from  the  indi- 
vidual. The  range  of  accommodation  is  also  diminished.  At  fortv 
years  the  near  point  is  about  22  cm.;  at  forty-five  years  it  has  receded 
to  28  cm.  This  would  indicate  that  the  lens  in  these  five  years  has 
lost  1  dioptry  of  refracting  power;  at  fifty  years  the  near  point  recedes 
to  43  cm.,  and  at  sixty  to  200  cm.,  indicating  a  loss  in  refracting  power 
on  the  part  of  the  lens  of  2  and  4  dioptries  respectively.  Convex 
lenses  placed  before  the  eyes  having  a  refracting  power  of  1,  2,  and  4 
dioptries  would  in  the  three  instances  return  the  near  point  to  its 
normal  position.  At  the  age  of  seventy  the  lens  is  incapable  of  any 
increase  during  an  accommodative  effort.  A  lens  of  4  dioptries 
would  therefore  be  required  by  such  a  man,  for  near  vision  at  10  inches. 

Myopia. — This  is  a  condition  of  the  eye  characterized  by  an  in- 
crease in  the  antero-posterior  diameter  or  a  hypernormal  refracting 
power  of  the  lens.  The  former  is  the  usual  condition.  Parallel  rays 
of  light  brought  to  a  focus  in  front  of  the  retina  again  diverge,  giving 
rise  to  diffusion-circles  and  indistinctness  of  the  image.  Divergent 
rays  alone  are  capable  of  being  focalized  on  the  retina  in  its  new 
position.  The  punctum  remotum  is  always  at  a  definite  distance, 
but  approaches  the  eye  as  the  myopia  increases.  The  near  point 
is  usually  much  nearer  the  eye  than  20  cm.  For  this  reason  the 
condition  is  termed  near  sight.     (Fig.  323.) 

The  increase  in  the  length  of  the  antero-posterior  diameter  may 
43 


674 


TEXT-BOOK  OF  PHYSIOLOGY. 


range  from  a  fraction  of  a  millimeter  up  to  10  mm.  With  an  increase 
of  0.16  mm.  the  far  point  is  but  200  cm.  distant;  and  with  an  increase 
of  3.2  mm.  it  is  but  10  cm.  distant.  Inasmuch  as  only  divergent 
rays  can  be  focalized  by  the  myopic  eye  normal  vision  can  be  restored 
by  the  use  of  a  biconcave  lens  with  a  diverging  power  in  the  first 
instance  of  0.5  dioptry  and  the  second  of  10  dioptries.     (Fig.  324.) 

Hypermetropia. — This  is  a  condition  of  the  eye  characterized  by 
decrease  of  the  normal  antero-posterior  diameter  or  by  a  subnormal 
refracting  power  of  the  lens.     The  former  is  the  usual  condition. 


Fig.  323. — Myopia.  Parallel  rays 
focus  at  F,  cross  and  form  diffusion- 
circles;  divergent  rays  from  A  focus 
on  the  retina. — (Hansell  and  Sweet.) 


Fig.  324. — Correction  of  Myopia 
by  a  Concave  Lens. — (Hansell  and 
Sweet.) 


Parallel  rays  of  light  do  not,  therefore,  come  to  a  focus  when  the 
accommodation  is  suspended.  Falling  on  the  retina  previous  to 
focalization,  they  give  rise  to  diffusion-circles  and  indistinctness  of  the 
image.  As  no  object  can  be  seen  distinctly  no  matter  how  remote, 
there  is  no  positive  far  point.  The  near  point  is  abnormally  distant— 
sometimes  as  far  as  200  cm.  For  this  reason  the  condition  is  termed 
far  sight.  A  hypermetropic  eye  without  accommodative  effort  can 
focalize  only  converging  rays  on  the  retina.  If  rays  of  light  were  to 
come  from  the  retina  of  such  an  eve,  they  would,  on  emerging,  take 


■/...:.":.-"-"-■--■»-  k 


I  [G.  325. — The  Hypermetropic  Fye.  Parallel  rays  (A,  B)  can  be  focused  only  at  a 
point  behind  the  eye,  as  at  /;  rays  of  light  coming  from  the  retina  take,  on  emerging  from 
the  eye,  a  divergent  direction,  C,  D.     K.  The  negative  punctum  remotum. 

a  divergent  direction,  as  shown  in  Fig.  325,  dotted  line  C  and  D. 
If  these  same  rays  were  to  be  prolonged  backward,  they  would  meet 
at  the  point  K,  which  is  the  punctum  remotum;  and  as  it  is  behind 
tin-  eye,  it  is  termed  negative.  Since  rays  coming  from  the  retina 
take  a  divergent  direction  on  emerging  from  the  eye,  it  is  evident 
that  only  converging  rays  can  be  focalized  by  a  passive  hyperme- 
tropic  eye.  As  there  are  no  convergent  rays  in  nature,  it  is  necessary 
for  distinct  vision  that  all  rays,  parallel  and  divergent,  shall  be  given 


THE  SENSE  OF  SIGHT. 


675 


a  convergent  direction  before  entering  the  eye.  This  is  done  by 
placing  before  the  eye  convex. lenses  the  converging  power  of  which 
is  proportional  to  the  degree  of  hypermetropia  (Figs.  326,  327). 

Astigmatism. — This  is  a  condition  of  the  eye  characterized  by 
an  inequality  of  curvature  of  its  refracting  surfaces  in  consequence  of 
which  not  all  of  a  homocentric  bundle  of  rays  are  brought  to  the 
same  focus.  The  inequality  may  be  either  in  the  cornea  or  lens,  or 
both,  though  usually  in  the  cornea. 


Fig.  326. — Hypermetropia.  Par- 
allel Rays  Focused  behind  the 
Retixa. — {Hansel!  and  Sweet.) 


Fig.  327. — Correction  of  Hyper- 
metropia by  a  Convex  Lens. 
— (Hanscll  and  Sweet.) 


In  the  normal  cornea  the  radius  of  curvature  in  the  vertical  meridian 
is  a  trifle  shorter,  7.6  mm.,  than  that  of  the  horizontal,  7.8  mm.,  and 
hence  its  focal  distance  is  slightly  shorter.  The  difference,  however, 
in  the  focal  distances  is  so  slight  that  the  error  in  the  formation  of  the 
image  is  scarcely  noticeable.  A  transection  of  a  cone  of  light  coming 
from  the  cornea  is  practically  a  circle.  If,  however,  the  vertical  cur- 
vature exceeds  the  normal  to  any  marked  extent,  the  rays  passing 
through  this  meridian  will  be  more  sharply  refracted  and  brought 
to  a  focus  much  sooner  than  the  rays  passing  through  the  horizontal 


Fig.  328. — Refraction  by  an  Astigmatic  Surface. — (Hanscll  and  Sweet.) 

meridian.  The  result  will  be  that  the  cone  of  light  will  be  no  longer 
circular,  but  more  or  less  elliptic.  The  variations  of  the  shape  of 
this  cone  are  shown  in  Fig.  328,  which  represents  the  appearances 
presented  on  cross-section  both  before  and  after  focalization  of  each 
set  of  rays.  Though  the  vertical  meridian  has  usually  the  sharper 
curvature,  it  not  infrequently  happens  that  the  reverse  is  true.  For 
the  reason  that  the  rays  from  one  point  do  not  all  come  to  the  same 
focus  or  point,  the  condition  is  termed  astigmatism. 


676  TEXT-BOOK  OF  PHYSIOLOGY. 

Spheric  Aberration. — When  the  rays  of  light  which  emanate 
from  a  point  fall  upon  a  spheric  lens,  they  do  not  after  passing  through 
it  reunite  at  one  point  because  of  the  fact  that  the  more  peripheral 
rays  have  a  shorter  focus  than  the  central  rays.  To  this  condition 
the  term  spheric  aberration  is  given.  Spheric  aberration  can  be  dem- 
onstrated in  the  human  eye.  That  this  condition  is  present  to  but 
a  slight  extent  in  the  normal  eye  is  due  to  the  presence  of  the  iris, 
which  intercepts  those  rays  which  would  otherwise  pass  through  the 
marginal  portions  of  the  refracting  media.  In  widely  dilated  eyes 
the  spheric  aberration  of  the  peripheral  parts  may  amount  to  as  much 
as  4.5  dioptries. 

Chromatic  Aberration. — When  a  beam  of  white  light  is  made 
to  pass  through  a  prism,  it  is  decomposed  into  the  primary  colors 
owing  to  a  difference  in  the  refrangibility  of  the  rays.  In  passing 
through  the  refracting  media  of  the  eye  the  different  rays  composing 
white  light  also  undergo  unequal  refraction  and  those  rays  which 
give  rise  to  one  color  are  brought  to  a  focus  at  a  point  somewhat 
different  from  those  which  give  rise  to  other  colors.  If  the  eye  is 
accommodated  for  one  set  of  rays,  it  is  not  for  another,  and  the  result 
is  a  fringe  of  colors  around  the  image.  This  defect  in  the  normal 
eve  is  so  slight  that  the  mind  fails  to  take  cognizance  of  it.  That  the 
eye  is  incapable  of  simultaneously  focalizing  rays  of  widely  different 
refrangibility,  as  those  which  give  rise  to  .the  blue  and  red  colors, 
is  shown  by  the  following  experiment:  The  eye  being  directed  to  a 
luminous  point,  a  plate  of  cobalt-glass  is  placed  between  the  light  and 
the  observer  close  to  the  eye.  This  substance  has  the  property  of 
intercepting  all  rays  but  the  red  and  the  blue  and  hence  these  alone 
will  be  seen.  The  center  of  the  image  produced  will  be  red  and  clearly 
defined,  the  periphery  blue  and  ill  defined.  The  reason  for  this  is 
clear.  The  eye  more  readily  accommodates  itself  for  the  red  rays, 
and  hence  their  focal  point  is  distinct.  The  blue  rays,  having  a 
higher  degree  of  refrangibility,  come  to  a  focus,  cross  and  diverge, 
and  give  rise  to  diffusion-circles.  If  a  biconcave  glass  be  placed  before 
the  cobalt,  the  blue  rays  can  be  focalized  on  the  retina,  while  the  red 
will  fall  on  the  retina  without  focalization.  The  image  will  now  be 
blue  and  distinct  in  the  center,  the  periphery  red  and  ill  defined. 
Wil  h  the  removal  of  the  minus  glass  the  reverse  condition  again  obtains. 

Imperfect  Centering. — From  a  purely  physical  point  of  view,  the 
eye  is  not  a  perfect  optic  instrument.  In  addition  to  the  defects 
noticed  in  the  foregoing  paragraphs,  there  is  yet  another,  viz.:  an 
imperfect  centering  of  the  refracting  surfaces.  In  first-class  optic 
instruments  the  lenses  are  centered — that  is,  their  exact  centers  are 
.situated  on  the  same  axis.  In  viewing  an  object  through  such  a 
system  the  visual  line  corresponds  with  the  axis  of  the  lens  system. 
This  is  not  the  case  with  the  refracting  system  of  the  eye.  A  line 
pas-ring  through  the  center  of  the  cornea  and  the  center  of  the  eye, 
Hi-  optic  axis  (O  A   in  Fig.  329),  does  not  pass  exactly  through  the 


THE  SENSE  OF  SIGHT. 


677 


2. 


center  of  the  lens  and  does  not  fall  into  the  point  for  most  distinct 
vision,  the  fovea.  This  has  led  to  the  recognition  of  other  lines  the 
relations  of  which  must  be  kept  in  mind  in  all  optic  discussions,  viz. : 
1.  The  visual  axis  or  visual  line  (V  L),  the  line  connecting  the  point 
viewed,  the  nodal  point  and  the  fovea  centralis. 
The  line  0}  fixation  or  line  of  regard  (V  C),  the  line  connecting  the 
point  viewed  with  the  center  of  rotation,  the  latter  being  situated 
6  mm.  behind  the  nodal  point  of  the  eye  and  9  mm.  before  the 
retina.  The  relation  of  these  lines  and  certain  angles  connected 
with  them  are  shown  in  Fig.  329.  The  angle  included  between 
the  line  D  D  (the  major  axis  of  the  corneal  ellipse)  and  the  visual 
line  is  the  angle  alpha,  amounting  on  the  average  to  50.  The 
angle  included  between  the  optic  axis  and  the  line  of  fixation  or 


temporal-  Sic£e 


Fig.   329. — Diagram  showing  the  Corneal  Axis  D  D,   the   Optic  Axis   O  A, 
the  Visual  Axis  V  L,  and  the  Line  of  Fixation  V  C;  also  the  Three  Adgels,  a,  /i,  y, 


regard  is  the  angle  gamma,  while  the  angle  between  the  optic 
axis  and  the  line  of  vision  is  the  angle  beta.     In  emmetropia  the 
angle  alpha  is  about  50.     In  hypermetropia  it  is  greater,  amount- 
ing to  70  or  8°,  giving  to  the  eye  an  appearance  of  divergence. 
In  myopia  it  is  much  smaller — 2° — or  in  extreme  cases  may  be 
abolished,  the  line  of  vision  corresponding  with  the  optic  axis 
or  even  passing  beyond  it.     The  angle  gamma  is  of  value  in  de 
termining  the  actual  deviation  of  the  eye  in  squint. 
Functions  of  the  Retina.  —Of  all  the  layers  of  the  retina,  the 
rods  and  cones  appear  to  be  the  most  essential  to  vision.     It  is  only 
this  layer  that  is  capable  of  receiving  the  light  stimulus  and  of  trans- 
forming it  into  some  specific  form  of  energy,  which  in  turn  arouses 
in  the  fibers  of  the  optic  nerve  the  characteristic  nerve  impulses. 
A  ray  of  light  entering  the  eye  passes  entirely  through  the  various 
layers  of  the  retina,  and  is  arrested  only  upon  reaching  the  pigmentary 
epithelium  in  which  the  rods  and  cones  are  embedded.     As  to  the 
manner  in  which  the  objective  stimuli — light  and  color,  so  called — 


67S  TEXT-BOOK  OF  PHYSIOLOGY. 

are  transformed  into  nerve  impulses,  but  little  is  known.  It  is  prob- 
able that  the  ether  vibrations  are  transformed  into  heat,  which  excites 
the  rods  and  cones.  These,  acting  as  highly  specialized  end-organs 
of  the  optic  nerve,  start  the  impulses  on  their  way  to  the  brain,  where 
the  seeing  process  takes  place.  As  to  the  relative  function  of  the 
rods  and  cones,  it  has  been  suggested,  from  the  study  of  the  facts 
of  comparative  anatomy,  that  the  rods  are  impressed  only  by  differ- 
ences in  the  intensity  of  light,  while  the  cones,  in  addition,  are  im- 
pressed by  qualitative  differences  in  color.  The  nerve-fibers  them- 
selves are  insensible  to  the  impact  of  the  ether  vibrations,  and  require 
for  their  excitation  some  intermediate  form  of  energy.  That  this  is 
the  case  was  shown  by  Donders,  who  reflected  a  beam  of  light  on 
the  optic  nerve  at  its  entrance  without  the  individual  experiencing  any 
sensation  of  light.  This  region,  occupied  only  by  the  optic-nerve 
fibers  and  devoid  of  any  special  retinal  elements,  is  therefore  an 
insensitive  or  blind  spot.  The  diameter  of  this  spot  is  about  1.5  mm., 
and  occupies  in  the  field  of  vision  a  space  of  about  6°.  It  is  situated 
about  3.5  mm.  to  the  nasal  side  of  the  visual  axis.     Its  existence  can 


Fig.  330. — Diagram  for  Observing  the  Situation  of  the  Blind  Spot. — 

(Helmholtz.) 

be  demonstrated  by  the  familiar  experiment  of  Mariotte,  which  con- 
sists in  placing  before  the  eye  two  objects  having  the  relation  to  each 
other  as  in  Fig.  330.  With  the  left  eye  closed  and  the  right  eye  directed 
to  the  cross,  both  objects  may  be  visible.  But  by  moving  the  figure 
away  from  or  toward  the  eye,  there  will  be  found  a  distance,  about 
30  cm.,  when  the  circle  will  be  invisible.  This  occurs  when  the  image 
falls  on  the  optic  nerve  at  its  entrance.  The  experiment  of  Purkinje 
as  described  in  the  following  paragraph  demonstrates  also  the  fact 
that  the  sensitive  portion  of  the  retina  is  to  be  found  only  in  the  layer 
of  rods  and  cones. 

It  is  well  known  that  the  blood-vessels  of  the  retina  are  situated 
in  its  innermost  layers  a  short  distance  behind  the  optic-nerve  fibers. 
Owing  to  this  anatomic  arrangement,  a  portion  of  the  light  coming 
through  the  pupil  will  be  intercepted  by  the  vessels  and  a  shadow 
projected  on  the  layer  of  rods  and  cones.  Ordinarily,  these  shadows 
are  not  perceived,  for  the  reason  that  the  shaded  parts  arc  more  sen- 
sitive, so  that  the  small  amount  of  light  passing  through  the  vessels 
produces  as  strong  an  impression  on  this  part  as  does  the  full  amount 
of  light  on  the  unshaded  parts  of  the  retina,  and  perhaps  because  the 
mind  has  learned  to  disregard  them.     But  if  light  be  made  to  enter 


THE  SENSE  OF  SIGHT.  679 

the  eye  obliquely,  the  position  of  the  shadows  will  be  changed,  when  at 
once  they  become  apparent.  This  can  be  shown  in  the  following  way: 
If  in  a  darkened  room  a  lighted  candle  be  held  several  inches  to  the 
side  and  to  the  front  of  the  eye,  and  then  moved  up  and  down,  there 
will  be  perceived,  apparently  in  the  field  of  vision,  an  arborescent 
figure  corresponding  to  the  retinal  blood-vessels.  This  is  due  to  the 
falling  of  the  shadows  on  unusual  portions  of  the  layer  of  rods  and 
cones. 

Excitability  of  the  Retina. — The  retina  is  not  equally  excitable 
in  all  parts  of  its  extent.  The  maximum  degree  of  sensibility  is  found 
in  the  macula  lutea,  and  especially  in  its  central  portion,  the  fovea. 
In  this  region  the  layers  of  the  retina  almost  entirely  disappear,  the 
layer  of  rods  and  cones  alone  remaining,  and  in  the  fovea  only  the 
latter  are  present.  That  this  area  is  the  point  of  most  distinct  vision 
is  shown  by  the  observation  that  when  the  eye  is  directed  to  any  given 
point  of  light,  its  image  always  falls  in  the  fovea.  Any  pathologic 
change  in  the  fovea  is  attended  by  marked  indistinctness  of  vision. 
The  sensibility  of  the  retina  gradually  but  irregularly  diminishes  from 
the  macula  toward  the  periphery.  This  diminution  in  sensibility 
holds  true  for  monochromatic  as  well  as  white  light. 

As  stated  above,  the  nature  of  the  molecular  processes  which  take 
place  in  the  retinal  tissue,  caused  on  one  hand  by  the  light  vibrations, 
and  on  the  other  hand  developing  nerve  impulses,  is  entirely  un- 
known. The  discovery  of  the  visual  purple  in  the  outer  segment  of 
the  rods  gave  promise  of  some  explanation  of  the  process,  especially 
when  it  was  shown  to  undergo  changes  when  exposed  to  the  action 
of  light.  But  as  the  pigment  is  wanting  in  the  cones,  and  especially 
in  the  fovea,  it  cannot  be  considered  essential  to  distinct  vision,  al- 
though that  it  plays  some  important  role  in  the  visual  process  is  highly 
probable.  It  was  observed  by  Van  Genderen  Stort,  that  when  an 
animal  is  kept  in  darkness  some  time  before  death,  the  cones  are  long 
and  filiform;  but  if  the  animal  has  been  exposed  to  light,  they  are 
short  and  swollen.  It  was  discovered  by  Boll  that  if  an  animal 
is  kept  in  darkness  an  hour  or  two  before  death  the  pigment  is 
massed  at  the  ends  of  the  rods  and  cones,  but  after  exposure  to  light 
it  becomes  displaced  and  extends  over  and  between  the  rods  almost 
to  the  external  limiting  membrane.  These  conditions  are  represented 
in  Fig.  331. 

The  Eye  a  Living  Camera.— In  its  construction,  in  the  arrange- 
ment of  its  various  parts,  and  in  their  mode  of  action  the  eye  may  be 
compared  to  a  camera  obscura.  Though  the  comparison  may  not  be 
absolutely  exact,  yet  in  a  general  way  it  is  true  that  there  are  many 
striking  points  of  similarity  between  them;  e.  g.,  the  sclera  and  chorioid 
may  be  compared  to  the  walls  of  the  camera;  the  combined  refracting 
media  to  the  single  lens,  the  action  of  which  results  in  the  focusing 
of  the  light  rays;  the  retina  to  the  sensitive  plate  receiving  the  image' 
formed  at  the  focal  point;  the  iris  to  the  diaphragm  for  the  regulation 


68o 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  the  amount  of  light  to  be  admitted,  and  for  the  partial  exclusion 
of  those  marginal  rays  which  give  rise  to  spheric  aberration;  the  ciliarv 
muscle  to  the  adjusting  screw,  by  means  of  which  the  image  is  brought 
to  a  focus  on  the  sensitive  plate,  notwithstanding  the  varying  distances 
of  the  object  from  the  lens.  The  presence  of  the  visual  purple  in  the 
rods  of  the  retina  capable  of  being  altered  by  light  makes  the  com- 
parison still  more  striking. 

Kiihne  even  succeeded  in  obtaining  a  fixed  image  or  an  optogram 
of  an  external  object  in  a  manner  similar  to  that  by  which  an  image 
is  fixed  on  the  sensitive  plate  of  a  camera.  An  animal  is  kept  in 
the  dark  for  about  ten  minutes  in  order  to  permit  the  retinal  pigment 
to  be  completely  regenerated.     The  animal,  with  the  eyes  covered, 


Fig.   331. — Section  of  the  Retina  of  a  Frog.     A.  In  darkness.     B.  In  light. — 
(After  Van  Genderen  Stort,  from  Tscherning's  "Physiologic  Optics.") 


is  then  brought  into  a  room  with  a  single  window.  While  the  head 
is  steadily  directed  to  the  window,  the  eye  is  exposed  for  several 
minutes.  The  eyes  are  again  covered,  the  animal  killed,  and  the 
eyes  removed  by  the  light  of  a  sodium  flame.  The  retina  is  then 
placed  in  a  4  per  cent,  solution  of  alum.  In  a  short  time  the  image 
of  the  window,  the  optogram,  will  be  fixed  (Fig.  332).  That  portion 
of  the  retina  corresponding  to  the  image  is  quite  bleached  in  appear- 
ance from  the  action  of  the  light  on  the  pigment.  During  life  the 
regeneration  of  the  visual  purple  must  take  place  with  extreme  rapidity. 
It  is  believed  to  be  derived  from  a  pigment  secreted  by  the  layer  of 
pigment  cells. 

Binocular  Vision.  -Though  two  images  are  formed,  one  on  each 
retina,  when  the  eyes  are  directed  to  a  given  object,  there  results  but 
one  sensation.     If  the  direction  of  either  visual  axis  be  changed  by 


THE  SENSE  OF  SIGHT. 


68 1 


Fig.  332. — Retina  of 
a  Rabbit.  Optogram 
of  a  Window  Four 
Meters  Distant  a. 
Yellow  spot.  b,  b. 
White  streak  of  nerve - 
fibers. — (Kiihne.) 


pressure  on  the  eyeball,  there  arise  two  sensations,  and  the  object 
appears  to  be  doubled.  The  reason  assigned  for  this,  in  the  first 
instance,  is  that  the  two  images  fall  into  the  foveae,  two  corresponding 
points;  wrhile  in  the  second  instance  they  fall  on  non-corresponding 
points.  It  would  appear,  therefore,  that  for  the  purpose  of  seeing 
an  object  singly  when  the  eyes  are  directed  toward  it,  the  rays  eman- 
ating from  it  must  fall  on  corresponding  parts  of  the  retina.  As  all 
portions  of  the  retina  are  sensitive  to  light,  though  in  varying  degrees, 
it  is  not  essential  that  the  images  always  fall  in 
the  fovese.  The  parts  of  the  retinae  which  cor- 
respond physiologicly  are  shown  in  Fig.  333. 
In  this  figure  the  retinal  area  is  divided  into 
quadrants  by  vertical  and  horizontal  lines  of 
separation,  as  they  are  termed.  If  one  retina  is 
placed  in  front  of  or  over  the  other,  it  will  be 
found  that  the  quadrants  bearing  similar  letters 
cover  each  other.  So  long  as  the  rays  of  light, 
entering  lhe  eye,  fall  on  corresponding  areas  the 
sensation  of  but  one  object  arises.  If,  however, 
they  fall  on  non-corresponding  areas,  two  sensa- 
tions  arise.     Normal   binocular  vision  enlarges 

very  considerably  the  area  of  the  visual  field,  permits  of  a  better  estima- 
tion of  the  size  and  distance  of  objects,  enables  the  mind  to  form  more 
readily  a  perception  of  depth,  increases  the  intensity  of  sensations  and 
makes  sensation  more  uniform  by  off-setting  retinal  rivalry. 

The  Horopter. — When  the  eyes  are  in  the  so-called  secondary 
position — that  is,  in  a  position  in  which  the  visual  axes  are  con- 
verged and  directed  to  a  point  in  front  of  and  in  the  middle  plane  of 

the  body — it  will  be  found  on 
examination  that  rays  of  light 
from  a  number  of  other  objects 
enter  the  eye,  pass  through  the 
nodal  point,  and  fall  on  corres- 
ponding parts  of  the  two  retinas 
and  give  rise  to  but  single  im- 
ages. All  such  points  lie,  for  the 
horizontal  line  of  separation,  on 
a  line  termed  the  horopter.  The  form  of  this  line  is  that  of  a  circle 
which  passes  through  the  fixation  point  and  the  two  nodal  points. 
Any  object  on  the  horopter  will  give  rise  to  but  a  single  image.  This 
is  shown  in  Fig.  334,  in  which  the  objects  I,  II,  III  project  their  rays 
into  both  eyes  which  fall  on  corresponding  areas. 

In  addition  to  the  horopter  for  the  horizontal  line  of  separation, 

there  is  also  an  horopter  for  the  vertical  line  of  separation.     At  a 

distance  of  two  meters  the  vertical  horopter  is  a  plane.     Within  this 

distance  it  is  concave  to  the  face;  beyond  this  distance  it  is  convex. 

An  object  which  lies  either  in  front  of  or  behind  the  fixation  point 


Fig.  ->>i2). — Corresponding  Areas  of  the 

Retina. 


682  TEXT-BOOK  OF  PHYSIOLOGY. 

will  project  its  rays  on  parts  of  the  retinae  which  do  not  correspond, 
and  hence  give  rise  to  double  images.  This  is  evident  from  examina- 
tion of  Fig.  335.  While  the  eyes  are  directed  to  figure  2,  of  which 
there  is  but  a  single  image,  the  objects  B  and  A  give  rise  to  double 
images,  for  reasons  already  given.  If  the  eyes  are  now  directed  to 
B,  double  images  will  be  formed  of  2  and  A. 

At  all  times,  therefore,  double  images  are  formed  on  the  retinae 
the  existence  of  which  is  scarcely  noticed  unless  the  attention  is  directed 
to  them.  This  is  due  to  the  fact  that  many  of  the  images  fall  on  the 
peripheral,  less  sensitive  parts  of  the  retinae.  At  the  same  time,  from 
a  want  of  accommodation  and  the  formation  of  diffusion-circles,  they 
are  indistinct.     For  these  reasons  they  are  readily  neglected. 


B 


Fig.  334. — Horopter  for  the 
Secondary  Position,  with  Con- 
vergence of  the  Visual  Axes. 
— (Landois.) 


Fig.  335.- — Scheme  of  Identical  and 
Non-identical  Points  of  the  Retina. — 
(Landois.) 


In  the  primary  position  of  the  eyes — that  is,  a  position  in  which 
the  visual  axes  are  parallel — the  horopter  is  a  plane  in  infinity.  In 
the  tertiary  position  the  horopter  is  a  curve  of  complex  form. 

Movements  of  the  Eyeball. — The  almost  spheric  eyeball  lies 
in  the  correspondingly  shaped  cavity  of  the  orbit,  like  a  ball  placed 
in  a  socket,  and  is  capable  of  being  moved  to  a  considerable  extent 
by  the  six  muscles  which  are  attached  to  it.  These  muscles  are 
the  superior  and  inferior  recti,  the  external  and  internal  recti,  and 
the  superior  and  inferior  obliqui  (Fig.  336).  The  four  recti  muscles 
arise  from  the  apex  of  the  orbit  cavity,  from  which  point  they  pass 
forward  to  be  inserted  into  the  sclera  about  7  to  8  mm.  from  the 
corneal  border.     The  superior  oblique  muscle  having  a  similar  origin 


THE  SENSE  OF  SIGHT. 


passes  forward  to  the  upper  and  inner  angle  of  the  orbit  cavity,  at 
which  point  its  tendon  passes  through  a  cartilaginous  pulley,  after 
which  it  is  reflected  backward  to  be  inserted  into  the  superior  sur- 
face of  the  sclera  about  16  mm.  behind  the  corneal  border.  The 
inferior  oblique  muscle  arises  from  the  inner  and  inferior  angle  of  the 
orbit  cavity.  It  then  passes  outward,  upward,  and  backward,  to  be 
inserted  into  the  upper,  posterior  and  temporal  portion  of  the  sclera 
about  4  or  5  mm.  from  the  optic  nerve  entrance. 

The  movements  of  each  eye  are  referred  to  three  fixed  lines  or 
axes,  which  have  their  origin  at  the  point  of  rotation  of  the  eyeball, 
this  point  lying  about  1.7  mm.  behind  the  center  of  the  globe.  If  the 
eye  looks  straight  forward  in  the  horizontal  plane  (the  head  being 
erect),  the  line  joining  the  center  of  rotation  with  the  object  looked  at 
is  the  line  of  fixation  or  .    7 

line  of  regard.      Around  i^t: 

this  antero-posterior  axis 
the  eye  may  be  regarded 
as  performing  its  circular 
rotation  or  torsion.  At 
right  angles  to  this  line, 
and  joining  the  center  of 
rotation  of  both  eyes,  is 
the  horizontal  or  trans- 
verse axis,  around  which 
the  movements  of  eleva- 
tion (up  to  34  degrees) 
and  depression  (down  to 
57  degrees)  take  place. 
At  right  angles  to  both 
of  these  lines  there  is  the 
vertical  axis,  around 
which  the  movements  of 
adduction  (toward  the 
nose  up  to  45  degrees) 
and  abduction  (toward  the  temple  up  to  42  degrees)  occur.  The 
six  muscles  may  be  divided  into  three  pairs,  each  of  which  has  a 
common  axis  around  which  it  tends  to  move  the  eyeball.  But 
only  the  common  axis  of  the  internal  and  external  recti  coincides 
with  one  of  three  axes  before  mentioned — namely,  with  the  vertical 
axis — thus  moving  the  ball  only  inwardly  or  outwardly — respec- 
tively. The  other  two  pairs,  however,  have  their  own  axes  of 
action,  and  their  movements  of  the  ball  must  be,  therefore,  analyzed 
with  regard  to  all  the  three  axes,  each  of  these  four  muscles  producing 
rotation,  elevation,  and  depression,  and  abduction  or  adduction.  The 
superior  and  inferior  recti  muscles,  forming  one  pair,  move  the  eye 
around  a  horizontal  axis  which  intersects  the  median  plane  of  the  body 
in  front  of  the  eyes  at  an  angle  of  63  degrees,  and  the  superior  and 


Fig.  336. — Muscles  or  the  Eye.  Tendon  or 
Ligament  of  Zinn.  i.  Tendon  of  Zinn.  2.  Ex- 
ternal rectus  divided.  3.  Internal  rectus.  4. 
Inferior  rectus.  5.  Superior  rectus.  6.  Superior 
oblique.  7.  Pulley  for  superior  oblique.  8.  In- 
ferior oblique.  9.  Levator  palpebrte  superioris. 
10,  10.  Its  anterior  expansion,  n.  Optic  ner,-e. — 
(Sappey.) 


684 


TEXT-BOOK  OF  PHYSIOLOGY. 


inferior  oblique  muscles  forming  the  third  pair  rotate  the  globe  around 
a  horizontal  axis  which  cuts  the  median  plane  of  the  body  behind  the 
eyes  at  an  angle  of  39  degrees.  Thus  it  is  that  each  muscle  moves 
the  eye  as  follows,  the  movement  for  practical  purposes  being  referred 
to  the  cornea:  The  rectus  externus  draws  the  cornea  simply  to  the 
temporal  side,  the  rectus  internus  simply  to  the  nose;  the  superior 
rectus  displaces  the  cornea  upward,  slightly  inward,  and  turns  the 
upper  part  toward  the  nose  (medial  torsion) ;  the  inferior  rectus  moves 
the  cornea  downward,  slightly  inward,  and  twists  the  upper  part  away 
from  the  nose  (lateral  torsion) ;  the  superior  oblique  displaces  the  cornea 
downward,  slightly  outward,  and  produces  medial  torsion;  while 
the  inferior  oblique  moves  the  cornea  upward,  slightly  outward,  and 
produces  lateral  torsion.  These  facts  show  that  for  certain  move- 
ments of  the  eye  at  least  three  muscles  are  necessary  (see  following 
table) : 


Inward, Rectus  internus. 

Outward, Rectus  externus. 

jj  .  ,  /  Rectus  superior. 

*  '  \  Obliquus  inferior. 

J  Rectus  inferior. 
\  Obliquus  superior. 
f  Rectus  internus. 

upward, \  Rectus  superior. 

(  Obliquus  inferior. 


Downward, . 
Inward  and 


Inward  and  [  Rectus  internus. 

downward, \  Rectus  inferior. 

[  Obliquus  superior. 
Outward  and  |  Rectus  externus. 

upward,   \  Rectus  superior. 

[  Obliquus  inferior. 
Outward  and  \  Rectus  externus. 

downward,  .  .  .  .  \  Rectus  inferior. 

[  Obliquus  superior. 


If  both  eyes  have  their  line  of  vision  in  the  horizontal  plane  parallel 
with  each  other  and  with  the  median  plane  of  the  body,  they  are 
said  to  be  in  the  primary  position.  All  other  positions  are  called 
^secondary.  Both  eyes  always  move  simultaneously,  which  is  called 
the  associated  movement  of  the  eyes.  There  are  three  forms  of  asso- 
ciated movements:  (1)  movement  of  both  eyes  in  the  same  direction; 
(2)  movements  of  convergence  by  which  the  visual  lines  are  con- 
verged on  a  point  in  the  middle  line  of  the  body;  (3)  movements  of 
divergence,  by  which  the  eyes  are  brought  back  from  convergence  to 
parallelism,  or  even  to  divergence,  as  in  certain  stereoscopic  exercises. 
A  combination  of  (1)  and  (2)  or  of  (1)  and  (3)  takes  place  for  certain 
positions  of  the  object  looked  at. 

Color-perception. — A  beam  of  sunlight  passed  through  a  glass 
prism  is  decomposed  into  a  series  of  colors — red,  orange,  yellow, 
green,  blue,  and  violet — the  so-called  spectral  colors,  so  well  exem- 
plified in  the  rainbow.  The  spectral  colors  are  termed  simple  colors, 
because  they  can  not  be  any  further  decomposed  by  a  prism.  Ob 
jectively,  the  spectral  colors  consist  of  very  rapid  transverse  vibrations 
of  the  ether,  from  about  400  millions  of  millions  per  second  for  red 
to  about  760  millions  of  millions  for  violet,  but  subjectively  they  are 
sensations  caused  by  the  impact  of  the  ether-waves  on  the  percipient 
layer  of  the  retina. 

It  is  possible  to  mix  or  blend  these  spectral  color-sensations  in 
the  eye  by  stimulating  the  same  area  of  the  retina  by  different  spectral 


THE  SENSE  OF  SIGHT. 


685 


colors,  cither  at  the  same  time  or  in  rapid  succession.  The  following 
table  shows  the  results  of  such  experiments  as  performed  by  v.  Helm- 
holtz  (Dk.   =  dark;  Wh.   =  whitish): 


Red. 

Orange. 

Yellow. 

Gr.  -yellow. 

Green. 

Bluish-green. 

Cyan-Blue. 


Violet. 


Purple. 

Dk.-rose. 

Wh.-rose. 

White. 

White-blue. 

Water-blue. 

Indigo. 


Indigo. 


Dk.-rose. 

Wh.-rose. 

White. 

Wh. -green. 

Water-blue. 

Water-blue. 


Cyan- 
blue. 


Wh.-rose. 
White. 
Wh. -green. 
WTh. -green. 
Bl. -green. 


Bluish- 
green. 


Green. 


Greenish- 
yellow. 


Yel- 
low. 


White.  Wh. -yellow.  Gold-yellow.  Orange. 

Wh. -yellow.  Yellow.  Yellow.  

Wh. -yellow.  Gr.-yellow.  

Green.  


These  are  the  mixed  colors.  But  it  is  to  be  observed  that  only  two 
new  color-sensations  can  be  produced,  white  and  purple,  the  remain- 
ing mixed  colors  already  finding  their  equivalent  in  the  spectrum. 
White  and  purple,  therefore,  are  color-sensations  which  have  no 
objective  equivalent  in  a  simple  number  of  ether- vibrations  like  the 
spectral  colors. 

Two  spectral  colors  which  by  their  mixture  produce  the  sensation 
of  white  are  called  complementary  colors.  Such  are  red  and  green- 
blue,  golden  yellow  and  blue,  green  and  violet.  The  mixture  of  all 
the  spectral  colors  produces  white  again.  This  is  the  result  of 
adding  two  or  more  color-sensations.  Different  results  are  obtained, 
however,  by  adding  color  pigments.  Yellow  and  blue,  for  example, 
produce  in  the  eye  white,  but  on  the  painter's  palette  green.  The 
colors  of  nature  are  usually  mixtures  of  simple  colors,  as  can  be  shown 
by  spectroscopic  analysis  or  by  a  synthesis  of  spectral  colors. 

In  all  color-sensations  wre  must  distinguish  three  primary  qualities : 
(1)  hue;  (2)  purity  or  tint;  (3)  brightness  or  luminosity.  The  first 
quality  gives  the  main  name  to  the  color — e.  g.,  red  or  blue — this  de- 
pending on  the  spectral  color  or  the  mixture  of  two  spectral  colors 
with  which  it  can  be  matched.  The  second  quality,  the  tint,  depends 
on  the  admixture  of  white  with  the  ground  color;  and  the  third  quality, 
brightness,  depends  on  the  objective  intensity  of  the  light  and  the 
subjective  sensitiveness  of  the  retina.  Color-perception  thus  far  refers 
only  to  the  most  sensitive  part  of  the  retina.  At  the  more  peripheral 
parts  of  the  retina  the  colors  are  seen  somewhat  differently,  as  is 
shown  by  the  following  table  giving  the  limits  up  to  which  the  colors 
are  recognized: 

White.  Blue.  Red.  Green. 

Externally oo°  So0  6s°  500 

Internally 6o°  S5°  So0  400 

Superiorly 4s0  4°°  3^°  30° 

Inferiority 700  6o°  450  35° 

Theories   of   Color-perception. — The   theory   of  v.   Helmholtz, 

originated  by  Thomas  Young  (1807),  assumes  in  its  latest  form  the 
existence  in  the  human  retina  of  three  different  kinds  of  end-organs. 


686  TEXT-BOOK  OF  PHYSIOLOGY. 

each  of  which  is  loaded  with  its  own  photo-chemical  substance  capable 
of  being  decomposed  by  a  certain  color,  and  thus  exciting  the  fiber 
of  the  optic  nerve. 

In  the  first  group  these  end-organs  are  loaded  with  a  red-sensitive 
substance,  which  is  affected  mainly  by  the  red  part  of  the  spectrum; 
the  second  group  has  its  end-organs  provided  with  a  green-sensitive 
substance,  which  is  mainly  excited  by  the  green  color;  while  the  third 
group  is  provided  with  a  blue-sensitive  substance,  this  latter  being 
mainly  affected  and  decomposed  by  the  blue-violet  portion  of  the 
spectrum.  All  these  three  different  end-organs  are  present  in  every 
part  of  the  most  sensitive  area  of  the  retina,  and  are  connected  by 
separate  nerve-fibers  with  special  parts  of  the  brain,  in  the  cells  of 
which  each  calls  up  its  separate  sensation  of  red  or  green  or  blue. 

Out  of  these  three  primary  color-sensations  all  other  color-sensa- 
tions arise.  If  a  light  mainly  excites  the  red-  or  green-  or  blue -sensi- 
tive substance  of  a  retinal  area,  we  term  it  red,  green,  or  blue,  re- 
spectively But  if  two  of  these  photo-chemical  substances  are  stimu- 
lated simultaneously,  quite  different  sensations  arise.  Thus  simul- 
taneous stimulation  of  the  red  and  green  substances  gives  rise  to  the 
sensation  of  yellow,  that  of  red  and  blue  to  the  sensation  of  purple, 
and  that  of  blue  and  green  to  the  sensation  of  blue-green.  Simul- 
taneous stimulation  of  all  three  substances  of  a  certain  area  produces 
the  sensation  of  white.  According  to  this  theory,  complementary 
colors  are  all  those  which  together  excite  all  the  three  substances. 
Color-blindness  is  explained  by  this  theory,  on  the  assumption  that 
two  of  the  photo-chemical  substances  have  become  similar  or  equal 
in  composition  to  each  other. 

The  theory  of  Hering,  brought  forward  in  1874,  has  the  under- 
lying assumption  that  the  process  of  restitution  in  a  nerve-element 
is  capable  of  exciting  a  sensation.  This  theory  asserts  that  there  are 
three  visual  substances  in  the  retina — a  white-black,  a  red-green,  and 
a  yellow-blue  visual  substance.  A  destructive  process  in  the  white- 
black  substance,  such  as  is  induced  not  only  by  white  light,  but  also 
by  any  other  simple  or  mixed  color,  produces  the  sensation  of  white, 
while  the  process  of  restitution  or  assimilation  in  this  substance  pro- 
duces the  sensation  of  black.  Similarly,  red  light  produces  clis- 
p,Ssimilation  or  decomposition  in  the  red-green  substance,  and  this, 
again,  the  sensation  of  red.  Green  light,  however,  favors  the  process 
<>\  restitution  or  assimilation  in  the  red-green  substances,  and  thus 
gives  rise  to  the  sensation  of  green.  In  the  same  way  the  sensation 
of  yellow  has  its  cause  in  the  decomposition  of  yellow-blue  substance 
induced  by  yellow  light,  while  the  sensation  of  blue  is  produced  by 
an  assimilative  process  in  the  same  substance.  Simultaneous  processes 
of  disassimilation  and  assimilation  in  the  same  visual  substance  an- 
tagonize each  other,  and  consequently  produce  no  color-sensation 
by  means  of  this  substance,  but  only  the  sensation  of  white,  by  reason 
of  decomposition,  by  both  colors,  in  the  white-black  substance.     Thus, 


THE  SENSE  OF  SIGHT. 


687 


yellow  and  blue,  impinging  on  the  same  retinal  area,  have  no  effect 
on  the  yellow-blue  substance,  because  they  are  antagonistic  in  their 
action  on  this  substance,  but  only  produce  the  sensation  of  white,  as 
both  yellow  and  blue  decompose  the  white-black  material.  Color- 
blindness is  explained  by  the  assumption  of  the  absence  of  either  the 
red-green  or  the  yellow-blue  visual  substance  in  the  retina. 

Accessory  Structures. — The  eyeball  is  protected  anteriorly  by 
the  eyelids  and  their  associated  structures,  the  Meibomian  glands, 
the  lachrymal  glands,  and  tears. 

The  eyelids  consist  of  a  central  framework  of  connective  tissue 
supporting  muscle  tissue  (the  orbicularis  palpebrarum  muscle)  and 
glands,  and  covered  externally  by  skin  and  internally  by  a  modified 
skin,  the  conjunctiva.     The  free  border  of  each  lid  is  strengthened 


Ai    IfHSte 


& 


Fig.  337. — The  Lacrimal  and  Meibomian  Glands,  and  Adjacent 'Organs  of  the 
Eye.  i,  1.  Inner  wall  of  orbit.  2,  2.  Inner  portion  of  orbicularis  palpebrarum.  3,  3. 
Attachment  to  circumference  of  base  of  orbit.  4.  Orifice  for  transmission  of  nasal  artery. 
5.  Muscle  of  Horner  (tensor  tarsi).  6,  6.  Meibomian  glands.  7.  7.  Orbital  portion  of 
lacrimal  gland.  8,9,10.  Palpebral  portion,  n,  11.  Mouths  of  excretory  ducts.  12,13. 
Lacrimal  puncta. — (Sappey.) 


by  a  semilunar  plate  of  dense  fibrous  tissue,  the  tarsus.  The  cuta- 
neous edge  of  the  lid  is  bordered  with  short  stiff  hairs.  At  the  inner 
extremity  each  eyelid  presents  a  small  opening,  the  pu  net  um  lacrimale, 
the  beginning  of  the  lachrymal  duct.  The  two  ducts  after  uniting 
open  into  the  nasal  duct. 

The  Meibomian  glands  are  modified  sebaceous  glands  imbedded 
in  the  posterior  portion  of  the  lids  (Fig.  337).  Their  ducts  open  on 
the  free  border  of  the  lid.  These  glands  secrete  an  oleaginous  ma- 
terial resembling  sebaceous  matter  which  accumulates  along  the 
margin  of  the  lid  and  prevents  the  tears  from  flowing  down  the  cheek. 

The  lachrymal  gland  is  situated  at  the  upper  and  outer  part  of  the 
orbit  cavity.     It  consists  of  a  series  of  compound  tubules  lined  by 


6S8  TEXT-BOOK  OF  PHYSIOLOGY. 

epithelium.  The  secretion  (the  tears)  is  conducted  from  the  gland 
to  the  outer  part  of  the  conjunctiva  by  seven  or  eight  ducts.  The 
lachrymal  secretion  consists  of  water  and  inorganic  salts.  It  is  dis- 
tributed over  the  corneal  surface  during  the  act  of  winking,  thus 
keeping  it  moist  and  free  from  foreign  particles.  It  eventually  passes 
into  the  lachrymal  ducts  and  then  into  the  nose.  The  lachrymal  glands 
receive  secretory  fibers  by  way  of  the  fifth  nerve  and  the  cervical 
sympathetic.  The  secretion  can  be  excited  reflexly  from  stimulation 
of  sensor  nerves  as  well  as  by  emotional  states. 


CHAPTER  XXVIII. 
THE  SENSE  OF  HEARING. 

The  physiologic  mechanism  involved  in  the  sense  of  hearing  in- 
cludes the  ear,  the  auditory  nerve,  its  cortical  connections,  and  nerve- 
cells  in  the  cortex  of  the  temporal  lobe. 

Peripheral  excitation  of  this  mechanism  develops  nerve  impulses 
which,  transmitted  to  the  cortex,  evoke  the  sensation  of  sound  and 
its  varying  qualities— intensity,  pitch,  and  timbre. 

The  specific  physiologic  stimulus;  to  the  terminal  organ,  the  organ 
of  Corti,  is  the  impact  of  atmospheric  undulations  of  varying  energy 
and  rapidity. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EAR. 

The  ear,  the  organ  of  hearing,  is  lodged  within  the  petrous  portion 
of  the  temporal  bone.  It  may,  for  convenience  of  description,  be 
divided  into  three  portions:  viz.,  the  external,  the  middle,  and  the 
internal  portions  (Fig.  338). 

The  external  ear  consists  of  the  pinna  or  auricle  and  the  external 
auditory  canal.  The  pinna  is  composed  of  a  thin  layer  of  cartilage 
which  presents  a  series  of  elevations  and  depressions.  It  is  attached 
by  fibrous  tissue  to  the  outer  edge  of  the  auditory  canal  and  covered 
by  a  layer  of  skin  continuous  with  that  covering  adjacent  structures. 
The  general  shape  of  the  pinna  is  concave.  Its  anterior  surface  pre- 
sents, a  little  below  the  center,  a  deep  depression — the  concha. 

The  external  auditory  canal  extends  from  the  concha  inward  for 
a  distance  of  from  25  to  30  mm.  It  is  directed  at  first  upward,  for- 
ward, inward,  and  then  downward  to  its  termination.  It  is  composed 
partly  of  bone  and  partly  of  cartilage  and  lined  by  a  reflection  of 
the  skin  covering  the  pinna.  At  the  external  portion  of  the  canal 
the  skin  contains  a  number  of  tubular  glands,  the  ceruminous  glands, 
which  resemble  in  their  conformation  the  perspiratory  glands.  They 
secrete  cerumen  or  ear-wax. 

The  middle  ear,  or  tympanum,  is  an  irregularly  shaped  cavity 
hollowed  out  of  the  temporal  bone  and  situated  between  the  external 
auditory  canal  and  the  internal  ear.  It  is  narrow  from  side  to  side, 
though  wider  above  than  below.  It  is  relatively  long  in  its  antero- 
posterior and  vertical  diameters.  The  upper  portion  is  known  as  the 
attic.  The  middle  ear  is  in  communication  posteriorly  with  the 
mastoid  cells,  anteriorly  with  the  pharynx  through  the  Eustachian 
tube. 

The  Eustachian    Tube. — The  passageway  between   the   tympanic 

44  689 


690 


TEXT-BOOK  OF  PHYSIOLOGY 


cavity  and  the  nasopharynx  is  known  from  its  discoverer  as  the 
Eustachian  tube.  It  is  composed  internally  of  bone,  externally  of 
cartilage,  and  is  lined  by  mucous  membrane  covered  with  ciliated 
epithelium.  Near  the  middle  of  its  course  the  tube  is  contracted, 
though  expanded  at  either  extremity  (Fig.  338).  It  measures  about 
40  mm.  in  length.  Its  general  direction  from  the  pharyngeal  orifice 
is  outward,  backward,  and  upward  at  an  a.ngle  of  about  45  degrees. 
The  middle  ear  cavity  is  separated  from  the  external  ear  by  a 
membrane — the  membrana  tympani — and  from  the  internal  ear  by 
an  osseo-membranous  partition  which  forms  a  common  wall  for  both 


Fig.  338. — The  Ear.  i.  Pinna,  or  auricle.  2.  Concha.  3.  External  auditory  canal. 
4.  Membrana  tympani.  5.  Incus.  6.  Malleus.  7.  Manubrium  mallei.  S.  Tensor  tym- 
pani. 9.  Tympanic  cavity,  to.  Eustachian  tube.  11.  Superior  semicircular  canal. 
12.  Posterior  semicircular  canal.  13.  External  semicircular  canal.  14.  Cochlea.  15. 
Internal  auditory  canal.  16.  Facial  nerve.  17.  Large  petrosal  nerve.  18.  Vestibular 
branch  of  auditory  nerve.      19.   Cochlear  branch. — (Sappey.) 


cavities.  The  interior  of  the  cavity  is  crossed  from  side  to  side  by  a 
chain  of  bones  and  lined  by  a  mucous  membrane  continuous  with 
that  lining  the  pharynx. 

The  membrana  tympani  is  a  thin,  translucent,  nearly  circular 
membrane,  measuring  about  10  mm.  in  diameter,  placed  at  the  inner 
termination  of  the  external  auditory  canal.  It  is  inclosed  in  a  ring 
of  bone  which  in  the  fetal  condition  can  be  easily  removed,  but  in 
the  adult  condition  can  not  be  removed,  owing  to  its  consolidation 
with  the  surrounding  bone.  This  membrane  consists  primarily  of 
a  layer  of  fibrous  tissue  which  is  covered  externally  by  a  thin  layer 
oi  skin  continuous  with  thai  lining  the  auditor}'  canal,  and  internally 


THE  SENSE  OF  HEARING. 


691 


by  a  thin  mucous  membrane.  The  tympanic  membrane  is  placed 
obliquely  at  the  bottom  of  the  auditory  canal,  inclining  from  above 
and  behind  downward  and  forward  at  an  angle  of  about  forty-five 
degrees.  The  external  surface  of  this  membrane  presents  a  funnel- 
shaped  depression,  the  sides  of  which  are  slightly  convex. 

The  Ear-bones. — Running  across  the  tympanic  cavity  and  form- 
ing an  irregular  line  of  joined  levers  is  a  chain  of  bones,  which  articu- 
late one  with  another  at  their  extremities.  These  bones  are  known 
as  the  malleus,  incus,  and  stapes.  The  form  and  arrangement  of  these 
bones  are  shown  in  Figs.  339,  340. 

The  malleus,  or  hammer  bone,  consists  of  a  head,  neck,  and  handle, 
of  which  the  latter  is  attached  to  the  inner  surface  of  the  membrana 
tympani.  The  incus  or 
anvil  bone  presents  a  con- 
cave articular  surface  which 
receives  the  head  of  the 
malleus.  The  stapes,  or 
stirrup-bone,  articulates  ex- 
ternally with  the  long  pro- 
cess of  the  incus,  and  in- 
ternally, by  its  oval  base, 
with  the  edges  of  an  oval 
opening,  the  foramen  ovale. 
The  entire  chain  is  partially 
supported  by  a  ligament 
attached  to  the  short  pro- 
cess of  the  incus  and  to  the 
walls  of  the  tympanic  cavity. 

The  Tensor  Tympani 
Muscle.  —  This  is  a  delicate 
muscle,  about  15  mm.  in 
length,  situated  in  a  nar- 
row groove  just  above  the 
Eustachian  tube  (Fig.  341). 
It  arises  from  the  cartila- 


Fig.  339. — Tympanic  Membrane  and  the  Audi- 
tory Ossicles  (Left)  seex  from  within,  i.  c 
from  the  Tympanic  Cavity.  M.  Manubrium 
or  handle  of  the  malleus.  T.  Insertion  of  the 
tensor  tympani.  h.  Head.  IF.  Long  process  of 
the  malleus,  a.  Tncus.  with  the  short  (K)  and  the 
long  (I)  process.  5.  Plate  of  the  stapes.  Ax, 
Ax,  is  the  common  axis  of  rotation  of  the  auditory 
ossicles.  Sz.  The  pinion-wheej  arrangement  be- 
tween the  malleus  and  iro;s. — (Landois.) 


ginous  portion  of  the  Eusta- 
chian tube  and  the  adjacent  portion  of  the  sphenoid  bone.  From 
this  origin  it  passes  nearly  horizontally  backward  to  the  tympanic 
cavity;  just  opposite  to  the  foramen  ovale,  its  tendon  bends  at  a  right 
angle  over  the  processus  cochleariformis  and  then  passes  outward 
across  the  tympanic  cavity  to  be  inserted  into  the  handle  of  the  malleus 
near  the  neck. 

The  stapedius  muscle  emerges  from  the  cavity  of  a  pyramid  of 
bone  which  projects  from  the  posterior  wall  of  the  tympanum.  Its  ten- 
don passes  forward  to  be  inserted  into  the  neck  of  the  stapes  bone 
near  its  point  of    articulation  with  the  incus. 

The   internal   ear,   or  labyrinth,  is   located  within  the  petrous 


6a: 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.    340. — Audi- 
tory   Ossicles,     i 
Head  of  malleus.     2 
Processus  brevis.     3 
Processus       gracilis 
4.  Manubrium.       5 
Long  process  of  in- 
cus.     6.  Articulation 
between    incus    and 
stapes.        7.    Stapes. 
— (Sappey.) 


portion  of  the  temporal  bone.     It  consists  of  an  osseous  and  a  mem- 
branous portion,  the  latter  contained  within  the  former. 

The  osseous  labyrinth  is  subdivided  into  vestibule,  semicircular 
canals,  and  cochlea. 

The  vestibule  is  a  small,  triangular-shaped  cavity 
between  the  semicircular  canals  and  the  cochlea.  It 
is  separated  from  the  cavity  of  the  middle  ear  by  an 
osseous  partition  which  presents  near  its  center  an 
oval  opening,  the  foramen  ovale.  In  the  living  con- 
dition this  opening  is  closed  by  the  base  of  the 
stapes  bone,  which  is  held  in  position  by  an  annular 
ligament.  The  inner  wall  presents  a  number  of 
openings  for  the  passage  of  nerve-fibers  (Fig.  342). 
The  semicircular  canals  are  three  in  number,  a 
superior  vertical,  an  inferior  vertical,  and  a  hori- 
zontal, each  of  which  opens  by  two  orifices  into  the 
cavity  of  the  vestibule,  with  the  exception  of  the  two 
vertical,  which  unite  at  one  extremity  and  then  open 
by  a  single  orifice.  Each  canal  near  its  vestibular 
orifice  is  enlarged  to  almost  twice  the  size  of  the 
rest  of  the  canal,  forming  what  is  known  as  the  ampulla. 

The  cochlea,  the  anterior  portion  of  the  labyrinth,  is  a  gradually 
tapering  canal,  about  35  mm.  in  length,  wound  spirally  two  and  a 
half  times  around  a  central  bony  axis,  the  modiolus.  The  cavity  of 
the  cochlea  is  partially  subdivided  into 
two  cavities  by  a  thin  spiral  plate  of  bone 
which  projects  from  the  inner  wall, 
known  as  the  lamina  osseous  spiralis.  In 
the  natural  condition  this  partition  is 
completed  by  a  connective-tissue  mem- 
brane, so  that  the  two  passages  are  com- 
pletely separated  from  each  other.  The 
upper  passage  or  scala  is  in  free  com- 
munication with  the  vestibule,  and  is 
known  as  the  scala  vestibuli;  the  lower 
passage  or  scala  in  the  dead  condition 
communicates  with  the  tympanum  by 
means  of  a  round  opening,  the  foramen 
rotundum,  and  is  therefore  known  as  the 
scala  tympani.      In  the  living  condition 

this  opening  is  completely  closed  by  a  membrane,  a  second  membrana 
tympani.  Both  the  scalse  vestibuli  and  tympani  communicate  at  the 
apex  of  the  cochlea  by  a  small  opening,  the  helicotrema.  The  modiolus, 
the  central  bony  axis,  is  perforated  from  base  to  apex  by  a  canal  for 
the  passage  of  the  auditory  nerve-fibers;  lateral  canals,  diverging  from 
the  central  canal,  pass  through  the  osseous  lamina  spiralis  and  transmit 
fibers  of  the  auditory  nerve.     The  interior  of  the  bony  labyrinth  is 


Fig.  341. — M,  The  Tensor 
Tympani  Muscle — the  Eus- 
tachian Tube  (Left). — 
(Landois.) 


THE  SENSE  OF  HEARING. 


6Q3 


Fig.  342. — Boxy  Cochlea,  i. 
Ampulla  of  superior  semicircular 
canal.  2.  Horizontal  canal.  3. 
Junction  of  superior  and  posterior 
semicircular  canals.  4.  The  pos- 
terior semicircular  canal.  5.  Fora- 
men rotundum.  6.  Foramen  ovale. 
7.  Cochlea. 


lined  by  periosteum  covered  by  epithelium  and  in  communication 
with  lymph-spaces  at  the  base  of  the  skull  by  means  of  the  aqueduct 
of  the  vestibule. 

The  membranous  labyrinth,  lying  within  the  osseous  labyrinth, 
corresponds  with  it  in  form,  though  it  is  smaller  in  size.  It  mav  be 
subdivided  into  vestibule,  semicircular  canals,  and  cochlea  (Fig.  343) 

The  vestibular  portion  consists  of  two 
small  sacs,  the  utricle  and  the  saccule, 
which  communicate  .with  each  other  by 
means  of  the  two  branches  of  a  duct 
passing  through  the  aqueduct  of  the 
vestibule — the  ductus  endolympkaticus. 

The  semicircular  canals  communicate 
with  the  utricle  in  the  same  manner  as 
the  bony  canals  communicate  with  the 
vestibule.  The  saccule  communicates 
with  the  membranous  cochlea  by  a  short 
canal,  the  canalis  reuniens.  The  walls 
of  the  utricle,  saccule,  and  semicircular 
canals  are  composed  of  connective  tissue 
lined  by  epithelium.  At  the  points  of 
entrance  of  the  auditory  nerve,  ihemaculce 

acusticce,  in  all  three  structures,  the  epithelium  undergoes  a  marked 
change  in  appearance  and  structure.  It  becomes  columnar  in  shape 
and  provided  with  stiff  hair-like  processes  or  threads,  which  projecl 
into  the  cavity.  In  the  saccule  and  utricle  the  hair-like  processes  are 
covered  by  a  layer  of  small  crystals  of  calcium  carbonate  held  together 
by  a  gelatinous  material.  The  crystals  are  known  as  otoliths  (Fig.  344). 
The  fibers  of  the  vestibular  nerve,  arising  from  the  cells  of  the 
ganglion  of  Scarpa  in  the  internal  auditory 
meatus,  send  their  peripherally  directed 
branches  through  the  foramina  in  the  inner 
wall  of  the  vestibule,  through  the  walls  of 
the  utricle  and  semicircular  canals  near  the 
ampulla.  As  the  fibers  approach  the  macula? 
acusticse  they  subdivide  into  delicate  fibrillar, 
which  ultimately  become  histologically  and 
physiologically  related  to  the  neuro-epithe- 
lium.  From  the  relation  of  the  nerve-fibers 
to  the  epithelium,  the  latter  must  be  re- 
garded as  the  highly  specialized  terminal. 
organ  of  the  vestibular  portion  of  the  auditory  nerve. 

The  cochlea  is  a  closed  membranous  tube  situated  between  the 
osseous  lamina  spiralis  and  the  outer  bony  wall.  A  transection  of  the 
entire  cochlea  shows  the  relation  of  the  osseous  and  membranous 
portions  (Fig.  345).  The  cochlear  tube  is  triangular  in  shape.  The 
base  is  attached  to  the  bony  wall,  the  apex  to  the  edge  of  osseous 


Fig.  343. — 1.  Utricle.  2. 
Saccule.  3.  Vestibular  end  of 
cochlea.  4.  Canalis  reuniens. 
5.  Membranous  cochlea.  6. 
Membranous  semicircular 

canal. — (Potters   "Anatomy.") 


694 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  344. —  Section  of  Wall  of 
Utricle  of  the  Internal  Ear, 
through  macular  region,  from 
rabbit,  shewing  otoliths  (0),  em- 
bedded within  granular  substance 
(g).  h.  Ciliated  cells  with  proc- 
esses, (p),  extending  between 
sustentacular  elements  (s).  m. 
Basement  membrane.  11.  Nerve- 
fibers  within  fibrous  tissue  (I) 
passing  toward  hair-cells  and 
becoming  non-medullated  at  base- 
ment-membrane.— (After  Piersol.) 


lamina  spiralis.     One  side  of  the  tube  forms  in  part  the  membrane  of 

Reissner,  the  other  side  forms  in  part  the  basilar  membrane.     The 

sides  of  the  cochlea  toward  the  scala  vestibuli  and  scala  tympani  are 

covered  with  epithelium.     The  triangular  cavity  of  the  cochlear  tube 

is  known  as  the  scala  media.  The  inner 
surface  of  the  cochlear  tube  is  lined  by 
epithelium,  which  becomes  extraordi- 
narily modified  and  specialized  along  the 
surface  of  the  basilar  membrane,  to  con- 
stitute what  is  known  as — 

The  Organ  of  Corti.— In  Fig.  345 
this  organ  is  represented  as  it  appears 
on  cross-section  of  the  cochlea.  It  con- 
sists primarily  of  an  arch  composed  of 
two  modified  epithelial  cells  known  as 
the  rods  or  pillars  of  Corti,  which  rest 
below  on  the  basilar  membrane,  but  meet 
and  interlock  above;  it  consists  second- 
arily of  a  series  of  columnar  epithelial 
cells  provided  with  hair-like  processes 
which  rest  upon  and  are  supported  by 
the  rods  both  on  the  inner  and  outer 
aspects  of  the  arch.  The  space  beneath 
the  arch  is  known  as  the  tunnel.     The 

inner  hair  cells  are  not  nearly  so  numerous  as  the  outer  hair  cells 

The  epithelial  cells  external  to  the  outer  and  inner  hair  cells  are  sup 

porting  or  sustentacular  in  character. 

The  organ  of  Corti  extends  the  entire  length  of  the  cochlea.     The 

number  of  rods  which,  standing 

side  by  side,  form  the  inner  limb 

of  the  arch  is  estimated  at  5600; 

the  number  which  form  the  outer 

limb  is  estimated  at  3850.     The 

outer  rods  are  broader  than  the 

inner  and  at  some  places  articu- 
late with  two  or  three  inner  rods. 

The  upper  edges  of  the  rods  are 

flattened,  elongated,  and  project 

outward,    forming  a  reticulated 

membrane    through  the   meshes 

.of  which  the  hair^ike  processes 

of  the  cells  project. 

From    the    connective-tissue 

thickening  on  the  upper  surface 

of  the  osseous  lamina  spiralis  there  extends  outward  over  the  organ  of 

Corti  a  thin  membrane,  the  membrana  lectoria.     The  common  cavity 

between  the  walls  of  the  osseous  and  membranous  labyrinth  in  the 


Fig.  345. — A  Transverse  Section  of  a 
Turn  of  the  Cochlea. 


THE  SENSE  OF  HEARING.  695 

vestibule,  the  semicircular  canals,  in  the  scala  vestibuli  and  scala 
tympani  of  the  cochlea,  is  filled  with  a  clear  fluid — the  perilymph;  the 
common  cavity  within  the  walls  of  the  entire  membranous  labyrinth 
is  also  filled  with  a  similar  fluid — the  endolymph.  The  hair-like  proc- 
esses of  the  epithelial  cells  covering  the  maculae  acusticae  and  the 
rods  of  Corti  are  consequently  bathed  by  endolymph.  Both  fluids 
are  in  relation  with  the  subarachnoid  lymph-spaces  at  the  base  of  the 
brain,  the  perilymph  through  the  aqueduct  of  the  vestibule,  the  endo- 
lymph through  the  endolymphatic  duct. 

The  fibers  of  the  cochlear  nerve,  arising  from  the  ganglion  cells 
of  the  spiral  ganglion  situated  in  the  osseous  lamina  spiralis  near 
the  modiolus,  send  their  peripheral  branches  to  the  saccule  and  to 
the  organ  of  Corti.  As  they  approach  this  structure  they  lose  their 
medullary  sheath  and  become  naked  axis-cylinders.  The  fibers  then 
divide  into  two  parts,  of  which  one  passes  to  the  inner  hair  cells;  the 
other  passes  between  the  inner  rods  and  crosses  the  tunnel  between 
the  outer  rods  to  the  outer  hair  cells.  The  exact  method  of  termina- 
tion of  these  fibers  in  the  hair  cells  is  unknown,  but  doubtless  it  is 
both  histologic  and  physiologic. 

From  the  relation  of  the  nerve-fibers  to  the  organ  of  Corti  the 
latter  must  be  regarded  as  the  highly  specialized  terminal  organ  of 
the  cochlear  division  of  the  auditory  nerve. 

THE  PHYSIOLOGY  OF  AUDITION. 

The  general  function  of  the  ear  is  the  reception  and  transmission 
of  atmospheric  vibrations  from  the  concha  to  the  percipient  elements 
— the  hair  cells — of  the  organ  of  Corti.  The  vibratory  excitation  of 
these  end-organs  thus  caused,  is  the  basis  of  auditory  perceptions. 
The  atmospheric  vibrations  are  collected  by  the  pinna  and  concha, 
conveyed  by  the  auditory  canal  to  the  tympanic  membrane,  trans- 
mitted by  the  chain  of  bones  to  the  labyrinth  to  pass  successively 
through  the  perilymph,  the  membranous  walls,  the  endolymph,  to 
the  hair  cells.  The  nerve  impulses  generated  by  these  vibrations  are 
then  transmitted  by  the  cochlear  nerve  to  the  auditory  centers  of  the 
cerebrum,  where  the  sensations  of  sound  are  evoked.  In  order  to 
appreciate  the  function  of  the  individual  structures  concerned  in  this 
general  function  there  must  be  kept  in  mind  a  few  of  the  character- 
istics of  atmospheric  vibrations. 

Atmospheric  Vibrations. — The  vibrations  of  the  atmosphere 
which  are  the  objective  causes  of  the  sensations  of  sound  are  com- 
municated to  it  by  the  vibrations  of  elastic  bodies  such  as  tuning- 
forks,  rods,  strings,  membranes,  etc.  These  produce  in  the  air  a 
to-and-fro  movement  of  its  particles,  resulting  in  a  succession  of 
alternate  condensations  and  rarefactions  which  are  propagated  in 
all  directions.  The  impact  of  a  rhythmic  succession  of  such  con- 
densations on  the  ear  gives  rise  to  musical  sounds;  the  impact  of  an 
arrhvthmic  or  irregular  succession  gives  rise  to  noises. 


696  TEXT-BOOK  OF  PHYSIOLOGY. 

If  a  writing  point  attached  to  a  tuning-fork  in  vibration  be  placed 
in  contact  with  a  traveling  recording  surface,  each  vibration  will  be 
recorded  in  the  form  of  a  wave.  For  this  reason  atmospheric  vibra- 
tions are  generally  spoken  of  as  sound-waves.  A  line  drawn  hori- 
zontally through  such  a  curve  indicates  the  position  of  rest  of  the  fork; 
the  extent  of  the  curve  on  each  side  of  this  line  indicates  the  excursion 
of  the  fork  or  the  amplitude  of  its  movement. 

The  sounds  which  physiologically  result  from  the  impact  and 
transmission  of  the  effects  of  sound-waves,  possess  intensity,  pitch,  and 
quality  or  tone. 

The  intensity  or  loudness  of  a  sound  depends  on  the  amplitude 
of  the  vibration  which  causes  it.  The  greater  the  amplitude  or 
swing  of  the  vibrating  body,  the  greater  is  the  energy  with  which  it 
strikes  the  ear. 

The  pitch  of  a  sound  depends  on  the  number  of  vibrations  which 
strike  the  ear  in  a  unit  of  time— a  second.  The  greater  the  number, 
the  higher  the  pitch.  Thus  while  the  pitch  of  the  sound  caused  by 
the  note  C,  on  the  first  leger  line  below,  of  the  music  scale,  corre- 
sponds to  256  vibrations,  the  pitch  of  the  sound  caused  by  the  note  C 
an  octave  above,  corresponds  to  512  vibrations.  The  lowest  rate  of 
vibration  which  can  produce  a  distinct  sound  varies  in  different 
individuals  from  14  to  18;  the  highest  rate  varies  from  35,000  to 
40,000  per  second.  Between  these  two  extremes  lies  the  range  of 
audibility,  which  embraces  about  11  octaves.  Vibrations  less  than 
14  per  second  can  not  be  perceived  as  a  continuous  sound;  vibrations 
beyond  40,000  also  fail  to  be  so  perceived.  In  the  ascent  of  the 
music  scale  from  the  lowest  to  the  highest  regions  there  is  a  gradual 
increase  in  the  vibration  frequency. 

The  quality  of  a  sound  depends  on  the  form  of  the  vibration.  It 
is  this  feature  which  gives  rise  to  those  differences  in  sensations  which 
permit  one  to  distinguish  one  instrument  from  another  when  both 
are  emitting  the  same  note.  The  form  of  the  sound-wave  in  any 
given  instance  is  the  resultant  of  a  combination  of  a  fundamental 
vibration  and  certain  secondary  vibrations  of  subdivisions  of  the 
vibrating  body.  These  secondary  vibrations  give  rise  to  what  is 
known  as  overtones.  By  their  union  with  and  modification  of  the 
fundamental  vibration  there  is  produced  a  special  form  of  vibration 
which  gives  rise  not  to  a  simple  but  a  composite  sensation.  It  is 
for  this  reason  that  the  same  note  of  the  piano,  the  violin,  and  the 
human  voice  varies  in  quality. 

The  Function  of  the  Pinna  and  External  Auditory  Canal.— 
In  those  animals  possessing  movable  ears  the  pinna  plays  an  im- 
portant part  in  the  collection  of  sound-waves.  In  man  it  is  doubtful 
if  ii  plays  a  part  at  all  necessary  for  hearing.  Nevertheless  an  indi- 
vidual with  defective  hearing  may  have  the  perception  of  sound 
increased  by  placing  the  pinna  at  an  angle  of  90  degrees  to  the  side 
of  the  head  or  by  placing  the  hand  behind  it.     The  external  auditory 


THE  SENSE  OF  HEARING.  697 

canal  transmits  the  sonorous  vibrations  to  the  tympanic  membrane 
From  the  obliquity  of  this  canal  it  has  been  supposed  that  the  vibra- 
tions, after  passing  the  concha,  undergo  a  series  of  reflections  on  their 
way  to  the  tympanic  membrane,  which,  owing  to  its  inclination,  would 
be  struck  by  them  in  a  much  more  effective  manner. 

The  Function  of  the  Tympanic  Membrane. — The  function  of 
the  tympanic  membrane  is  the  reception  of  the  atmospheric  vibrations 
which  are  transmitted  to  it.  This  it  does  by  vibrating  in  unison  with 
them.  The  vibrations  which  the  membrane  exhibits  correspond  in 
amplitude,  in  frequency,  and  in  form  to  those  of  the  atmosphere. 
That  this  membrane  actually  reproduces  all  vibrations  within  the 
range  of  audibility  has  been  experimentally  demonstrated.  The 
membrane  not  being  fixed,  as  far  as  its  tension  is  concerned,  does 
not  possess  a  fixed  fundamental  note,  like  a  stationary  fixed  mem- 
brane, and  is  therefore  just  as  well  adapted  for  the  reception  of  one 
set  of  vibrations  as  another.  This  is  made  possible  by  variations  in 
its  tension  in  accordance  with  the  pitch  of  frequency  of  the  atmos- 
pheric vibrations.  In  the  absence  of  vibration  the  membrane  is  in 
a  condition  of  relaxation;  with  the  advent  of  sound-waves  possessing 
a  gradual  increase  of  pitch,  as  in  the  ascent  of  the  music  scale,  the. 
tension  of  the  membrane  increases  until  its  maximum  is  reached  at 
the  upper  limit  of  the  range  of  audibility.  By  this  change  in  tension 
certain  tones  become  perceptible  and  distinct,  while  others  become 
imperceptible  and  indistinct. 

The  Function  of  the  Tensor  Tympani  Muscle. — The  function 
of  this  muscle  is,  as  its  name  indicates,  to  change  and  to  fix  the  tension 
of  the  tympanic  membrane,  so  that  it  can  most  readily  vibrate  in  unison 
with  vibrations  of  varying  degrees  of  rapidity.  The  tendon  of  this 
muscle  playing  around  the  processus  cochleariformis  is  attached 
almost  at  a  right  angle  to  the  handle  of  the  malleus.  Hence  as  the 
muscle  contracts  it  exerts  its  traction  from  the  process  and  draws  the 
handle  of  the  malleus  inward,  thus  increasing  the  convexity  of  the 
tympanic  membrane  and  at  the  same  time  its  tension.  With  the 
relaxation  of  the  muscle  the  handle  of  the  malleus  passes  outward, 
and  the  convexity  and  tension  diminish. 

In  the  ascent  of  the  music  scale,  each  note  corresponding  to  an 
increase  in  vibration  frequency,  requires  for  its  perception  an  increase 
in  tension  and  an  increase  in  the  force  of  the  contraction  of  the  tensor 
muscle.  In  the  descent  of  the  music  scale  the  reverse  conditions 
obtain.  The  contraction  of  the  muscle  is  of  the  nature  of  a  single 
twitch,  and  of  just  sufficient  force  and  duration  to  tense  the  membrane 
for  a  given  rate  of  vibration. 

The  contraction  of  the  muscle  is  excited  rerlexly.  The  afferent 
path  is  through  fibers  of  the  trigeminal  nerve  distributed  to  the  tym- 
panic membrane;  the  efferent  path  is  through  fibers  in  the  small  root 
of  the  trigeminal.  The  stimulus  is  sudden  pressure  on  the  tympanic 
membrane.     The  more  frequently  and  forcibly  the  stimulus  is  applied. 


698  TEXT-BOOK  OF  PHYSIOLOGY. 

the  greater  is  the  muscle  response.  The  tensor  tympani  muscle  may 
therefore  be  regarded  as  an  accommodative  apparatus  by  which  the 
tympanic  membrane  is  adjusted  for  the  reception  of  vibrations  of 
varying  degress  of  frequency. 

The  Function  of  the  Chain  of  Bones. — The  function  of  the 
chain  of  bones  is  to  transmit  the  effects  of  the  atmospheric  vibrations 
to  the  fluid  of  the  labyrinth.  The  manner  in  which  this  is  accom- 
plished becomes  evident  from  the  relation  which  the  bones  of  this 
chain  bear  to  one  another  and  to  the  tympanic  membrane  on  the 
one  hand  and  to  the  fluid  of  the  labyrinth  on  the  other. 

When  pressure  is  made  on  the  outer  surface  of  the  tympanic  mem- 
brane it  is  at  once  pushed  inward,  carrying  with  it  the  handle  of 
the  malleus,  the  head  at  the  same  time  rotating  outward  around  an 
axis  corresponding  to  its  ligamentous  attachments.  As  the  handle 
moves  inward  a  small  ledge  of  bone  just  below  the  malleo-incudal 
joint  locks  with,  and  hence  pushes  inward,  the  long  process  of  the 
incus.  Since  this  process  is  united  at  almost  a  right  angle  to  the 
stapes  bone,  the  latter  is  forced  toward  and  into  the  foramen  ovale, 
thus  producing  a  pressure  on  the  perilymph.  With  the  cessation  of 
the  pressure  the  elastic  forces  of  the  membrane  and  of  the  ligaments 
return  the  handle  of  the  malleus  to  its  former  position;  by  the  un- 
locking of  the  malleo-incudal  joint  the  entire  chain  also  returns  to 
its  former  position  without  exerting  undue  traction  on  the  basal  attach- 
ment of  the  stapes. 

As  the  long  process  of  the  incus  is  shorter  than  the  handle  of  the 
malleus,  and  as  the  movement  between  them  takes  place  around  an 
axis  from  before  backward,  it  follows  that  the  excursion  of  the  incus 
and  stapes  will  be  less  than  that  of  the  malleus,  while  the  force  will 
be  greater.  Hence  as  the  vibrations  are  transferred  from  the  tym- 
panic membrane  of  large  area  to  the  base  of  the  stapes  of  small  area 
(20  to  1.5),  they  lose  in  amplitude  but  increase  in  force.  Their  pres- 
sure on  the  perilymph  is  therefore  thirty  times  greater  than  on  the 
membrana  tympani.  In  addition  to  its  function  as  a  transmitter  of 
vibrations,  the  chain  of  bones  serves  as  a  point  of  attachment  for 
muscles  which  regulate  the  tension  of  the  tympanic  membrane  and 
the  pressure  on  the  labyrinth. 

The  Function  of  the  Stapedius  Muscle. — The  function  of  the 
stapedius  muscle  is  a  subject  of  much  discussion.  According  to 
Henle,  its  function  is  to  so  adjust  the  stapes  bone  that  it  will  be  pre- 
vented from  exerting  an  undue  pressure  on  the  perilymph  during  the 
inward  excursions  of  the  incus  process.  According  to  Toynbee, 
its  function  is  to  press  the  posterior  part  of  the  stapes  inward,  make- 
it  a  fixed  point,  and  place  the  anterior  part  in  such  a  position  that 
it  will  vibrate  freely  and  accurately. 

The  Function  of  the  Eustachian  Tube.— In  order  that  the  tym- 
panic membrane  may  vibrate  freely  it  is  essential  that  the  air  pressure 
on  both  sides  shall  be  equal  at  all  times.     This  is  made  possible  by 


THE  SENSE  OF  HEARING.  699 

the  Eustachian  tube.  Were  it  not  for  this  passageway,  with  each 
inward  swing  of  the  membrane  the  air  in  the  tympanic  cavity  would 
be  condensed  and  its  pressure  raised,  in  consequence  of  which  the 
movement  of  the  membrane  would  be  retarded;  with  each  outward 
swing,  the  air  would  be  rarefied  and  its  pressure  lowered  below  that 
of  the  atmosphere,  and  in  consequence  the  movement  outward  would 
be  retarded;  the  maximum  response,  therefore,  of  the  membrane  to 
a  given  vibration  could  not  be  attained  and  the  resulting  sound  would 
be  muffled  and  indistinct.  But  as  with  each  vibration  of  the  mem- 
brane the  air  can  pass  into  and  out  of  the  tympanum  through  this 
tube,  inequalities  of  pressure  are  prevented  and  a  free  vibration  per- 
mitted. 

The  impairment  in  the  acuteness  of  hearing  which  is  caused  by 
either  a  rise  or  fall  of  pressure  in  the  middle  ear  can  be  shown — 

1.  By  closing  the   moutH   and  nose   and   then  forcing  air  from  the 

lungs  through  the  Eustachian  tube  into  the  tympanum,  thus  in- 
creasing the  pressure. 

2.  By  closing   the    mouth    and   nose  and  then  making  an  effort  of 

deglutition.     As  this  act  is  attended  by  an  opening  of  the  phar- 
yngeal end  of  the  Eustachian  tube,  the  air  in  the  tympanum  is 
partly  withdrawn  and  the  pressure  lowered.     In  each  instance 
hearing  is  impaired.     After  either  experiment  the  normal  con- 
dition is  restored  by  swallowing  with  the  nasal  passages  open. 
The    Functions    of    the    Internal    Ear. — From    the    anatomic 
arrangement  of  the  structures  of  the  internal  ear  it  is  evident  that  if 
the  vibrations  of  the  stapes  bone  are  to  reach  the  peripheral  organs — 
the  hair  cells — of  both  the  vestibular  and  cochlear  nerves,  they  must 
traverse  successively  the  perilymph,  the  membranous  walls,  and  the 
endolymph.     As  the  perilymph  is  incompressible,  the  inward  move- 
ment of  the  stapes  would  be  prevented  were  it  not  for  the  elastic 
character   of   the   membrane   closing  the   foramen   rotundum.     The 
pressure  wave  occasioned  by  each  inward  movement  of  the  stapes 
is  transmitted  through  the  scala  vestibuli,  the  helicotrema,  the  scala 
tympani,  to  this  membrane,  which  by  virtue  of  its  elasticity  is  pressed 
into  the  tympanic  cavity.     With  the  outward  movement  of  the  stapes, 
equilibrium  is  at  once  restored. 

The  Functions  of  the  Cochlea. — The  cochlea  is  the  portion 
of  the  internal  ear  which  is  concerned  in  the  perception  of  tones. 
The  arrangement  of  the  histologic  elements  of  the  organ  of  Corti 
indicates  that  they  in  some  way  respond  to  the  vibrations  of  varying 
frequency  and  form,  and  through  the  development  of  nerve  impulses, 
evoke  the  sensations  of  pitch  and  quality.  The  manner  in  which 
this  is  accomplished  is  largely  a  matter  of  speculation.  While  many 
theories  have  been  offered  in  explanation  of  the  power  to  distinguish 
the  pitch  and  the  quality  or  timbre  of  a  tone,  most  physiologists  prefer 
that  of  Helmholtz,  who  regarded  the  transverse  fibers  of  the  basilar 
membrane   as  the  elements  immediately  concerned,   and   compared 


7oo  TEXT-BOOK  OF  PHYSIOLOGY. 

them,  both  in  their  arrangement  and  power  of  sympathetic  vibration, 
with  the  strings  of  a  piano.  He  said:  "If  we  could  so  connect  every 
string  of  a  piano  with  a  nerve-fiber  that  the  nerve-fiber  would  be 
excited  as  often  as  the  string  vibrated,  then,  as  is  actually  the  case 
in  the  ear,  every  musical  note  which  affected  the  instrument  would 
excite  a  series  of  sensations  exactly  corresponding  to  the  pendulum- 
like vibrations  into  which  the  original  movements  of  the  air  can  be 
resolved;  and  thus  the  existence  of  each  individual  overtone  would 
be  exactly  perceived,  as  is  actually  the  case  with  the  ear.  The  per- 
ception of  tones  of  different  pitch  would,  under  these  circumstances, 
depend  upon  different  nerve-fibers,  and  hence  would  occur  quite 
independently  of  each  other.  Microscopic  investigation  shows  that 
there  are  somewhat  similar  structures  in  the  ear.  The  free  ends  of 
all  the  nerve-fibers  are  connected  with  small  elastic  particles  which 
we  must  assume  are  set  into  sympathetic  vibration  by  sound-waves." 
(Stirling.) 

The  mechanism  might  be  regarded,  therefore,  somewhat  as 
follows:  The  sound-waves  received  by  the  membrana  tympani  and 
transmitted  by  the  chain  of  bones  to  the  fenestra  ovalis  produce 
variable  pressures  in  the  fluids  of  the  internal  ear;  these  pressures 
varv  in  intensity,  in  number,  and  in  quality,  and  correspond  with 
the  intensity,  pitch,  and  quality  of  the  tones.  If,  therefore,  a  com- 
pound wave  of  pressure  be  communicated  by  the  base  of  the  stapes, 
it  will  be  resolved  into  its  constituents  by  the  different  transverse 
fibers  of  the  basilar  membrane,  each  picking  out  its  peculiar  portion  of 
the  wave  and  thus  stimulating  corresponding  nerve  filaments.  Thus 
different  nerve  impulses  are  transmitted  to  the  brain,  where  they  are 
fused  in  such  a  manner  as  to  give  rise  to  a  sensation  of  a  particular 
quality,  but  still  so  imperfectly  fused  that  each  constituent,  by  a 
strong  effort  of  attention,  may  be  still  recognized.  The  transverse 
fibers  of  the  basilar  membrane  vary  in  length  from  0.04155  mm.  at 
the  base  of  the  cochlea  to  0.495  mm-  at  the  aPex>  and,  according  to 
Retzius,  are  about  24,000  in  number.  As  the  human  ear  usually 
cannot  distinguish  more  than  11,064  tones,  it  is  evident  that  there 
is  a  sufficient  anatomic  basis  for  this  theory. 

The  functions  of  the  semicircular  canals  have  already  been 
stated  in  connection  with  the  chapter  relating  to  the  functions  of  the 
cerebellum. 


CHAPTER  XXIX. 
REPRODUCTION. 

Reproduction  is  the  process  by  which  a  new  individual  is  initiated 
and  developed  and  the  species  to  which  it  belongs  is  preserved.  Re- 
production is  the  result  of  the  union  and  subsequent  development 
of  germ-  and  sperm-cells.  These  cells  are  produced  and  their  union 
accomplished  by  the  cooperation  of  the  reproductive  organs  charac- 
teristic of  the  two  sexes. 

Embryology  is  a  department  of  anatomic  science  which  has  for 
its  object  the  investigation  of  the  successive  stages  that  the  new  being 
passes  through  during  its  gradual  development  prior  to  birth. 

THE  REPRODUCTIVE  ORGANS  OF  THE  FEMALE. 

The  reproductive  organs  of  the  female  comprise  the  ovaries, 
Fallopian  tubes,  uterus,  and  vagina  (Fig.  346). 

The  Ovaries. — The  ovaries  are  two  small,  flattened  bodies, 
measuring  about  40  mm.  in  length  and  20  in  breadth.  They  are 
situated  in  the  cavity  of  the  pelvis,  one  on  either  side,  and  embedded 
in  a  fold  of  the  peritoneum,  known  as  the  broad  ligament.  A  section 
of  the  ovary  shows  that  it  consists  externally  of  a  thin,  firm,  connective- 
tissue  membrane  and  internally  of  a  fine  connective-tissue  stroma, 
supporting  blood-vessels,  non-striated  muscle-fibers  and  nerves,  and 
containing  in  its  meshes  a  very  large  number  of  spheric  sacs  named 
after  their  discoverer,  de  Graaf ,  the  Graafian  sacs  or  follicles.  These 
follicles  are  very  numerous  and  are  present  in  all  portions  of  the 
ovary,  though  they  are  most  abundant  toward  its  peripheral  portions. 
It  is  estimated  that  the  human  ovary  contains  from  20,000  to  40,000 
follicles.  The  follicles  vary  considerably  in  size;  while  many  are  visible 
to  the  unaided  eye,  others  require  for  their  detection  high  powers  of 
the  microscope.  Although  the  follicles  are  present  in  the  ovary  at 
the  time  of  birth,  it  is  not  until  the  period  of  puberty  that  they  assume 
functional  activity. 

From  this  time  on  to  the  catamenial  period  there  is  a  constant 
growth  and  development  of  these  follicles.  Each  follicle  consists  of 
an  external  investment  of  fibrous  tissue  and  blood-vessels,  and  an 
internal  investment  of  cells,  the  membra  na  granulosa.  At  the  lower 
portion  of  this  membrane  there  is  an  accumulation  of  cells,  the  pro- 
ligerous  disc  (Fig.  347).  The  cavity  of  the  follicle  contains  a  slightly 
yellowish,  alkaline,  albuminous  fluid,  a  transudate  in  all  probability 
from  the  blood-vessels.     The  Graafian  follicle  is  of  especial  interest, 

701 


702 


TEXT-BOOK  OF  PHYSIOLOGY. 


for  it  is  in  this  structure,  and  more  especially  in  the  proligerous  disc, 
that  the  true  germ-cell  or  ovum  is  developed. 

The  ovum  is  a  spheric  body  measuring  about  0.3  mm.  in  diameter. 
It  consists  of  a  mass  of  living,  protoplasmic  material,  cytoplasm,  a 
nucleus  or  germinal  vesicle,  and  a  nucleolus  or  germinal  spot.  The 
cvtoplasm  presents  toward  its  central  portion  a  quantity  of  granular 
material,  partly  fatty  in  character,  the  deutoplasm  or  vitellus.  The 
peripheral  portion  of  the  cytoplasm  is  surrounded  by  a  delicate  radially 
striated  border,  the  zona  pellucida  or  radiata  (Fig.  348). 

The  nucleus  consists  of  a  nuclear  membrane  enclosing  contents. 
The  latter  consist  of  an  amorphous  material  in  which  is  embedded 
a  network,  some  of  the  threads  of  which  have  a  strong  affinitv  for  cer- 


Fig.  346. — Uterus,  Fallopian  Tubes  axd  Ovaries;  Posterior  View.  1,1.  Ovaries. 
2,  2.  Fallopian  tubes.  3,  3.  Fimbriated  extremity  of  the  left  Fallopian  tube  seen  from 
its  concavity.  4.  Opening  of  the  left  tube.  5.  Fimbriated  extremity  of  the  right  tube, 
posterior  view.  6,  6.  Fimbriae  which  attach  the  extremity  of  each  tube  to  the  ovary. 
7,  7.  Ligaments  of  the  ovary.  8,  8,  9,  9.  Broad  ligament.  10.  Uterus.  11.  Cervix 
uteri.     12.  Os  externum.     13,  13.  Vagina. 

tain  staining  materials,  and  hence  are  known  as  chromatin,  while  others 
stain  less  deeply  and  are  known  as  achromatin. 

The  Fallopian  Tubes. — The  Fallopian  tubes  are  about  12  centi- 
meters in  length  and  extend  from  the  upper  angles  of  the  uterus  to 
the  ovaries.  Each  tube  is  somewhat  trumpet-shaped,  the  narrow 
portion  being  close  to  the  uterus,  the  wide  portion  close  to  the  ovary. 
The  outer  extremity  of  the  tube  is  expanded  and  subdivided,  and 
presents  a  series  of  processes  termed  fimbriae,  one  of  which  is  attached 
to  the  ovary.  The  tube  consists  of  three  coats — an  external  or  serous; 
a  middle  or  muscular,  the  fibers  of  which  are  arranged  longitudinally 
and  transversely;  and  an  internal  or  mucous.  The  surface  of  the 
mucous  coat  is  covered  with  a  layer  of  ciliated  epithelial  cells,  the 
motion  of  which  is  toward  the  uterus. 

The  Uterus. — The  uterus  is  pyriform  in  shape  and  divided  into 
a  bodv  and   neck.     It   measures,  before  the  first  pregnancy,  about 


PLATE  III. 


1 IIAGRAM  OF  FCETAL  CIRCULATION. 


W  Preyer  del 


REPRODUCTION. 


703 


US 


-d 


^7 


7  cm.  in  length,  5  cm.  in  breadth  and  2\  cm.  in  thickness.  A  frontal 
section  of  the  uterus  shows  a  central  cavity  which  in  the  body  is  tri- 
angular in  shape,  in  the  neck  oval  or  fusiform  (Fig.  349).  At  the 
upper  angles  of  the  uterus  the  cavity  is  continuous  with  the  cavity  of 
each  Fallopian  tube.  At  the  junction  of  the  body  and  the  neck,  the 
cavity  presents  a  constriction,  the  internal  os.  The  constriction  at 
the  end  of  the  neck  is  known  as  the  external  os.  The  walls  of  the 
uterus  are  extremely  thick  and  composed  of  non-striated  muscle- 
fibers  arranged  in  a  very  j, 
complicated  manner. 
The  interior  of  the  uterus 
is  lined  by  mucous  mem- 
brane covered  with  cylin- 
dric  ciliated  epithelial 
cells,  the  motion  of  which 
is  toward  the  external  os. 
Tubular  glands  are  found 
in  large  numbers  in  the 
mucous  membrane  lin- 
ing the  cavity,  while 
racemose  glands  are 
found  in  the  mucous 
membrane  lining  the 
neck.  Owing  to  the 
flattening  of  the  uterus 
from  before  backward 
the  walls  are  almost  in 
contact  and  the  cavity 
almost  obliterated. 

The  Vagina. — The 
vagina  is  a  musculo- 
membranous  canal,  from 
12  to  18  cm.  in  length, 
situated  between  the  rec- 
tum and  bladder.  It 
extends  from  the  surface 
of  the  body  to  the  brim 
of  the  pelvis,  and  em- 
braces at  its  upper  extremity  the  neck  of  the  uterus. 

Ovulation. — After  the  establishment  of  puberty  a  Graafian  follicle 
develops  and  ripens  or  matures  periodically,  usually  every  twenty- 
eight  days.  During  the  time  of  maturation  the  follicle  increases  in 
size,  from  an  augmentation  of  its  fluid  contents,  and  approaches  the 
surface  of  the  ovary,  where  it  forms  a  projection  varying  from  6  to 
12  mm.  in  size.  When  maturation  is  complete  the  vesicle  ruptures, 
and  the  ovum  and  liquid  contents  are  discharged.  The  ovum,  by  a 
mechanism  not  fullv  understood,  is  received  bv  the  fimbriated  ex- 


W8k 


w* 


^■ss^zSssM^s^y^Sis 


Or 


Fig.  347. — Section  of  Cortex  of  Cat's  Ovary, 
Exhibiting  Large  Graafian"  Follicles,  a.  Per- 
ipheral zone  of  condensed  stroma,  b.  Groups  of  im- 
mature follicles,  c.  Theca  of  follicle,  d.  Membrana 
granulosa,  e.  Discus  proligerus.  /.  Zona  pellucid  a. 
g.  Yitellus.  /;.  Germinal  vesicle,  i.  Germinal  spot. 
k.  Cavity  of  liquor  folliculi. — (After  Piersol.) 


■04 


TEXT-BOOK  OF  PHYSIOLOGY. 


tremity  of  the  Fallopian  tube  and  enters  its  cavity.  The  ovum  is 
then  transferred  through  the  tube  by  the  peristaltic  contraction^of 
its  muscle-fibers  and  by  the  action  of  the  cilia  of  its  lining  epithelium. 
The  time  occupied  in  the  transference  of  the  ovum  from  the  ovary 
to  the  interior  of  the  uterus  has  been  estimated  to  be  from  four  to 
ten  days. 

Either  at  the  time,  or  very  shortly  after,  its  discharge  from  the 
follicle,  the  ovum,  and  more  especially  the  nucleus,  undergoes  a  series 
of  histologic  changes  which  eventuates  in  an  extrusion  of  a  portion 
of  the  chromatin  material.  The  extruded  portions  are  known  as  the 
polar  bodies.      The  non-extruded  portion  of  the  chromatin  material 


vl 


:  \ 


Fig.  348.  —  Ovum  of  a  Cow.  i.  Zona 
pellucida.  2.  Cytoplasm,  vitellus.  3.  Nu- 
cleus, germinal  vesicle.  4.  Nucleolus,  germ- 
inal spot.  5.  Corona  radiata.  The  radial 
striation  of  the  zona  pellucida  can  not  be 
seen. — (Stohr.) 


Fig.  349. — Frontal  Sec- 
tion of  the  Uterus.  1.  Cav- 
ity of  the  body.  2,  3.  Lateral 
walls.  4,4.  Cornua.  5.  Os 
internum.  6.  Cavity  of  the 
cervix.  7.  Arbor  vitae  of  the 
cervix.  8.  Os  externum.  9. 
Vagina. — (Sappey.) 


is  known  as  the  female  pronucleus.  The  succession  of  changes  which 
the  nucleus  undergoes  is  termed  maturation.  As  the  nucleus  is 
regarded  as  the  part  of  the  ovum  which  transmits  parental  character- 
istics it  is  assumed  that  the  extrusion  of  a  portion  of  the  nuclear 
material  is  a  means  by  which  an  excess  of  inherited  substance  is 
prevented. 

Menstruation. — Menstruation  is  a  periodic  discharge  of  blood 
and  mucus  from  the  surface  of  the  mucous  membrane  of  the  uterus, 
and  occurs  about  every  twenty-eight  days.  The  duration  of  the 
menstrual  period  extends  over  four  or  five  clays  and  the  amount  of 
blood  discharged   varies  from   180  c.c.  to  200  c.c.     Menstruation  is 


REPRODUCTION. 


705 


usually  an  accompaniment  of  ovulation,  though  the  latter  process  may 
take  place  independently  of  the  former.  It  is  characterized  by  both 
local  and  systemic  changes.  The  local  changes  are  most  marked 
in  the  uterus,  the  mucous  membrane  of  which  increases  in  thickness 
from  a  proliferation  of  the  connective  tissue  and  a  hyperemic  condi- 
tion of  the  blood-vessels.  Subsequently  to  these  changes  the  epithe- 
lial surface,  as  well  as  the  more  superficial  portions  of  the  connective 
tissue,  undergo  degeneration  and  exfoliation,  after  which  the  finer 
blood-vessels  rupture  and  permit  of  an  escape  of  blood  into  the  uterine 
cavity.  At  the  end  of  the  menstrual  period  regenerative  changes  set 
in  which  continue  until  the  normal  condition  of  the  mucous  mem- 
brane is  reestablished. 

The  Corpus  Luteum. — With  the  rupture  of  the  Graafian  follicle 
there  is  an  effusion  of  blood  into  the  follicular  cavity  which  soon 
coagulates,  loses  its  color  and  assumes  the  characteristics  of  fibrin. 
The  walls  of  the  follicle,  which  have  become  thickened  from  the 
deposition  of  a  reddish-yellow  glutinous  substance,  now  become  con- 
voluted and  undergo  a  still  further  hypertrophy,  until  they  encroach 
upon  and  almost  obliterate  the  follicular  cavity.  In  a  few  weeks  the 
mass  loses  its  red  color  and  becomes  decidedly  yellow,  when  it  is 
known  as  the  corpus  luteum.  With  the  continuance  of  reparative 
changes  this  body  gradually  disappears  until  at  the  end  of  two  months 
nothing  remains  but  a  small  cicatrix  on  the  surface  of  the  ovary. 
Such  are  the  changes  in  the  follicle  if  the  ovum  has  not  been  impreg- 
nated. 

The  corpus  luteum,  after  impregnation  has  taken  place,  undergoes 
a  much  slower  development,  becomes  larger,  and  continues  during 
the  entire  period  of  gestation.  The  difference  between  the  corpus 
luteum  of  the  unimpregnated  and  pregnant  condition  is  expressed  in 
the  following  table  by  Dalton: 


Corpus  Luteum  of  Menstruation. 


Corpus  Luteum  of  Pregnancy. 


At  the  end  of  three 

weeks. 
One  month. 


Two  months 
Four  months. 
Six  months. 
Nine  months. 


Three-quarters   of   an   inch  in   diameter;    central  clot  reddish; 
convoluted  wall  pale. 

Smaller;  convoluted 
wall  bright  yellow;  clot  still 
reddish. 

Reduced  to  the  condition 
of  an  insignificant  cicatrix. 


Absent  or  unnoticeable. 


Absent. 


Absent. 


Larger;  convoluted  wall  bright  yel- 
low; clot  still  reddish. 


Seven-eighths  of  an  inch  in  di- 
ameter; convoluted  wall  bright  yellow; 
clot  perfectly  decolorized. 

Seven-eighths  of  an  inch  in  diame- 
ter; clot  pale  and  fibrinous;  convo- 
luted wall  dull  yellow. 

Still  as  large  as  at  the  end  of  second 
month;  clot  fibrinous;  convoluted  wall 
paler. 

Half  an  inch  in  diameter;  central 
clot  converted  into  a  radiating  cicatrix: 
externa]  wall  tolerably  thick  and  con- 
voluted, but  without  any  bright  yellow 
color. 


jo6 


TEXT-BOOK  OF  PHYSIOLOGY. 


THE  REPRODUCTIVE  ORGANS  OF  THE  MALE. 

The  reproductive  organs  of  the  male  comprise  the  testicles,  vasa 
deferentia,  vesicular  seminales,  and  penis. 

The  Testicles. — The  testicles  are  oblong  glands,  about  40  mm.  in 
length,  30  mm.  in  breadth  and  20  mm.  in  thickness,  and  contained 
within  the  cavity  of  the  scrotum.  A  section  of  the  testicle  (Fig.  350) 
reveals  the  presence  externally  of  a  dense  fibrous  membrane,  the  tunica 
albuginea,  and  internally  a  connective-tissue  framework  consisting 
mainly  of  septa,  which  enter  the  organ  on  its  posterior  aspect  at  the 
mediastinum  testis,  passing  inward  in  a  diverging  manner.  The 
spaces  between  the  septa  are  occupied  by  the  true  gland  substance, 
the  seminiferous  tubules. 

The  seminiferous  tubules  are  very  numerous,  the  estimate  as  to 

their  number  varying  from  800  to  1000. 
When  unraveled  they  measure  from  30 
to  40  cm.  in  length  and  0.3  mm.  in 
diameter.  At  their  peripheral  extremities 
the  tubules  are  very  much  convoluted, 
but  as  they  pass  toward  the  mediastinum 
testis,  the  convolutions  disappear,  and 
after  uniting  with  one  another  terminate 
in  from  twenty  to  thirty  straight  tubes, 
the  vasa  recta,  which  pass  through  the 
mediastinum  and  form  the  rete  testis. 
At  the  upper  part  of  the  mediastinum 
the  tubules  unite  to  form  from  nine  to 
thirty  small  ducts,  the  vasa  efferentia, 
which  soon  become  very  much  convo- 
luted. After  a  short  course  they  unite 
to  form  a  single  tortuous  tube,  about  7 
meters  in  length  and  0.4  mm.  in  diameter, 
which  descends  behind  the  testicle  to  its 
lower  border.  This  tube  is  known  as 
the  epididymis.  The  seminal  tubule  con- 
sists of  a  basement  membrane  lined  by  granular  nucleated  epithelium. 
The  vas  deferens,  the  excretory  duct  of  the  testicle,  is  about  60  cm. 
in  length  and  from  2  to  3  mm.  in  diameter,  and  extends  upward  from 
the  epididymis  to  the  inguinal  canal,  through  which  it  passes  into  the 
abdominal  cavity  and  then  to  the  under  surface  of  the  base  of  the 
bladder,  where  it  unites  with  the  duct  of  the  vesicula  seminalis  to  form 
the  ejaculatory  duct 

The  vesiculae  seminales  are  two  lobulated  pyriform  bodies,  about 
40  mm.  in  length,  situated  on  the  under  surface  of  the  bladder.  Each 
vesicula  seminalis  consists  of  an  external  fibrous  coat,  a  middle,  mus- 
cular coat,  and  an  external  mucous  coat.  The  mucous  coat  contains 
a)  number  of  small  tubular  albumin-producing  glands  which  secrete 
a  characteristic  fluid. 


Fig.  350. — Diagram  of  a  Ver- 
tical Section  throvgh  a  Tes- 
ticle. 1.  Mediastinum  testis.  2, 
2.  Trabecular.  3.  One  of  the 
lobules.  4,  4.  Vasa  recta.  5. 
Globus  major  of  the  epididymis. 
6.  Globus  minor.  7.  Vas  def- 
erens.— {H  olden.) 


REPRODUCTION. 


The  ejaculatory  duct,  formed  by  the  union  of  the  vas  deferens  and 
the  duct  of  the  vesicula  seminalis,  opens  into  the  prostatic  portion 
of  the  urethra  (Fig.  351). 

The  prostate  gland  is  a  musculo-glandular  mass  situated  at  the 
posterior  extremity  of  the  urethra.  It  contains  a  large  number  of 
tubules,  more  or  less  branched  and  convoluted,  and  lined  by  columnar 
epithelium.  They  secrete  a  fluid  which  is  poured  into  the  urethra 
at  the  time  of  the  ejaculation  of  semen. 

The  penis  consists  of  three  parts:  the  corpus  spongiosum  below, 
through  which  passes  the  urethra,  and  the  two  corpora  cavernosa, 
one  on  either  side  and  above. 
The  corpus  spongiosum  termi- 
nates anteriorly  in  a  conic-shaped 
structure,  the  glans  penis;  the 
corpora  cavernosa  consist  ex- 
ternally of  a  fibrous  investment 
and  internally  of  erectile  tissue. 
These  bodies  are  abundantly 
supplied  with  blood,  which  after 
entering  their  substance  by  the 
arteries,  passes  into  sinuses  or 
reservoirs,  from  which  it  is  car- 
ried away  by  veins.  These  ves- 
sels pass  to  the  dorsum  of  the 
penis  and  unite  to  form  a  large 
vein  by  which  the  blood  is  re- 
turned to  the  general  circulation. 
By  virtue  of  the  erectile  tissue 
in  the  corpora  cavernosa  the 
penis  becomes  erect  and  rigid 
when  the  blood  supply  is  in- 
creased. This  takes  place  in 
response  to  peripheral  stimula- 
tion or  emotional  states,  or  both 
combined.  When  these  conditions  are  established  nerve  impulses 
pass  outward  through  nerves,  the  nervi  erigentes,  which  have  their 
origin  in  the  lumbar  region  of  the  spinal  cord,  and  bring  about  an 
active  dilatation  of  the  arteries  and  a  relaxation  of  the  non-striated 
muscle-fibers  in  the  corpora  cavernosa.  With  these  events  there  is  a 
rapid  influx  of  blood  and  a  distention  and  an  erection  of  the  organ. 
This  condition  is  furthered  and  maintained  by  a  partial  compression 
of  the  dorsal  vein  by  the  fibrous  capsule. 

Semen. — The  semen  is  a  complex  fluid  composed  of  the  secretions 
of  the  testicles,  the  vesiculae  seminales,  the  prostatic  tubules,  and 
urethral  glands.  It  is  grayish-white  in  color,  mucilaginous  in  con- 
sistence, characteristic  in  odor,  and  somewhat  heavier  than  water. 
In  response  to  appropriate  stimulation  the  muscle-fibers  in  the  walls 


Fig.  351. — Vas  Deferens,  Vesicul.e 
Seiiinales,  and  Ejaculatory  Ducts. 
a.  Vas  deferens,  b.  Seminal  vesicle. 
c.  Ejaculatory  duct.  d.  Termination  of 
the  ejaculatory  duct.  e.  Opening  of  the 
prostatic  utricle.  /,  g.  Veru  montanum. 
h,  I.  Prostate. — (Liegeois.) 


7o8 


TEXT-BOOK  OF  PHYSIOLOGY 


of  the  vasa  deferentia,  vesiculae  seminales,  and  prostatic  tubules 
contract  and  discharge  their  contents  into  the  urethra,  from  which 
they  are  forcibly  ejected  by  the  rhythmic  contraction  of  the  ejaculatory 
muscles,  the  ischio-  and  bulbo  caver  no  si.  The  amount  of  semen  dis- 
charged at  each  ejaculation  varies  from  i  to  5  c.c. 

Spermatozoa. — The  spermatozoa  are  peculiar  morphologic  ele- 
ments which  arise  within  the  seminiferous  tubules  as  a  result  of  com- 
plex histologic  changes  in  the  lining  epithelium. 
An  adult  spermatozoon  consists  of  a  conoid  slightly 
flattened  head,  from  the  posterior  part  of  which 
there  projects  a  short  straight  rod,  provided  with  a 
long  filamentous  tail  or  cilium  and  an  end-piece 
(Fig.  352).  The  head  contains  a  nucleus  of  chro- 
matin material.  The  total  length  of  a  spermato- 
zoon varies  from  50  to  80  micromillimeters.  The 
characteristic  physiologic  feature  of  spermatozoa  is 
incessant  locomotion  when  in  a  suitable  medium. 
So  long  as  they  are  confined  to  the  vas  deferens 
they  are  quiescent,  but  with  their  advent  into  the 
vesicula  seminalis  and  dissemination  in  its  contained 
fluid,  they  become  extremely  active  and  move  around 
with  considerable  rapidity.  The  power  of  locomo- 
tion depends  on  the  possession  of  the  tail  which,  by 
lashing  the  surrounding  fluid  now  in  this  and  now 
in  that  direction,  propels  the  head  from  place  to 
place.  The  vitality  of  spermatozoa  is  such  as  to 
enable  them  to  retain  their  physiologic  activities  in 
the  uterus  for  more  than  eight  days. 

The  development  of  spermatozoa  from  testicular 
cells  as  observed  in  lower  animals  indicates  that 
each  cell  gives  rise  to  four  embryonic  forms — sper- 
matids— which  subsequently  develop  into  adult 
spermatozoa.  In  this  process  the  primary  nuclear 
chromatin  undergoes  a  division,  so  that  each  sper- 
matozoon receives  but  a  fractional  amount.  The  changes  by  which 
this  condition  is  brought  about  are  comparable  to  the  changes  exhibited 
by  the  ovum,  and  have  for  their  result  a  reduction  in  the  quantity  of 
hereditary  substance  to  be  transmitted. 

Fecundation. — Fecundation  is  the  union  of  the  spermatozoon 
(the  sperm-cell)  with  the  ovum  (the  germ-cell)  and  takes  place  in  the 
great  majority  of  instances  in  the  Fallopian  tube.  After  the  intro- 
duction of  the  spermatozoa  into  the  vagina  during  the  act  of  copulation, 
they  soon  begin  to  pass  upward,  into  and  through,  the  uterine  cavity 
and  out  into  the  Fallopian  tube,  where  they  accumulate  in  large 
numbers  and  retain  their  vitality  for  some  days.  The  migration  is 
effe<  led  by  the  propelling  power  of  the  filamentous  tail. 

From    observations    made   on   the   behavior   of   the   spermatozoa 


Fig.  352. — Human 
Spermatozoon.  i. 
Front  view,  2,  side 
view,  of  the  head. 
k.  Head.  m.  mid- 
dle piece.  /.  Tail. 
e.  Terminal  fila- 
ment. — (After  Rel- 
zins.) 


REPRODUCTION. 


709 


toward  the  ovum  in  lower  animals,  and  on  the  manner  by  which  their 
union  is  effected,  the  inference  may  be  drawn  that  a  similar  procedure- 
takes  place  in  mammals.  In  lower  animals  the  spermatozoa  on 
approaching  an  ovum  take  on  increased  activity,  swimming  around 
it  in  all  directions  and  apparently  seeking  a  point  of  entrance.  In 
fish  and  molluscs  the  zona  pellucida  presents  a  distinct  opening,  the 
micropyle,  through  which  the  spermatozoon  passes.  Inasmuch  as 
the  mammalian  ovum  is  devoid  of  such  an  opening,  the  mechanism  of 
entrance  of  the  spermatozoon  is  not  clearly  understood.  Notwith- 
standing their  enormous  numbers  it  is  generally  believed  that  but  a 
single  spermatozoon  effects  an  entrance  into  the  ovum.  With  the 
accomplishment  of  this,  however,  the  spermatozoon  loses  its  vitality, 
after  which  the  body  and  tail  disappear.  The  head,  which  in  this 
instance  also  is  the  transmitter  of  the  inherited  material,  advances  to 


Fig.  353. — Impregnated  Uterus, 
with  Folds  of  Decidua  Growing  up 
Around  the  Egg.  The  narrow  open- 
ing, where  the  folds  approach  each 
other,  is  seen  over  the  most  prominent 
portion  of  the  egg. — (Dalton.) 


Fig.  354. — Impregnated  Uterus; 
showing  the  connection  between  the 
villosities  of  the  chorion  and  the  decidual 
membranes. — (Dalton.) 


meet  and  unite  with  the  nucleus  of  the  ovum.  A  series  of  histologic 
changes  now  arise,  which  eventuate  in  the  production  of  a  new  cell,  a 
parent  cell,  possessing  all  the  features  of  cell  structure  and  the  physio- 
logic activities  and  characteristics  of  both  ancestral  cells.  From  this 
parent  cell  the  new  being  develops  through  successive  division,  multi- 
plication, and  differentiation  of  cells. 

The  Fixation  of  the  Ovum. — If  the  ovum  is  to  develop  into  a 
new  being  it  is  essential  that  it  be  retained  within  the  cavity  of  the 
uterus.  This  is  accomplished  by  the  development  of  specialized 
structures  on  the  surface  of  the  uterine  mucosa  and  on  the  surface 
of  the  ovum.  With  the  fertilization  of  the  ovum,  the  mucous  mem- 
brane of  the  uterus  takes  on  an  increased  growth;  it  becomes  hyper- 
trophied  and  vascular,  and  develops  small  elevations  known  as  villi. 
Inasmuch  as  this  membrane  is  detached  and  discharged  at  the  birth 
of  the  fetus,  it  is  known  as  the  decidua  vera.     With  the  fertilization 


710 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  the  ovum,  the  zona  pellucida  or  radiata  also  develops  villosities, 
and  as  it  passes  from  the  Fallopian  tube  into  the  uterus  the  villi  inter- 
digitate,  and  its  further  progress  is  retarded.  (Figs.  353  and  354.) 
In  a  short  time  a  portion  of  the  decidua  vera  grows  up  on  all  sides 
and  encloses  the  ovum.  Its  retention  is  thus  secured.  That  portion 
of  the  decidua  which  grows  around  the  ovum  is  termed  the  decidua 
reflexa;  while  the  portion  to  which  the  ovum  attaches  itself  is  termed 
the  decidua  serotina,  and  is  of  interest  for  the  reason  that  it  becomes 
the  seat  of  development  of  the  placenta,  the  organ  by  which  the  fetus 
is  nourished.  As  development  advances  the  decidua  reflexa  also 
increases  in  size  and  extent,  and  about  the  end  of  the  fourth  month 
comes  into  contact  with  the  decidua  vera,  with  which  it  ultimately 
fuses. 

DEVELOPMENT  OF  FETAL  ACCESSORY  STRUCTURES. 

Segmentation  of  the  Ovum. — Shortly  after  the  formation  of  the 
parent  cell,  segmentation  of  the  nucleus  and  cytoplasm  takes  place 
in  accordance  with  karyokinetic  methods.  The  two  new  cells  thus 
formed  undergo  a  similar  division  into  four,  the  four  into  eight,  the 

eight  into  .sixteen, 
and  so  on  until  the 
space  within  the 
zona  pellucida  is 
completely  filled 
with  a  large  number 
of  small  cells,  each 
possessing  the  char- 
acteristic cell  struc- 
tures. The  peri- 
pheral cells  then  ar- 
range themselves  in 
the  form  of  a  mem- 
brane, and  as  they 
are,  at  the  same 
time,  subjected  to 
mutual  pressure 
they  assume  a  poly- 
hedral shape,  and 
give  to  the  mem- 
brane a  mosaic  ap- 
pearance (Fig.  355).  The  central  cells  then  undergo  liquefaction.  At 
some  point  on  the  inner  surface  of  the  membrane,  cells  accumulate 
which  by  their  division  and  multiplication  form  a  second  membrane. 
The  two' together  are  known  as  the  external  and  internal  blastodermic 
membranes. 

Germinal  Area. — At  about  this  period  there  is  an  accumulation 
of  cells  at  a  certain  spot  in  the  substance  of  the  blastodermic  mem- 


Fig.  355. — Primitive  Trace  of  the  Embryo,  a. 
Primitive  trace,  b.  Area  pellucida.  c.  Area  opaca.  d. 
Blastodermic  cells,  e.  Villi  beginning  to  appear  on  the 
surface  of  the  zona  pellucida. — (Lugois.) 


REPRODUCTION. 


711 


branes  which  marks  the  position  of  the  future  embryo.  This  spot, 
at  first  circular,  soon  becomes  elongated.  A  slight  indentation  now 
develops  into  what  is  known  as  the  primitive  trace.  Around  this  area 
there  is  a  clear  space,  the  area  pellucida,  which  is  in  turn  surrounded 
by  a  darker  region,  the  area  opaca.  The  primitive  trace  soon  dis- 
appears and  the  area  pellucida  becomes  guitar-shaped.  A  second 
groove,  the  medullary  groove,  is  now  formed,  which  develops  from 
before  backward  and  becomes  the  neural  medullary  canal. 

Blastodermic  Membranes. — The  embyro,  at  this  period,  con- 
sists of  three  layers — viz.,  the  external  and  the  internal  blastodermic 
membranes  and  a  middle  membrane  formed  by  a  genesis  of  cells 
from    their    internal    surfaces.      These 

layers  are  known  from  without  inward  as  ^ 

the  epiblast,  mesoblast,  and  hypoblast. 

The  epiblast  gives  rise  to  the  central 
nerve  system,  the  epidermis  and  its  ap- 
pendages, and  the  primitive  kidneys. 

The  mesoblast  gives  rise  to  the  dermis, 
muscles,  bones,  nerves,  blood-vessels, 
sympathetic  nerve  system,  connective 
tissue,  the  urinary  and  reproductive  ap- 
paratus, and  the  walls  of  the  alimentary 
canal. 

The  hypoblast  gives  rise  to  the  epi- 
thelial lining  of  the  alimentary  canal  and 
its  glandular  appendages,  the  liver  and 
pancreas,  and  the  epithelium  of  the 
respiratory  tract. 

Dorsal  Laminae. — As  development 
advances,  the  true  medullary  groove 
deepens,  and  there  arise  two  longitudinal 
elevations  of  the  epiblast — the  dorsal 
lamina,  one  on  either  side  of  the  groove 
— which  grow  up,  arch  over,  and  unite  so 
as  to  form  a  closed  tube,  the  primitive 
central  nerve  system. 

The  Chorda  Dorsalis. — Just  beneath  the  neural  canal  there  arises 
a  group  of  hypoblastic  cells  which  arrange  themselves  in  the  form  of  a 
cylindric  rod,  which  marks  out  the  position  of  the  future  bony  axis 
of  the  body.     This  rod  is  known  as  the  chorda  dorsalis  or  notochord. 

Primitive  Vertebrae. — On  either  side  of  the  neural  canal  the 
cells  of  the  mesoblast  undergo,  a  longitudinal  thickening,  which 
develops  and  extends  around  the  neural  canal  and  the  chorda  dorsalis, 
and  forms  the  arches  and  the  bodies  of  vertebrae.  They  become 
divided  transversely  into  segments. 

The  mesoblast  now  separates  into  two  layers:  the  external,  joining 
with  the  epiblast,  forms  the  somatopleural  the  internal,  joining  with 


-  -pp 


Fig.  356. — Diagram  Repre- 
senting the  Relation  of  Pri 
mary  Structures  in  a  Develop- 
ing Chicken;  Vertical  Trans- 
verse Section.  The  medullar}' 
groove  and  chorda  dorsalis  are  seen 
in  section;  the  alimentary  canal 
pinched  off  from  the  yolk-sac  is 
completely  closed,  a.  Amnion,  a, 
c.  Amniotic  cavity  filled  with 
amniotic  fluid,  pp.  Space  be- 
tween amnion  and  chorion  con- 
tinuous with  the  pleuro-peritoneal 
cavity  inside  the  body.  vt.  Vitel- 
line membrane,  or  zona  pellucida. 
ys.  Yolk-sac,  or  umbilical  vesicle. 
— (Foster  and  Balfour.) 


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TEXT-BOOK  OF  PHYSIOLOGY 


the  hypoblast,  forms  the  s  plane  Jaw  pleura;  the  space  between  them 
constitutes  the  pleuro- peritoneal  cavity  (Fig.  356). 

Visceral  Laminae. — The  walls  of  the  pleuro-peritoneal  cavity 
are  formed  by  a  downward  prolongation  of  the  somatopleura  (the 
visceral  lamina),  which,  as  they  extend  around  in  front,  pinch  off 
a  portion  of  the  yolk-sac  (formed  by  the  splanchnopleura),  which 
becomes  the  primitive  alimentary  canal;  the  lower  portion,  remaining 
outside  of  the  body  cavity,  forms  the  umbilical  vesicle. 

The  Fetal  or  Embryonic  Membranes. — With  the  appearance 
of  the  visceral  laminae  two  membranes  develop  in  succession,  both 
of  which  play  an  important  part  in  the  subsequent  life  of  the  embryo. 
These  are  known  as  the  amnion  and  the  allantois. 

The  amnion  is  formed  by  folds  of  the  epiblast  and  the  external 
layer  of  the  mesoblast  rising  up  in  front,  behind,  and  at  the  sides. 
These  folds  gradually  extend  over  the  back  of  the  embryo  to  a  certain 


Fig.  357. — Diagram  of  Fecundated 
Egg.  a.  Umbilical  vesicle,  b.  Amniotic 
cavity,     c.  Allantois. — (Dalton.) 


Fig.  358. — Fecundated  Egg  with 
Allantois  nearly  Complete,  a.  Inner 
layer  of  amniotic  fold.  b.  Outer  layer 
of  ditto,  c.  Point  where  the  amniotic 
folds  come  in  contact.  The  allantois 
is  seen  penetrating  between  the  outer  and 
inner  lavers  of  the  amniotic  folds. — 
(Dalton.) 


point  where  they  meet,  coalesce,  and  enclose  a  cavity  known  as  the 
amniotic  cavity.  The  membranous  partition  between  the  folds  is 
absorbed,  after  which  the  outer  layer  recedes  and  becomes  blended 
with  the  primitive  enveloping  membrane  of  the  ovum  and  thus  assists 
in  the  formation  of  the  chorion — the  external  covering  of  the  embryo. 

The  cavity  enclosed  by  the  amnion  is  at  first  quite  small,  but  soon 
enlarges  from  the  accumulation  of  a  clear,  transparent  fluid,  the 
amniotic  fluid.  It  gradually  increases  in  amount  up  to  the  latter 
period  of  gestation,  when  its  volume  reaches  about  one  liter.  This 
fluid  is  derived  mainly  from  the  blood,  as  it  contains  albumin,  sugar, 
fatty  matter,  and  inorganic  salts.  Traces  of  urea  indicate  that  some 
of  its  constituents  arc  derived  from  the  embryo  itself. 

The  allantois  is  primarily  a  pouch  or  diverticulum  which  develops 
from  the  posterior  portion  of  the  alimentary  canal.  As  il  develops 
it  enlarges,  and  in  its  growth  inserts  itself  between  the  two  layers  of 
the  amnion,  coming  into  contact  more  especially  with  the  external  layer. 


REPRODUCTION. 


7i3 


It  finally  completely  surrounds  the  embryo,  after  which  its  edges 
become  fused  together  (Figs.  357  and  358). 

The  allantois  is  of  especial  interest  and  importance,  as  it  is  the 
means  by  which  the  blood  of  the  embryo  is  brought  into  relation 
with  the  blood  of  the  mother.  As  it  develops,  two  arteries,  the  hypo- 
gastrics, one  from  each  internal  iliac,  pass  out  of  the  abdominal  cavity 
within  the  walls  of  the  allantois,  and  follow  it  in  its  course  around 
the  embryo.  The  ultimate  branches  of  these  arteries  penetrate  the 
villous  processes  which  develop  on  the  surface  of  the  chorion  and 
which  take  part  in  the  formation  of  the  placenta.  A  single  large 
vein  emerges  from  the  placenta  and  returns  the  blood  to  the  embryo. 
In  its  course  it  winds  around  the  arteries  in  a  spiral  manner  a  number 
of  times.  These  vessels — the  umbilical  arteries  and  vein — are  en- 
closed by  the  walls  of  the  allantois  and  amnion,  and  together  constitute 
the  umbilical  cord  which  at  the 
end  of  gestation  is  about  60  cm. 
in  length.     (Figs.  359,  360). 

The  Chorion. — The  chorion, 
the  external  investment  of  the 
embryo,  is  formed  by  the  fusion 
of  the  primitive  egg  membrane 
— the  zona  pellucida — the  ex- 
ternal layer  of  the  amnion,  and 
the  allantois.  Very  early  in  de- 
velopment its  external  surface 
becomes  covered  with  homogen- 
eous, granular,  club-shaped  proc- 
esses, which  by  continued  bud- 
ding and  growth,  give  to  the 
membrane  a  shaggy  appearance. 

At  about  the  end  of  the  second  month  these  processes  begin  to  atrophy 
and  disappear  from  the  surface  of  the  chorion,  with  the  exception  of 
that  portion  which  is  in  contact  with  the  decidua  serotina.  At  this 
point  the  processes  or  villi  continue  to  grow  and  develop,  and  insert 
themselves  more  deeply  into  the  mucous  membrane.  Corresponding 
processes  from  the  mucous  membrane  insert  themselves  between 
the  villi  of  the  chorion,  which  by  their  growth  and  fusion  secure,  among 
other  things,  the  retention  of  the  embryo. 

The  Nutrition  of  the  Embryo. — Coincident ly  with  the  develop- 
ment of  the  amnion,  allantois,  and  chorion,  there  arises  within  the 
body  of  the  embryo  the  early  forms  of  many,  if  not  all,  of  the  future 
viscera.  The  nutritive  material  required  for  their  growth  is  partly 
contained  within  the  umbilical  vesicle  lying  without  the  body  cavity. 
That  this  material  may  be  utilized,  blood-vessels  emerge  from  the 
body  and  ramify  within  the  walls  of  the  vesicle.  The  capillaries  to 
which  these  vessels  give  rise  come  into  close  relation  with  and  absorb 
the  food  material,  after  which  it  is  carried  bv  veins  to  the  heart,  bv 


Fig.  359. — Human  Embryo  and  its  En- 
velopes at  the  End  of  the  Third  Month. 


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TEXT-BOOK  OF  PHYSIOLOGY. 


which  it  is  distributed  to  all  parts  of  the  embryo.  These  vessels 
are  collectively  known  as  the  omphalo-mesenteric  arteries  and  veins. 
This  primitive  or  vitelline  circulation  is  of  short  duration  in  mam- 
mals, as  the  nutritive  material  in  the  vesicle  is  small  in  amount  and 
is  soon  exhausted.  In  birds,  however,  it  is  of  primary  importance. 
The  main  supply  of  nutritive  material,  however,  is  derived  from 
the  mother  by  means  of  a  highly  developed  and  specialized  organ — 
The  Placenta. — Of  all  the  embryonic  structures  the  placenta 
is  the  most  important.  It  is  formed  by  the  end  of  the  third-  month, 
after  which  it  gradually  increases  in  size  up  to  the  end  of  the  eighth 
month,  by  which  time  it  is  fully  developed.  It  then  measures  from 
18  to  24  cm.  in  diameter  and  weighs  from  400  to  600  grams.     It  is 

most  frequently  situated  at 
the  upper  and  back  part  of 
the  uterine  cavity.  Though 
exceedingly  complex  in 
structure  it  consists  essen- 
tially of  two  portions,  a 
fetal  and  a  maternal. 

The  fetal  portion  con- 
sists primarily  of  those  villi 
on  the  chorion  in  relation 
with  the  decidua  serotina. 
These  structures  gradually 
increase  in  size  and  num- 
ber, and  receive  the  ulti- 
mate branches  of  the  um- 
bilical arteries.  The  ma- 
ternal portion  consists 
primarily  of  the  decidua 
serotina.  As  gestation  ad- 
vances the  chorionic  villi 
rapidly  increase  in  size  and 
number,  and  receive  the 
branches  of  the  umbilical 
arteries.  At  the  same  time  the  decidua  serotina  becomes  hyper- 
trophied  and  vascular.  With  the  continued  growth  and  development 
of  these  two  structures  they  gradually  fuse  together  and  finally  become 
inseparable.  In  accordance  with  the  needs  of  the  embryo,  the  decidua 
serotina  and  its  contained  blood-vessels  undergo  certain  histologic 
changes  which  result  in  the  formation  of  large  cavities,  sinuses,  or 
lakes,  into  which  the  blood  of  the  uterine  vessels  is  emptied.  Coin- 
<  irlcntly  the  villi  of  the  chorion  grow  and  give  off  numerous  branches, 
which  project  themselves  in  all  directions  into  the  blood  of  uterine 
sinuses  (Figs.  361,  362).  As  the  placenta  develops,  the  structures 
separating  the  blood  of  the  mother  from  that  of  the  child  gradually 
become   modified    until   they   are   represented  by  a  thin  cellular  or 


Fig.  360. — Human  Embryo,  with  Amnion  and 
Allantois,  in  the  Third  Week.  There  are  as 
yet  no  limbs;  the  embryo  and  its  appendages  are 
surrounded  by  the  tufted  chorion. — (Haeckel.) 


REPRODUCTION. 


7^3 


homogeneous  membrane.  The  conditions  now  are  such  as  to  permit 
of  a  free  exchange  of  material  between  the  mother  and  child.  Whether 
by  osmosis  or  by  an  act  of  secretion,  the  nutritive  materials  of  the 
maternal  blood  pass  through  the  intervening  membrane  into  the 
fetal  blood  on  the  one  hand,  while  waste  products  pass  in  the  reverse 
direction  into  the  maternal  blood  on  the  other  hand.  Inasmuch 
as  oxygen  is  absorbed  and  carbon  dioxid  exhaled  by  the  same  struc- 
tures, the  placenta  is  to  be  regarded  as  both  a  digestive  and  a  respira- 
tory organ.  So  long  as  these  exchanges  are  permitted  to  take  place 
in  a  normal  manner  the  nutrition  of  the  embrvo  is  secured. 


Fig.  361. — Diagram  showing  the  Relations  of  the  Fetal  Membranes.  Ant. 
Amnion.  Ch.  Chorion.  M.  Muscle  wall  of  uterus.  R.  Decidua  reflexa.  5.  Serotina. 
V.  Decidua  vera.     Y.  Yolk  stalk. — (McMurrich.) 


The  Fetal  Circulation. — The  composition  of  the  blood  as  well 
as  the  course  it  pursues  through  the  heart  and  vascular  apparatus 
presents  peculiarities  which  have  arisen  in  consequence  of  the  neces- 
sity of  obtaining  nutritive  material  through  the  placenta  and  the 
almost  impervious  condition  of  the  pulmonary  capillaries.  On  re- 
turning from  the  placenta,  the  blood  in  the  umbilical  vein  is  relatively 
rich  in  nutritive  material  and  scarlet  red  in  color  from  the  presence 
of  oxygen.  As  it  passes  into  the  abdominal  cavity  a  portion  of  the 
blood  is  directed  by  the  duct  us  venosus  into  the  vena  cava,  while 
another  portion  is  emptied  into  the  branches  of  the  portal  vein,  by 
which  it  is  distributed  to  the  liver  and  from  which  it  emerges  bv  the 


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TEXT-BOOK  OF  PHYSIOLOGY. 


hepatic  veins  and  poured  into  the  vena  cava.  The  blood  in  the  vena 
cava  is  thus  a  mixture  of  venous  blood  from  the  lower  extremities 
and  liver,  and  oxygenated  blood  from  the  placenta.  After  its  dis- 
charge into  the  right  auricle  the  blood  is  directed  by  a  fold  of  the 
lining  membrane,  the  Eustachian  valve,  through  an  opening  in  the 
interauricular  septum,  the  foramen  ovale,  into  the  left  auricle.  It  then 
flows  through  the  auriculo-ventricular  opening  into  the  left  ventricle, 
thence  into  the  aorta,  and  by  its  branches  is  distributed  to  all  parts 
of  the  body. 

The  blood  from  the  head  and  upper  extremities  is  emptied  by 
the  superior  vena  cava  into  the  right  auricle,  but  as  it  passes  in  front 
of  the  Eustachian  valve,  it  flows  directly  into  the  right  ventricle  and 
then   into   the   pulmonary  artery.     On   account   of  the   unexpanded 


Amnion. 
Chorion. 


■  Chorionic  villi. 


HI 

Intervillous  spaces. 
^^.S^l}^1,'  Floal 


,-  Attached  villi. 
J 

Vein. 


Fig.  362. — Diagram  of  Human  Placenta  at  the  Close  of  Pregnancy. — (Schaper.) 


Muscularis 


condition  of  the  lungs  and  the  almost  impervious  condition  of  the 
pulmonary  capillaries,  but  a  small  portion  of  the  blood  passes  through 
them,  while  the  larger  portion  by  far  passes  into  the  aorta  directly 
through  a  duct,  the  ductus  arteriosus,  which  enters  at  a  point  below 
the  origin  of  the  left  carotid  and  subclavian  arteries.  A  comparison 
of  the  blood  distributed  to  the  head  and  upper  extremities,  with  that 
distributed  to  the  lower  extremities,  will  show  a  larger  percentage 
of  nutritive  material  and  oxygen  in  the  former  than  in  the  latter, 
a  fact  which  has  been  offered  as  an  explanation  of  the  more  rapid 
growth  of  the  upper  half  of  the  body.  As  the  blood  passes  through 
the  aorta,  a  portion  is  directed  from  the  main  current  by  the  hypogas- 
tric  and   umbilical   arteries  to   the   placenta,   where   it   loses  carbon 


REPRODUCTION. 


m 


dioxid  and  gains  oxygen,  and  changes  in  color  from  a  bluish  red  to  a 
scarlet  red. 

Parturition. — At  the  end  of  gestation — approximately  280  days 
from  the  time  of  conception — a  series  of  changes  occur  in  the  uterine 
structures  which  lead  to  an  expulsion  of  the  child,  the  placenta,  and 
decidua  vera.  To  this  process  in  its  entirety  the  term  parturition 
is  given.  At  this  time,  from  causes  not  clearly  defined,  the  uterine 
walls  begin  to  exhibit  throughout  their  extent  a  series  of  slight  con- 
tractions which  are  somewhat  peristaltic  in  character;  these  con- 
tractions, which  gradually  increase  in  frequency  and  vigor,  bring 
about  a  dilatation  of  the  internal  os  and  a  descent  of  the  membranes 
into  the  cervical  canal.  The  pressure  exerted  by  these  membranes 
during  the  time  of  the  contraction  materially  assists  in  the  relaxation 
of  the  circular  fibers  and  a  dilatation  of  the  external  os.  When  the 
dilatation  has  so  far  advanced  that  the  diameter  of  the  external  os 
attains  a  measure  of  7  or  8  cm.,  the  tension  of  the  membranes  becomes 
sufficiently  great  to  lead  to  their  rupture  and  to  a  partial  escape  of 
the  amniotic  fluid.  With  this  event,  the  presenting  part  of  the  child, 
usually  the  head,  descends  into  the  cervical  canal.  After  a  short 
period  of  rest  the  uterine  contractions  return  and  rapidly  increase  in 
vigor  and  duration.  As  a  result  of  the  pressure  thus  exerted  from  all 
sides  on  the  body  of  the  child,  the  head  gradually  descends  into  the 
vagina  and  finally  emerges  through  the  vulva  to  be  followed  in  a  short 
time  by  the  expulsion  of  the  trunk  and  limbs,  and  a  discharge  of  the 
remaining  amniotic  fluid.  With  the  expulsion  of  the  child  the  uterine 
contractions  cease  for  a  period  of  ten  or  fifteen  minutes,  when  they 
again  recur,  wTith  the  result  of  detaching  the  placenta  and  expelling 
it  into  the  vagina.  It  is  then  removed  by  the  cooperative  action  of  the 
abdominal  and  perineal  muscles.  The  hemorrhage  which  would 
naturally  occur  with  the  detachment  of  the  placenta  and  the  laceration 
of  the  maternal  vessels  is  prevented  by  the  firm  continuous  contraction 
of  the  uterine  walls,  by  wrhich  the  vessels  are  compressed  and  perma- 
nently closed. 

The  Establishment  of  Inspiration  and  the  Adult  Circulation. 
— After  the  birth  of  the  child  and  the  detachment  of  the  placenta, 
there  speedily  occurs  a  decrease  in  the  quantity  of  oxygen  and  an 
increase  in  the  quantity  of  carbon  dioxid  in  the  blood,  a  condition 
which  causes  a  discharge  of  nerve  energy  from  the  inspiratory  center, 
a  contraction  of  the  inspiratory  muscles,  an  expansion  of  the  thorax, 
and  an  inflow  of  air  into  the  lungs. 

In  the  later  months  of  intrauterine  life  the  vascular  apparatus 
undergoes  certain  anatomic  changes  which  favor  the  transition  from 
the  placental  to  the  adult  circulation.  Thus  the  ductus  venosus  con- 
tracts, and  shunts  a  larger  volume  of  blood  into  and  through  the 
liver;  the  Eustachian  valve  diminishes  in  size  and  at  the  time  of  birth 
has  almost  disappeared;  a  membranous  fold  grows  upward  and 
backward  from  the  edge  of  the  foramen  ovale  on  the  left  side;  the 


7i8  TEXT-BOOK  OF  PHYSIOLOGY.  ' 

ductus  arteriosus  also  contracts.  With  the  first  inspiration  and  the 
expansion  of  the  lungs,  the  blood  which  enters  the  pulmonary  arterv 
passes  through  the  pulmonary  capillaries  in  large  volume  and  is 
returned  by  the  pulmonary  veins  to  the  left  auricle.  The  entrance 
of  the  blood  into  this  cavity  presses  the  membranous  fold  against 
the  margins  of  the  foramen  ovale  and  thus  prevents  the  further  flow 
of  blood  from  the  right  auricle.  The  blood  entering  the  right  auricle 
by  the  inferior  vena  cava  now  flows  into  the  right  ventricle,  which 
is  favored  by  the  small  size  of  the  Eustachian  valve.  The  foramen 
ovale  is  permanently  closed  at  the  end  of  a  week  or  ten  days;  the 
ductus  arteriosus  at  the  end  of  four  days.  The  umbilical  vein  and 
ductus  venosus,  at  the  end  of  four  or  five  days,  have  also  become 
almost  impervious  from  the  contraction  of  their  walls.  The  hypo- 
gastric arteries  remain  open  and  carry  blood  to  the  walls  of  the  bladder. 

Lactation. — As  pregnancy  advances  the  mammary  glands  in- 
crease in  size,  partly  from  a  deposition  of  fat  and  connective  tissue 
and  partly  from  a  multiplication  of  the  secreting  acini.  The  lining 
epithelial  cells  at  the  same  time  increase  in  size,  and  toward  the  end 
of  pregnancy  begin  to  exhibit  functional  activity.  At  the  time  of 
birth,  or  within  a  day  or  so  after  birth,  the  acini  are  filled  with  a  fluid 
which  in  its  qualitative  composition  resembles  milk  and  is  known  as 
colostrum.  It  is  distinguished  from  milk  more  especially  in  the  fact 
that  it  contains  in  large  quantity  a  proteid  which  coagulates  on  boiling, 
and  certain  inorganic  salts  which  have  a  laxative  effect  on  the  new- 
born child.  Normal  lactation  and  the  phenomena  which  accompany 
it  are  fully  established  by  the  end  of  the  second  or  third  day. 

The  composition  of  milk  and  the  mechanism  of  its  production 
have  been  stated  in  the  chapter  on  Secretion. 

Physiologic  Activities  of  the  Embryo. — During  intrauterine 
life  the  evolution  of  structure  is  accompanied  by  an  evolution  of 
function.  The  relatively  simple  and  uniform  metabolism  of  the 
undifferentiated  blastodermic  membranes  gradually  increases  in 
complexity  and  variety,  as  the  individual  tissues  and  organs  make 
their  appearance  and  assume  even  a  slight  degree  of  functional  activity. 
As  to  the  periods  at  which  different  organs  begin  to  functionate,  but 
little  is  positively  known. 

The  primitive  heart,  in  all  probability,  begins  to  pulsate  very 
early,  as  in  an  embryo  from  fifteen  to  eighteen  days  old  and  measuring 
but  2.2  mm.  in  length,  Coste  found  the  amnion,  the  allantois,  the 
omphalo-mesenteric  vessels,  and  the  two  primitive  aortae  developed. 
In  the  earlier  weeks,  all  products  of  metabolism  are  doubtless  elimi- 
nated by  the  placental  structures;  but  as  metabolism  increases  in 
complexity  the  liver  and  kidney  assume  excretory  activity.  Thus, 
at  the  end  of  the  third  month  the  intestine  contains  a  dark,  greenish, 
viscid  material — meconium — composed  of  bile  pigments,  bile  salts, 
and  desquamated  epithelium;  the  amniotic  fluid,  as  well  as  the  fluid 
within  the  bladder,   contains  urea  at  the  end  of  the  sixth  month, 


REPRODUCTION.  719 

indicating  the  establishment  of  both  hepatic  and  renal  activity.  Con- 
tractions of  the  skeletal  muscles  of  the  limbs  begin  about  the  fifth 
month,  from  which  it  may  be  inferred  that  the  mechanism  for  muscle 
activity,  viz.,  muscles,  efferent  nerves,  and  spinal  centers,  has  become 
anatomically  developed  and  associated,  and  capable  of  coordinate 
activity.  These  contractions  are,  in  all  probability,  automatic  or 
autochthonic  in  character  due  to  stimuli  arising  within  the  spinal 
centers.     The  remaining  organs  remain  more  or  less  inactive. 

After  birth,  with  the  first  inspiration  and  the  introduction  of  food 
into  the  alimentary  canal,  the  physiologic  mechanisms  which  sub- 
serve general  metabolism  begin  to  functionate  and  in  the  course  of  a 
week  are  fully  established.  At  this  time  the  cardiac  pulsation  averages 
about  135  a  minute;  the  respiratory  movements  vary  from  30  to  35 
a  minute,  and  are  diaphragmatic  in  type;  the  urine,  which  was  at  first 
scanty,  is  now  abundant  and  proportional  to  the  food  consumed; 
the  digestive  glands  are  elaborating  their  respective  enzymes,  digestion 
proceeding  as  in  the  adult.  The  hepatic  secretion  is  active  and  the 
lower  bowel  is  emptied  of  its  contents;  the  coordinate  activities  of  the 
nerve-,  muscle-,  and  gland-mechanisms  are  entirely  reflex  in  character. 
Psychic  activities  are  in  abeyance  by  reason  of  the  incomplete  develop- 
ment of  the  cerebral  mechanisms. 


APPENDIX. 


PHYSIOLOGIC  APPARATUS. 

The  study  of  the  physical  and  physiologic  properties  of  muscles  and 
nerves  necessitates  the  employment  of  some  stimulus  which,  when 
applied  to  either  tissue,  will  call  forth  a  contraction  of  the  muscle,  or 
the  development  of  a  nerve  impulse  in  the  nerve.  The  most  convenient 
stimulus  is  electricity,  for  the  reason  that,  with  appropriate  apparatus, 
its  intensity  and  duration  can  be  graduated  with  the  utmost  nicety. 
Moreover,  it  does  not  destroy  the  tissues,  as  do  many  chemic,  physical, 
and  mechanic  stimuli. 

It  is  therefore  necessary  that  the  student  should  have  a  practical 
acquaintance  with  those  appliances  by  means  of  which 
electricity  is  generated,  applied  and  controlled. 

The  electric  cell  is  an  apparatus  composed  of 
different  elements,  which,  by  virtue  of  chemic  actions 
taking  place  among  them,  generate  and  conduct  elec- 
tricity. In  its  simplest  form  an  electric  cell  consists 
of  two  metals — zinc  and  copper,  or  carbon,  or  platinum, 
etc.,  immersed  in  an  exciting  fluid,  usually  dilute  sul- 
phuric acid  (Fig.  363). 

The  zinc  element  is  the  one  acted  on  chemically 
by  the  sulphuric  acid,  and  at  the  expense  of  which  the 
electricity  is  maintained.  It  is  known  as  the  gener- 
ating element.  The  copper  is  the  collecting  and  con- 
ducting element. 

With  the  immersion  of  these  elements  in  a  solution  of  H2S04  a 
chemic  action  at  once  takes  place  between  the  zinc  and  the  acid,  with 
the  formation  of  zinc  sulphate  and  the  liberation  of  hydrogen,  as 
expressed  in  the  following  formula: 

Zn  +  H2S04  =  ZnS04  +  H2. 

The  zinc  sulphate  passes  into  the  solution,  while  the  hydrogen  accu- 
mulates on  the  surface  of  the  copper  element. 

As  all  chemic  action  is  accompanied  by  the  development  of  elec- 
tricity, it  can  be  shown  by  appropriate  means  that  this  is  the  case  at 
the  surface  of  the  zinc.  Such  a  combination  is  the  means  of  establish- 
ing a  difference  of  potential  between  two  points;  the  point  of  highesl 
potential  being  the  surface  of  the  zinc  or  the  positive  clement,  the 
point  of  lowest  potential  being  the  copper  or  the  negative  element. 

46  72  T 


Fig. 
Electric  Cell. 


TEXT-BOOK  OF  PHYSIOLOGY. 


So  long  as  the  elements  remain  unconnected  there  is  no  movement 
of  electricity,  no  current. 

If  the  ends  of  the  elements  projecting  beyond  the  fluid  are  connected 
by  a  copper  wire,  a  pathway  or  circuit  is  established,  and  a  movement  of 
the  electricity  takes  place.  As  electricity  flows  from  the  point  of  high  to 
the  point  of  low  potential,  it  follows  that  inside  the  cell  the  current  flows 
from  the  zinc  to  the  copper,  and  outside  the  cell  from  the  copper  to  the 
zinc.  Such  a  current  is  termed  a  continuous,  a  galvanic  or  a  voltaic 
current.  Inasmuch  as  there  is  a  progressive  fall  in  potential  between 
the  highest  and  lowest  points,  it  follows  that  any  two  points  in  the 
circuit  will  exhibit  a  similar  difference,  of  potential.  For  this  reason 
the  projecting  end  of  the  copper  element  is  at  a  higher  potential  than 
the  projecting  end  of  the  zinc  element.  The  end  of  the  copper  is, 
therefore,  termed  the  positive,  +  pole  or  anode,  the  end  of  the  zinc  the 
negative,  —  pole  or  kathode. 

Electric  Units. — Owing  to  the  difference  of  the  electric  potential 
in  the  cell,  the  electricity  leaves  the  cell  under  a  certain  degree  of 
pressure,  termed  the  "electro-motive  force."  As  it  passes  through  the 
circuit  it  meets  with  resistance,  the  amount  of  which  will  depend  on 

the  nature  of  the  circuit 


»-} 


£^\ 


ShGJ  -* 


H2SO 


Fig.  364. 
ix  Series. 


—Two  Simple  Electric  Cells  Joined 
C.  Copper.     Z.  Zinc. 


material,  its  length,  and 
the  area  of  its  cross-sec- 
tion. In  accordance 
with  the  resistance  will 
depend  the  quantity  of 
electricity  that  a  given 
electro-motive  force  will 
press  through  in  a  unit 
of  time.  The  strength 
of  the  current  will  there- 
fore not  depend  entirely 
on  the  ratio  between  the 


on  the   electro-motive  force,   but,   rather, 
electro-motive  force  and  the  resistance. 

For  the  measurement  of  electric  quantities,  a  system  of  units  has 
been  devised.  The  unit  of  electro-motive  force  is  the  volt;  the  unit  of 
resistance  is  the  ohm,  i.  e.,  the  resistance  offered  by  a  column  of  mer- 
cury 106.3  cm-  l°n§  and  T  scl-  mm-  m  section  at  o°  C;  the  unit  of 
quantity  is  the  coulomb;  the  unit  of  time  is  one  second.  One  volt 
is  the  electro-motive  force  which,  when  steadily  applied,  will  press 
through  a  resistance  of  one  -ohm,  one  coulomb  of  electricity  in  one 
second  of  time  yielding  a  current  strength  of  one  ampere. 

This  relation  may  be  expressed  in  the  following  formula,  Ohm's 
law: 


C  (current  strength)  = 


[■.!■  i  tro  mol  iv,    fon  c  (F    M.  1  . ) 


-or  Ampere 


Voll 
(  >hm 


l-'.i  -1   lam  i-  i  R  ) 

In  practical  work  it  is  often  necessary  to  increase  the  strength  of  the 
current.     This  is  done  bv  uniting  two  or  more  cells  in  series,  *'.  e., 


PHYSIOLOGIC  APPARATUS. 


723 


uniting  the  copper  of  one  cell  to  the  zinc  of  a  second,  and  so  on  (Fig. 
364).  If  the  resistance  remains  the  same  the  total  voltage  is  that  of 
one  cell  multiplied  by  the  number  of  cells  united. 

The  cell  as  above  described  cannot  maintain  a  current  of  constant 
strength  for  any  length  of  time,  for  the  following  reasons: 

1.  The  sulphuric  acid  solution,  in  consequence  of  its  chemic  action, 
soon  becomes  nothing  more  than  a  saturated  solution  of  zinc  sulphate, 
after  which  its  chemic  activity  ceases.  The  current,  therefore,  soon 
diminishes  in  strength. 

2.  The  accumulation  of  hydrogen  bubbles  on  the  surface  of  the 
copper  hinders  the  passage  of  the  electricity.  In  a  short  time  they 
develop  a  current  in  the  opposite  direction,  which  also  tends  to  weaken 
the  original  current.     This  action  is  termed  polarization  of  the  elements. 

Cells  of  this  character  are 
not  suited  for  physiologic 
work,  in  which  constancv  in 
the  strength  of  the  current 
is  absolutely  necessary.  To 
overcome  these  disadvantages, 
cells  have  been  devised  which 
are  less  violent  in  action, 
which  prevent  polarization, 
and  which  maintain  a  current 
of  constant  strength  for  a  long 
period  of  time.  One  of  the 
most  generally  used  for  physi- 
ologic purposes  is— 

The  Daniell  cell.  This 
consists  of  a  porous  cup  con- 
taining a  saturated  solution 
of  CuS04,  copper  sulphate, 
in  which  is  immersed  a  copper 

plate  or  rod.  This  combination  is  placed  in  a  glass  vessel  containing 
a  solution  of  H2S04  (1:15).  In  this  solution  is  immersed  a  roll  of 
sheet  zinc  (Fig.  365).  Each  of  the  plates  is  provided  with  a  binding 
screw.  When  the  cell  is  in  action  the  sulphuric  acid  attacks  the 
zinc,  forming  zinc  sulphate,  and  liberates  hydrogen;  the  cup  being 
porous,  the  hydrogen  passes  into  the  copper  sulphate  solution,  where 
it  combines  with  the  sulphuric  acid  radicle,  and  liberates  metallic 
copper.  Polarization  of  the  copper  is  thus  prevented.  The  metallic 
copper  is  deposited  on  the  copper  plate,  which  is  thus  kept  bright. 
The  copper  sulphate  solution  is  kept  at  the  point  of  saturation  by 
packing  around  the  copper  cylinder  a  quantity  of  the  crystals  of  the 
salt.  The  sulphuric  acid  passes  back  into  the  porous  cup,  to  take  the 
place  of  that  used.  This  cell  is  remarkably  constant  for  these  reasons, 
and  well  adapted  for  physiologic  as  well  as  other  purposes  where  a 
current  of  uniform  strength  is  necessary. 


Fig.  365. — Daxiell  Cell. 


724 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  projecting  ends  of  the  copper  and  zinc  plates  are  termed  respec- 
tively the  positive  pole  or  anode,  and  the  negative  pole  or  kathode. 
The  electro-motive  force  of  a  Daniell  cell  is  practically  i  volt;  but  when 
the  two  poles  are  connected  by  a  wire  of  i  ohm  resistance,  the  current 
strength  will  be  less  than  i  ampere,  possibly  only  0.7,  owing  to  the 
resistance  offered  to  the  flow  of  electricity  by  the  fluids  between  the 
zinc  and  the  copper.  In  all  measurements,  the  internal  resistance 
of  the  cell  must  be  taken  into  consideration. 

The  Dry  Cell. — The  commercial  dry  cell  is  a  convenient  source  of 
electricity  for  general  laboratory  work.  It  consists  of  a  cup  of  zinc, 
the  inner  surface  of  which  is  covered  over  with  a  thick  layer  of  a  paste 
of  plaster-of-Paris,  saturated  with  ammonium  chlorid.     In  the  center 

of  the  cup  there  is  a  rod  of  carbon. 
Surrounding  this  rod  and  occupying 
the  space  between  it  and  the  plaster- 
of-Paris  paste,  is  a  mixture  of  man- 
ganese dioxid  and  charcoal.  The 
upper  surface  of  the  cell  is  sealed  to 
prevent  evaporation.  The  electricity 
is  generated  at  the  surface  of  the  zinc 
cup  by  the  chemic  action  of  the  chlorin 
which  arises  from  the  dissociation  of 
the  ammonium  chlorid.  When  the 
plates  are  united  by  a  conjunctive  wire 
the  current  within  the  cell  flows  from 
the  zinc  (the  positive  element)  to  the 
carbon  (the  negative  element),  and 
without  the  cell  from  the  carbon  (the 
positive  pole)  to  the  zinc  (the  nega- 
tive pole). 

Leads. — By   means    of    insulated 

wires  attached  to  the  poles  of  a  cell, 

the  electricity  may  be  conducted  from 

the    cell    and    used    for    exciting   or 

stimulating   purpose.       As   the   wires 

thus  become  practically  prolongations  of  the  plates  their  ends  become 

the  corresponding  poles.      In  experimental  work  the  ends  of  the  wires 

are  provided  with  special  devices,  termed — 

Non-polarizable  electrodes.  The  necessity  for  the  employmen 
of  such  electrodes  arises  from  the  fact  that  when  the  ends  of  the  wires 
from  a  cell  are  placed  in  direct  contact  with  the  tissues  chemic  changes 
arc  produced  in  a  short  time,  which  lead  to  their  polarization.  As  a 
result,  a  current  opposite  in  direction  to  that  of  the  cell  is  developed, 
which  tends  to  weaken  or  neutralize  it.  This  polarization  current 
vitiates  the  result  of  many  experiments  made  with  highly  irritable 
tissue  such  as  nerve-tissue.  Whether  for  stimulating  purposes  or  for 
I  he   purpose  of  detecting  the   existence  of  electric   currents  in   living 


Fig.  366. — Non-polarizable  Elec 
trodes.  1.  Du  Bois-Reymond's.  2 
Yon  Fleischl's.     3.  d'Arsonval's. 


PHYSIOLOGIC  APPARATUS. 


725 


tissues,  it  is  essential  that  the  electrodes  used  shall  be  non-polarizable. 
The  earliest  electrodes  of  this  character  were  made  by  du  Bois-Rey- 
mond  and  were  based  on  the  fact  discovered  by  Regnault  that  a  strip 
of  chemically  pure  zinc  or  amalgamated  zinc  (Matteucci)  immersed 
in  a  saturated  solution  of  zinc  sulphate  would  not  polarize.  One 
form  made  by  du  Bois-Reymond  is  shown  in  Fig.  366.  It  consists 
of  a  flattened  glass  tube  attached  to  a  universal  joint  and  supported 
by  an  insulated  brass  stand.  The  lower  end  of  the  tube  is  closed  with 
kaolin  or  China  clay  made  into  a  paste  with  a  0.6  per  cent,  solution  of 
sodium  chlorid.  It  can  be  moulded  into  any  desired  shape.  The 
interior  of  the  tube  is  partially  filled  with  a 
saturated  solution  of  sulphate  of  zinc  in 
which  is  immersed  the  strip  of  amalga- 
mated zinc.  To  the  upper  end  of  the  zinc 
the  conducting  wire  is  attached. 

The  v.  Fleischl  brush  electrode  is  similar 
to  the  preceding  except  that  the  end  of  the 
tube  is  closed  by  the  brush  of  a  camel's-hair 
pencil. 

The  d'Arsonval  electrode  consists  of  a 
glass  tube  containing  a  silver  rod  coated 
with  fused  silver  chlorid.  The  interior  of 
the  tube  is  filled  with  normal  salt  solution 
0.6  per  cent,  and  the  end  closed  with  a 
thread  or  plug  of  asbestos  which  is  made 
to  project  beyond  the  tube  for  a  short  dis- 
tance. 

Any  one  of  these  three  electrodes  is 
suitable  for  physiologic  experimentation,  as 
their  free  ends  neither  corrode  the  tissues 
nor  develop  electric  currents. 

Keys. — Muscle  and  nerve-tissues  are 
conductors  of  electricity.  When,  therefore, 
the  terminals  (the  non-polarizable  elec- 
trodes) of  the  wires  of  a  cell  are  placed  in 
contact  with  either  a  muscle  or  a  nerve  a 
circuit  is  made  through  which  a  current  of  electricity  flows;  when  one 
or  both  are  removed,  the  circuit  is  broken  and  the  current  ceases.  In 
practical  work  it  is  often  necessary  to  keep  the  electrodes  in  contact 
with  the  tissues  for  a  variable  length  of  time.  The  circuit,  however, 
may  be  alternately  made  and  broken  at  will  by  interposing  along  the 
return  wire  a  mechanic  contrivance  known  as  a  key,  of  which  there 
are  many  forms. 

The  du  Bois-Reymond  Friction  Key. — This  consists  of  a  plate 
of  vulcanite  attached  to  a  screw  clamp  by  which  it  can  be  fastened  to 
the  edge  of  a  table  (Fig.  367).  The  surface  of  the  vulcanite  plate 
carries  two  rectangular  blocks  of  brass,  each  of  which  has  two  holes 


Fin.  367. — Du  Rois-Rey- 
mond  Friction  Key. 


726 


TEXT-BOOK  OF  PHYSIOLOGY. 


2. 


drilled  through  it,  for  the  insertion  of  wires,  which  are  held  in  position 
by  small  screws.  A  movable  bridge  of  brass,  provided  with  an  ebonite 
handle,  serves  to  make  connection  between  the  blocks.  There  are 
two  ways  of  interposing  this  key  in  the  circuit. 

1.  As  a  Simple  Key. — For  this  purpose  one  of  the  wires,  usually  the 
negative,  is  carried  from  the  cell  to  one  block  and  then  continued 
from  the  second  block.  When  the  bridge  is  down,  the  circuit  is 
made  and  the  current  passes;  when  it  is  up,  the  circuit  is  broken. 
As  a  Short-circuiting  Key. — When  used  for  this  purpose,  the  wires 
of  the  cell  are  carried  to  the  inner  holes  of  each  block  and  then 
continued  from  the  outer  holes  to  the  tissues  or  to  some  form  of 
apparatus  which  it  is  desired  to  actuate.  When  the  key  is  closed 
i.  e.,  when  the  bridge  is  down,  the  current  on  reaching  the  key, 
will  divide,  one  portion  passing  across  the  bridge  and  so  back  to 

the    cell,  the  other  portion 
passing  to  the  tissue  or  ap- 
paratus.     The    amount    of 
the  current  which  is  returned 
to  the  cell  through  the  short 
circuit  will  be  proportional 
to    the    resistance    of    the 
longer  circuit.     As  the  latter 
is  usually  great  in  compar- 
ison with  the  former;  prac- 
tically   all    the    current    is 
short-circuited.      When  the 
bridge  is  lowered,  therefore, 
the  current  is  short-circuited ; 
when  it  is  raised,  the  current  flows  into  the  longer  circuit  through 
the  tissue  or  apparatus. 
The  Mercury  Key. — In  this  form  the  connection  is  established  by 
means  of  mercury.     It  consists  of  a  circular  block  in  the  center  of  which 
there  is  a  cup  containing  mercury   (Fig.  368).     At  opposite  points 
there  are  binding  posts,  one  of  which  is  provided  with  a  rigid  fixed 
copper  rod  passing  into  the  mercury;  the  other  is  provided  with  a 
movable  bent  rod  which  may  be  made  to  dip  into  or  be  withdrawn 
from  the  mercury  by  the  ebonite  handle. 

The  effect  of  a  constant  or  galvanic  current  on  a  muscle  or  nerve 
will  depend  to  some  extent  on  its  strength.  This  may  be  accurately 
regulated  by  means  of  an  apparatus  known  as — 

The  Rheocord.  With  this  apparatus  an  electric  current  may  be 
divided,  one  portion  continuing  through  a  conductor  back  to  the 
battery,  the  other  portion  being  sent  off  through  the  nerve.  The 
strengths  of  these  two  currents  are  inversely  proportional  to  the 
resistances  of  their  circuits.  A  simple  form  of  rheocord  (Fig.  369) 
consists  of  a  long  wire  arranged  for  convenience  in  parallel  lines  on  a 
small  wooden  base  and  connected  at  its  two  ends  with  binding  posts 


Fig.  368. — A  Mercury  Key. 


PHYSIOLOGIC  APPARATUS. 


727 


A  and  B.  The  resistance  of  this  wire,  1.6  ohms,  can  be  increased  by 
the  introduction  of  small  resistance  coils,  between  D  and  B,  varying 
from  5  to  20  ohms. 

The  two  binding  posts  A  and  B  are  connected  with  the  positive  and 
negative  poles  of  an  electric  cell  respectively.  A  simple  key  is  placed  in 
the  circuit. 

From  A,  a  wire  passes  to  one  of  the  electrodes  on  which  the  muscle 
or  nerve  rests.  A  second  wire  passes  from  the  second  electrode  to  a 
clamp  S,  by  way  of  the  binding  post  C,  which  can  be  fastened  to  the 
long  wire  at  any  given  point.  The  current,  on  reaching  A,  will  divide 
into  two  branches,  one  of  which  will  pass  along  the  wire  A,  B,  and 
thence  back  to  the  cell;  the  other  will  pass  through  the  nerve  and  back 
to  S  and  thence  also  to  the 
cell.  The  amount  of  current 
passing  through  the  nerve 
circuit  will  be  inversely  pro- 
portional to  the  resistance  of 
the  nerve  and  directly  propor- 
tional to  the  difference  of 
potential  between  A  and  S. 
If  S  is  close  to  A,  the  differ- 
ence of  potential  is  slight. 
If  S  is  removed  from  A  to- 
ward B,  the  difference  of 
potential  is  increased  and  the 
current  sent  through  the  nerve 
circuit  is  increased. 

In  many  experiments  it  is 
necessary  to  reverse  the  direc- 
tion of  the  current,  in  other 
experiments  to  deflect  it,  with- 
out changing  the  position  of 
the  electrodes.  Both  these 
results  may  be  accomplished  by  the  use  of — 

Pohl's  commutator.  This  is  a  round  block  of  wood  with  six 
cups,  each  of  which  is  in  connection  with  a  binding  post  (Fig.  370).  In 
each  of  the  two  cups  marked  1  and  2,  +  and  — ,  is  inserted  one  end 
of  a  copper  wire  bent  at  right  angles.  The  other  ends  of  the  wires 
are  supported  and  insulated  by  a  hard-rubber  handle.  To  the  top 
of  each  wire  is  soldered  a  semicircular  copper  wire.  This  arrange- 
ment permits  of  a  rocking  movement,  whereby  the  opposite  ends  of 
the  semicircular  wires  can  be  made  to  dip  into  cups  3  and  4,  and  into 
cups  5  and  6  alternately.  Two  wires  crossed  in  the  middle  of  the 
block  serve  to  connect  opposite  pairs  of  cups.  When  in  use,  the 
cups  are  filled  with  clean  mercury.  The  method  of  using  the  com- 
mutator is  as  follows: 
1.   As  a  Current  Reverser. — The  positive  and   negative   poles  of   the 


Fig.  369. — Rheocord. 


72S  TEXT-BOOK  OF  PHYSIOLOGY. 

electric  cell  are  connected  by  wires  with  binding  posts  i  and  2 
respectively.  A  key  is  interposed  in  the  circuit.  Wires  are  then 
carried  from  binding  posts  3  and  4  to  the  electrodes  in  connection 
with  the  muscle  or  nerve.  The  rocker  of  the  commutator  is  so 
turned  that  the  ends  of  the  semicircular  wires  dip  into  cups  3  and  4. 
The  direction  of  the  current  will  be  on  the  closure  of  the  circuit 
from  1  to  3,  then  from  3  along  a  wire  to  and  through  the  tissue 
and  back  to  4,  and  thence  to  the  cell.  If  the  position  of  the 
rocker  be  now  reversed  so  that  the  opposite  ends  of  the  semi- 
circular wires  dip  into  cups  5  and  6,  the  direction  of  the  current 
through  the  tissue  will  be  reversed.  The  positive  current,  after 
entering  binding  post  1,  will  flow  to  5;  then  along  one  of  the  cross 
wires  to  4;  then  along  a  wire  to  and  through  the  tissue  and  back 
to  3,  along  the  opposite  cross  wire  to  6.  thence  to  2  and  so  back 
to  the  cell. 


Fig.  370. — Pohl's  Commutator. 


A.  Arranged  as  a  current  reverser;  B,  as  a  cur- 
rent deflector. 


2.  As  a  Current  Deflector. — When  it  is  desirable  to  deflect  the  current 

to  two  pairs  of  electrodes  differently  situated,  wires  are  carried 

from  binding  posts  3  and  4  to  one  pair,  and  from  5  and  6  to  the 

other  pair.     The  cross  wires  are  then  removed.     According  to 

the  position  of  the  rocker  the  current  will  be  deflected  to  one  or 

the  other. 

The  Inductorium. — This  is  an  apparatus  designed  for  the  purpose 

of  obtaining  single  or  rapidly  succeeding  electric  currents  by  induction. 

Its  construction   is  based  on  facts  discovered  by  Faraday,  some  of 

which  are  the  following: 

If  two  circuits,  a  primary  and  a  secondary,  are  placed  parallel  to 
each  other,  the  former  connected  with  a  galvanic  cell,  the  latter  with 
a  galvanometer,  it  is  found  that,  at  the  moment  the  primary  circuit  is 
made,  and  at  the  moment  it  is  broken,  a  current  is  induced  in  the 
secondary  circuit,  as  shown  by  a  momentary  deflection  of  the  galvano- 
meter needle.  During  the  continuous  How  of  the  current  through  the 
primary  circuit   there   is  no  evidence  of  a  current   in   the  secondary 


PHYSIOLOGIC  APPARATUS. 


729 


circuit.  The  induced  current  is  but  of  momentary  duration.  The 
current  flowing  through  the  primary  circuit  is  termed  the  inducing, 
the  current  flowing  through  the  secondary  circuit  the  induced  current. 

The  induced  current  is  opposite  in  direction  to  that  of  the  inducing 
current  when  the  circuit  is  made  or  closed;  it  is  in  the  same  direction, 
however,  when  the  circuit  is  broken  or  opened. 

If  the  circuits  are  arranged  in  the  form  of  coils,  it  is  found  that, 
other  things  being  equal,  the  strength  of  the  induced  currents  will  be 
proportional  to  the  number  of  turns  in  the  coils. 

If  the  coils  are  placed  at  varying  distances  from  each  other,  the 
strength  of  the  induced  current  varies,  increasing  as  the  coils  are 
approximated,  decreasing  as  they  are  separated. 

Approximation  or  separation  of  the  coils  while  the  current  is  flowing 
through  the  primary  circuit  develops  an  induced  current,  which  dis- 
appears, however, 
the  moment  the 
movement  of  the 
coil  ceases.  A 
sudden  increase  or 
decrease  in  the 
strength  of  the  in- 
ducing current  also 
develops  an  in- 
duced current. 

When  the  coils 
are  approximated 
or  the  primary  cur- 
rent increased  in 
strength,  the  in- 
duced current  is 
opposite  in  direc- 
tion to  that  of  the 
inducing  cur-rent ; 
with  the  reverse  conditions,  the  induced  current  has  the  same  direction. 

The  induced  currents  have  been  termed,  in  honor  of  their  discoverer, 
Faradic  currents. 

The  du  Bois-Reymond  inductorium,  based  on  the  foregoing 
facts,  consists  essentially  of  two  coils  of  insulated  copper  wire,  termed 
primary  and  secondary  (Fig.  371). 

The  primary  coil,  R',  consists  of  thick  copper  wire  wound  around 
a  wooden  spool  attached  to  a  vertical  support.  The  beginning  of  this 
coil  is  at  the  binding  post  S",  its  termination  either  at  binding  post 
P"  or  S'".  In  the  course  of  this  primary  wire  or  circuit,  there  are 
placed  two  vertical  bars  of  soft  iron,  B',  connected  at  their  bases  to 
form  a  horseshoe  magnet,  around  the  ends  of  which  the  wire  is  coiled. 
The  object  of  this  device  will  be  explained  later. 

Inside  the  primary  coil  there  is  placed  a  bundle  of  soft  iron  wires, 


Fiq.  371- — Ixductoritjji  of  du  Bois-Reyiioxd.     R',  Pri- 
mary, R".  secondary  spiral.     B.   Board  on  which  R"  moves. 

1.  Scale.      -\ .  Wires    from    battery.     P',    P".  Pillars.     H. 

Neef's  hammer.  B'.  Electro-magnet.  S'.  Binding  screw 
touching  the  steel  spring  (H).  S"  and  S'".  Binding  screws  to 
which  to  attach  wires  where  Xeef's  hammer  is  not  required. 


73o  TEXT-BOOK  OF  PHYSIOLOGY. 

which,  as  soon  as  the  circuit  is  made,  become  magnetized,  with  the 
effect  of  increasing  the  action  of  the  inducing  current. 

The  secondary  coil,  R",  consists  of  a  much  greater  number  of  turns 
of  a  finer  copper  wire,  the  ratio  being  about  40  to  1,  also  wound  around 
a  spool,  having  a  tunnel  sufficiently  large  to  enable  it  to  slide  over  the 
primary.  By  these  means  the  strength  of  the  induced  current  is 
increased.  As  a  result  of  the  construction  of  the  inductorium,  the  low 
electro-motive  force  of  the  cell  is  transformed  into  the  high  electro- 
motive force  characteristic  of  the  induced  current.  As  the  number 
of  turns  of  wire  in  the  secondary  coil  is  to  the  number  in  the  primary, 
so  are  the  electro-motive  forces  in  the  secondary  coil  to  those  in  the 
primary  coil. 

The  secondary  coil  slides  along  a  track,  B,  which  permits  it  to  be 
moved  toward  or  away  from  the  primary.  The  distance  between  the 
two  coils  can  be  measured  and  the  strength  of  the  induced  current 
again  reproduced,  other  things  being  equal,  by  means  of  a  centimeter- 
millimeter  scale  pasted  on  the  edge  of  B. 

The  ends  of  the  wire  of  the  secondary  coil  are  fastened  to  two 
binding  posts  to  which  conducting  wires  provided  with  hand  electrodes 
can  be  attached. 

The  inductorium  may  be  used  for  obtaining  either  a  single  current 
or  a  series  of  rapidly  repeated  induced  currents. 

The  Single  Induced  Current. — On  account  of  its  high  electro-motive 
force,  its  penetrative  power,  and  short  duration,  the  single  induced 
current  is  a  most  convenient  and  suitable  form  of  stimulus  for  many 
purposes.  In  order  to  obtain  such  a  current,  the  positive  wire  of  the 
cell  is  carried  to  binding  post  S",  and  the  negative  wire  either  to  S'" 
or  P".  A  key  is  placed  in  the  primary  circuit.  The  course  of  the 
current  will  then  be  on  the  closure  of  the  circuit  from  the  cell  to  S", 
thence  around  R'  to  S'",  and  so  back  to  the  cell;  or  if  the  negative 
wire  is  connected  with  P",  the  course  of  the  current  on  leaving  R' 
will  be  through  the  coils  surrounding  the  two  vertical  bars  B',  thence 
to  P",  and  so  back  to  the  cell.  If  the  secondary  coil  be  placed  close 
to  the  primary  and  the  wires  of  the  secondary  brought  into  contact 
with  a  muscle,  it  will  be  found  that  with  both  the  make  and  the  break 
of  the  primary  circuit  a  current  is  induced  in  the  secondary,  as  shown 
by  a  short  quick  pulsation  of  the  muscle;  but  during  the  time  of  closure 
of  the  circuit,  the  induced  current  is  wanting,  as  shown  by  the  quiescent 
condition  of  the  muscle.  It  will  be  apparent,  however,  from  the 
energy  of  the  contraction  that  the  break  induced  current  is  a  more 
efficient  stimulus  than  the  make  induced  current.  That  this  is  the 
case  is  made  evident  by  removing  the  secondary  to  the  end  of  the  slide- 
way  and  then  gradually  bringing  it  toward  the  primary  half  a  centi- 
meter at  a  time,  making  and  breaking  the  circuit  after  each  move- 
ment until  a  pulsation  of  the  muscle  occurs.  It  will  be  found  to 
occur  first  on  the  break  of  the  circuit.  As  the  secondary  approaches 
the  primary  a  position  will  be  reached  when  a  pulsation  occurs  on 


PHYSIOLOGIC  APPARATUS.  731 

the  make  as  well  as  on  the  break  of  the  circuit,  though  it  will  be  less 
pronounced. 

The  explanation  offered  for  this  difference  in  the  strength  of  the  two  induced  currents 
is  as  follows:  With  the  make  of  the  circuit  and  the  passage  of  the  battery  current  through 
the  primary  coil  there  is  induced  in  the  neighboring  and  parallel  turns  of  the  wire  an  extra 
current  opposite  in  direction  to  the  primary  current.  This  extra  or  self-induced  current 
antagonizes  and  prevents  the  current  from  attaining  its  maximum  development  as  quickly 
as  it  otherwise  would,  and  therefore  its  efficiency  as  an  inducer  of  a  current  in  the  secondary 
is  diminished.  On  the  break  of  the  circuit  the  primary  current  disappears  quickly,  and 
as  there  is  nothing  to  retard  its  disappearance  its  efficiency  as  an  inducer  of  a  current  in 
the  secondary  coil  is  not  diminished.  It  is  not  infrequently  stated  that  the  disappearance 
of  the  primary  current  induces  in  the  neighboring  coils  a  break  extra  current  corresponding 
in  direction  which  assists  in  the  development  of  the  induced  current.  This  is  not  the  case, 
however,  as  no  break  extra  current  is  developed  in  the  inductorium  as  ordinarily  used 
when  actuated  by  a  battery  current  of  moderate  strength. 

As  it  is  not  so  much  the  intensity  of  the  current  as  it  is  rapid  variations  in  intensity 
•that  produce  effects,  it  is  readily  apparent  why  the  induced  current  developed  at  the 
break  of  the  primary  is  more  effective  as  a  stimulus  than  the  induced  current  developed 
at  the  make  of  the  primary  circuit.  The  quantity  of  the  electricity  is,  however,  the  same 
in  both  cases. 

If  the  secondary  be  pushed  further  along  the  slideway  until  it 
largely  covers  the  primary  coil,  a  position  will  be  reached  when  the 
make  induced  current  equals  in  its  efficiency  as  a  stimulus  the  break 
induced  current;  and  if  the  secondary  be  yet  further  advanced,  a 
position  is  reached  when  the  make  induced  current  becomes  more 
powerful  and  efficient  than  the  break  induced  current,  as  shown  by 
the  greater  contraction  of  the  muscle.  This  result  is  explained  by 
the  fact  that  the  make  extra  current  is  now  able  of  itself  to  induce  a 
current  in  the  secondary  coil,  on  account  of  its  proximity,  which,  added 
to  that  induced  by  the  battery  current,  produces  a  current,  greater 
than  that  induced  on  the  break  of  the  circuit.* 

Rapidly  Repeated  Induced  Currents. — As  the  single  induced  current 
is  of  extremely  short  duration,  it  is  inefficient  as  a  stimulus  in  the  con- 
duct of  many  experiments.  It  is  necessary,  therefore,  to  develop  it 
with  a  frequency  that  is  sufficient  to  give  rise  to  a  summation  of  effects. 
The  duration  of  the  stimulation  may  be  thus  considerably  prolonged. 
This  is  accomplished  by  introducing  in  the  primary  circuit  close  to  the 
primary  coil  an  automatic  interrupter,  usually  Neef's  modification  of 
Wagner's  hammer  (Fig.  371).  This  consists  of  a  vertical  post,  P', 
to  the  top  of  which  is  fastened  a  metallic  spring  carrying  at  its  opposite 
end  a  steel  or  iron  hammer,  H,  which  hangs  over,  but  does  not  touch, 
the  two  vertical  bars  of  soft  iron  around  which  the  wire  of  the  primary 
coil  is  wound.  About  the  middle  of  the  spring  on  its  upper  surface 
there  is  a  small  plate  of  platinum  which  is  in  contact  with  an  adjustable, 
platinum-tipped  screw,  S',  carried  by  a  plate  of  brass  in  connection 
with  binding  post  S". 

For  the  purpose  of  interrupting  the  primary  circuit  frequently  in  a 
unit  of  time,  and  thus  developing  induced  currents  in  quick  succession, 
the  apparatus  is  arranged  in  the  following  way:     The  positive  and 

*  "  On  certain  peculiarities  of  the  inductorium,"  Prof.  Colin  C.  Stewart,  "  Univ.  Pa. 
Medical  Bulletin,"  Feb.,  1904. 


732  TEXT-BOOK  OF  PHYSIOLOGY. 

negative  poles  of  the  electric  cell  are  connected  by  wires  with  binding 
posts  P'  and  P",  a  key  being  interposed  in  the  circuit.  If  the  screw 
S'  is  in  contact  with  the  spring,  the  current  on  the  closure  of  the  circuit 
will  enter  P',  pass  along  the  spring  to  S',  thence  into  and  through  the 
primary  coil  R',  to  the  coils  surrounding  the  vertical  bars  B',  then  to 
P",  and  so  back  to  the  cell. 

As  the  current  passes  around  the  vertical  bars,  they  are  magnetized. 
The  magnetization  draws  down  the  hammer,  and,  in  so  doing,  breaks 

the  circuit  at  the  tip  of  the  screw,  S'. 
The  vertical  bars  are  at  once  demag- 
netized, and  the  hammer  is  restored 
to  its  original  position  by  the  elasticity 
of  the  spring.  The  circuit  is  thus 
re-established,  the  current  flows 
through  the  coils,  the  bars  are  again 
magnetized,  the  hammer  is  drawn 
down,  to  be  followed  by  a  second  break 
of  the  circuit. 

The  number  of  times  the  circuit  is 
thus  made  and  broken  per  second  will 
vary  with  the  length  of  the  spring. 

As  each  interruption  of  the  primary 
circuit  develops  an  induced  current, 
it  follows  that  the  latter  must  succeed 
each  other  with  a  frequency  corres- 
ponding with  the  frequency  of  the 
former.  If  while  the  primary  circuit 
is  thus  being  interrupted  the  wires  of 
the  secondary  coil  be  placed  in  con- 
tact with  a  muscle,  the  induced  current  will  give  rise  to  contractions 
which  will  succeed  each  other  so  rapidly  that  they  fuse  together, 
producing  a  spasm  or  tetanus  of  the  muscle.  For  this  reason  these 
currents  are  frequently  spoken  of  as  tetanizing  currents,  and  the  pro- 
cedure as  tetanization  or  Faradization.  These  currents  also  increase 
in  strength  as  the  secondary  approaches  the  primary. 


Fig.  372. — Helmholtz's  Modifi- 
cation of  Neef's  Hammer.  As 
long  as  c  is  not  in  contact  with  d, 
g  h  remains  magnetic;  thus  c  is  at- 
tracted to  d  and  a  secondary  circuit, 
a,  b,  c,  d,  e,  is  formed;  c  then  springs 
back  again,  and  thus  the  process 
goes  on.  A  new  wire  is  introduced 
to  connect  a  with  /.     K.  Battery. 


Helmholtz's  Modification  of  the  Inductorium. — With  a  view  of  equalizing  the 
strengths  of  the  induced  currents,  Helmholtz  suggested  a  device  the  adoption  of  which 
accomplishes  this  to  a  certain  extent.  It  consists  (Fig.  372)  in  connecting  with  a  wire 
binding  posts  P'  and  S",  and  in  providing  binding  post  P"  with  an  adjustable  screw  which 
can  be  raised  until  the  spring  comes  in  contact  with  it,  when  the  hammer  is  drawn  down 
by  the  electromagnet  B'.  This  latter  arrangement  is  practically  a  short-circuiting  key  by 
which  a  portion  of  the  current  is  returned  to  the  cell  without  ever  entering  the  primary  coil. 
'I  b.i  nne  arrangement,  though  differently  lettered,  is  shown  in  Fig.  371  By  the  use  of 
the  entire  device  the  changes  in  the  primary  coil  are  made  not  by  making  and  breaking 
the  primary  current,  but  by  alternately  long-  and  short-circuiting  the  current.  "When  the 
short  -<  ircuiting  key  is  opened,  the  full  volume  of  the  primary  current  flows  through  the  pri- 
mal 1  oil.  When  the  short-circuiting  key  is  closed,  most  of  the  current  fails  to  enter  the 
coil,  taking  tin-  easier  path  through  the  key.  Some  of  the  current,  however,  always  flows 
through  (he  coil  and  is  never  diverted.  The  cycle  of  changes  in  the  electric  condition  of 
the  primary  coil  is  thus  altered  for  two  reasons: 


PHYSIOLOGIC  APPARATUS. 


733 


"First,  we  no  longer  have  an  alternation  between  a  full  primary  current  and  Done  at  al 
— rather  an  alternation  between  a  full  primary  current  and  a  weaker  one.     The  difference 
in  the  phases  is  thus  lessened,  the  extent  of  the  change  on  making  and  breaking  is  lessened , 
and  correspondingly  the  efficiency  of  the  make  and  break  currents  induced  in  the  secondary 
coil  is  slightly  decreased. 

"Second,'on  making  the  primary  current,  as  in  the  ordinary  coil,  the  sudden  appearance 
of  the  primary  current  is  antagonized  by  the  opposing  make  extra  current,  with  the  result 
that  the  make  induced  current  is  still  further  reduced;  while  on  breaking  the  current  the 
break  extra  current  can  now  flow  through  the  primary  coil  across  the  short-circuiting  key. 
This  current,  trailing  behind  the  disappearing  primary  current  in  the  same  direction,  pro- 
duces the  same  effect  as  if  the  primary  current  itself  were  to  disappear  slowly.  As  a  result 
the  disappearance  of  the  primary  current  loses  its  former  efficiency  as  an  inducer  of  second- 
ary currents,  and  the  break  induction  current  is  reduced  to  about  the  efficiency  of  the  make. 
'  "This  so-called  'equalizing'  of  the  make  and  break  induced  currents  is  never  perfect, 
if  for  no  other  reason,  because  the  make  extra  current  must  take  the  long  circuit  through 
the  battery,  while  the  break  extra  current  has  an  easier  path  through  the  short-circuiting 
kev,  and  is  thus  greater  than  the  make  extra  current."     (C.  C.  Stewart."! 


THE  GRAPHIC  METHOD. 

The  term  graphic  is  applied  to  a  method  by  which  curves  or  tracings 
are  obtained  which  represent  the  extent,  duration,  and  time  relations  of 
the  movements  accompanying  physiologic  processes.     If  these  move- 
ments can  be  trans- 
lated  in   one  direc- 
tion,   they    may    be 
recorded  in  different 
ways : 

i.  By  attaching  the 
moving  struc- 
ture —  e.  g . , 
heart,  muscle, 
etc. — to  a  deli- 
cate lever  the 
free  extremity 
of  which  is  pro- 
vided   with     a 


Fig. 


-A  Receiving  Tambour. 


writing  point. 
2.  By  transmitting  the  movement  through  a  column  of  air  enclosed 
in  a  rubber  tube  the  two  ends  of  which  are  attached  to  a  metallic 
capsule,  covered  by  a  rubber  membrane,  termed  a  drum  or  tam- 
bour. When  the  membrane  of  the  first  tambour  is  pressed  or 
driven  inward,  the  air  is  forced  through  the  rubber  tube  into  the 
second  tambour  and  its  membrane  is  pushed  outward.  As 
soon  as  the  primary  pressure  is  removed,  the  membranes  return 
to  their  former  condition.  If  the  membrane  of  the  first  tambour 
is  drawn  outward,  the  air  in  the  system  is  rarefied  and  the  mem- 
brane of  the  second  tambour  is  pressed  inward.  For  the  purpose 
of  registering  the  movement  transmitted  by  the  column  of  air, 
the  second  tambour  is  provided  with  a  light  lever  supported  by  a 
vertical  bearing  resting  on  a  small  metallic  disk.  The  mem- 
brane of  the  first  tambour  is  frequently  provided  with  a  button, 


734 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  is  placed  over  the  moving  structure.  The  inward  move- 
ment of  the  membrane  of  the  first  tambour  produces  an  outward 
movement  of  the  membrane  of  the  second  tambour,  indicated, 
though  magnified,  by  the  rise  of  the  free  end  of  the  lever.  The 
reverse  movement  of  the  membrane  is  attended  by  a  fall  of  the 
lever.  The  first  tambour  is  termed  the  receiving,  the  second  the 
recording  tambour  (Figs.  373,  374). 
3.  By  enclosing  an  organ — e.  g.,  kidney,  spleen,  arm,  finger,  etc. — 
in  a  rigid  glass  or  metal  vessel  which  at  one  point  is  in  communic- 
ation with  a  recording  apparatus — e.  g.,  (1)  a  piston  provided 
with  a  lever  (page  483);  or  (2)  a  tambour  and  lever  (page  336); 
or  (3)  a  mercurial  manometer  carrying  a  float  and  pen  (page  341). 
The  space  between  the  part  investigated  and  the  vessel  is  filled 
with  fluid.  The  variations  in  volume  of  the  organ  cause  a  dis- 
placement of  the  fluid  and  give  rise  to  a  to-and-fro  movement 
which  is  taken  up  and  reproduced  by  the  recording  apparatus. 
The  writing  point  may  be  (1)  some  form  of  pen  carrying  ink  which 
records  the  movement  on  a  white  paper  surface,  or  (2)   a  piece  of 

metal,  glass,  or  paper 
wdiich  records  the 
movement  on  smoked 
paper  or  glass. 

The  Recording 
Surface. — The  sur- 
face which  receives 
and  records  the  move- 
ments of  a  pen  or 
lever  is  usually  that  of 
a  cylinder  which  is  covered  with  glazed  paper  and  coated  with  a  thin 
layer  of  soot,  obtained  by  passing  the  cylinder  through  the  flame  of 
a  gas  burner.  The  axis  of  the  cylinder  is  supported  by  a  metal  frame- 
work. If  the  writing  point  of  the  lever  be  placed  against  the  cylinder 
and  a  movement  be  imparted  to  it,  a  portion  of  the  soot  is  rubbed  off, 
leaving  a  white  line  behind.  If  the  cylinder  be  stationary,  the  rise  and 
fall  of  the  lever  are  recorded  as  a  vertical  line.  Such  a  record  shows 
only  the  extent  of  a  movement.  If  the  cylinder  is  traveling,  however, 
at  a  uniform  rate,  the  rise  and  fall  of  the  lever  are  recorded  in  the  form 
of  'a  curve  the  width  of  the  two  arms  of  which  will  depend  partly  on 
the  rapidity  of  the  movement  of  the  lever  and  partly  on  the  rate  of 
movement  of  the  cylinder.  The  cylinder  movement  is  initiated  and 
maintained  by  clock-work  or  by  the  transmission  of  power  by  belting 
to  a  system  of  pulleys  in  connection  with  its  axis.  As  the  tracing  is 
wave-like  in  form,  the  cylinder  is  frequently  spoken  of  as  a  kymograph 
or  wave  recorder  (Fig.  375). 

From  the  record  thus  obtained  it  is  possible  to  determine  not  only 
the  extent  but  also  the  duration,  the  form,  and  the  rate  of  recurrence 
of  any  given  movement. 


Fig.  374. — A  Recording  Tambour. — (Marey.) 


PHYSIOLOGIC  APPARATUS. 


735 


The  Extent  of  a  Movement. — As  the  lever  not  only  takes  up  and 
reproduces  a  movement,  but  at  the  same  time  magnifies  it,  it  is  essential 
that  the  degree  of  magnification  be  known,  in  order  to  determine  the 
actual  extent  of  the  movement.  The  magnification  of  the  lever  is 
readily  determined  by  dividing  the  distance  between  the  axis  of  the 
lever  and  its  writing  point  by  the  distance  between  the  axis  and  the 
point  of  attachment  of  the  structure,  and  then  dividing  the  height  of 
the  tracing  by  this  quotient.  The  final  quotient  represents  the  extent 
of  the  movement. 

The  Time  Relations  of  a  Movement. — When  recorded  in  the  form 
of  a  curve,  the  duration  of  the  entire  movement,  or  of  any  one  portion 
of  it,  can  be  determined  by 
means  of  a  time  marking  or 
chronographic  apparatus,  con- 
sisting of  (i)  a  small  signal 
magnet  provided  with  a  mov- 
able armature,  to  which  is 
attached  a  writing  style;  (2) 
an  automatic  interrupter;  and 
(3)  an  electric  cell. 

The  Signal  Magnet.— 
The  magnet  (Fig.  376)  is 
actuated  by  the  electric  cur- 
rent made  and  broken  at 
regular  and  known  intervals 
by  an  automatically-acting  in- 
terrupter placed  in  the  circuit. 
With  each  make  and  break  of 
the  circuit  the  armature  and 
style  move  alternately  down- 
ward and  upward.  The  ex- 
cursion of  the  style  can  be 
readily  recorded  on  a  travel- 
ing surface.  The  character 
and  number  of  the  interrup- 
tions per  second  will  deter- 
mine the  character  of  the 
tracing.  If  they  occur  in  a 
rhythmic  manner,  the  tracing 
will  be  sinus  oidal  or  wave- 
like in  form.  If  the  time  of  interruption  is  of  short  duration  as 
compared  with  the  time  of  closure  of  the  circuit,  the  tracing  will  be  a 
horizontal  line  with  short  vertical  elevations  at  regular  intervals. 

The  Automatic  Interrupter. — The  circuit  may  be  interrupted  by 
vibrating  reeds,  tuning-forks,  metronomes,  etc.  A  well-known  form 
of  vibrating  reed  is  shown  in  Fig.  377.  This  consists  of  a  metallic 
frame  carrving  a  coil  of  wire  in  the  center  of  which  there  is  a  core  of 


Fig.    375. — Kymograph. 
zold,  Leipzig.) 


(Boruttau's,    Pet- 


736 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  376. — Signal  Magnet. 


soft  iron.  To  the  vertical  part  of  the  frame  there  is  fastened  the  reed, 
the  distal  end  of  which  is  bent  to  dip  into  an  adjustable  mercury  cup. 
When  in  circuit  the  current  enters  the  coil,  then  flows  into  and  through 
the  frame  and  the  reed  to  the  mercury,  and  thence  back  to  the  cell. 
On  the  closure  of  the  circuit  and  the  magnetization  of  the  iron  core 
the  reed  is  withdrawn  from  the  mercury,  the  circuit  broken,  and  the 
core  demagnetized.  The  elasticity  of  the  spring  returns  it  to  the 
mercury,  when  the  circuit  is  again  restored.  The  reed  may  be  so 
constructed  that  it  will  be  raised  and  lowered  50,  100,  or  200  times  a 

second.     The   armature 
of  the  signal  magnet  un- 
dergoes a  corresponding 
number     of     elevations 
and  depressions.     If  the 
reed  vibrates  100   times 
in  a  second,  the  distance 
from  crest  to  crest  of  the 
wave  tracing  will  represent  y^  of  a  second.     Interrupters  of  various 
kinds  have  been  devised  which  make  and  break  the  circuit  from  1  to 
250  times  a  second. 

Moist  Chamber. — In  many  experiments,  it  is  necessary  to  keep  the 
nerve  or  muscle  preparation  in  a  uniformly  moist  atmosphere.  To  se- 
cure this,  a  moist  chamber  is  employed  (Fig.  378).  This  consists  of  a 
hard-rubber  platform,  supported  by  a  piece  of  brass,  which  slides  up 
and  down  a  vertical  rod,  and  which  can  be  clamped  at  any  height.  By 
means  of  a  short  lever  the 
vertical  rod  can  be  turned, 
carrying  the  platform  from 
side  to  side.  The  rod  is 
secured  to  a  firm  iron  base. 
Six  double  binding  posts 
for  the  attachment  of  wires 
pass  through  the  platform. 
Near  the  side  of  the  upper 
surface  of  the  platform 
there  rises  a  vertical  rod, 
carrying  a  clamp  for  hold- 
ing the  femur  of  a  nerve- 
muscle  preparation,  as  well 
as  a  horizontal  rod  for  supporting  three  pairs  of  non-polarizablc 
electrodes.  A  groove  around  the  outer  edge  of  the  platform  receives  a 
glass  shade,  which  covers  the  whole.  The  air  of  the  chamber  is  kept 
moist  by  placing  in  it  pieces  of  blotting-paper  saturated  with  water. 

From  the  under  surface  of  the  platform  there  descends  a  rod,  which, 
by  means  of  a  double  binding  screw,  supports  a  horizontal  rod,  modi- 
fied at  one  end  to  carry  the  delicate  axis  of  a  light  stiff  recording  lever. 
The  end  of  this  lever  is  pointed,  to  enable  it  to  write  on  a  smoked  glass 


377 


-Page's  Vibrating  Reed. 
modification.) 


(Reichert's 


PHYSIOLOGIC  APPARATUS. 


737 


or  paper.  Beneath  the  axis  is  a  strip  of  brass,  carrying  a  screw,  which 
gives  support  to  the  lever  until  the  instant  the  contraction  of  the 
muscle  begins.  This  screw,  the  after-loading  screw,  also  enables 
the  lever  to  be  placed  in  a  horizontal  position.  The  portion  of  the 
lever  near  the  axis  is  provided  with  a  double  hook,  the  lower  portion 
of  which  serves  for  the  attachment  of  the  weight  by  which  the  muscle 
is  counterpoised. 

In  some  experiments,  as  in  the  registration  of  a  muscle  contraction 
under  varying  conditions,  it  is  necessary  to  give  the  lever  mass  by 
attaching  weights  directly  beneath  the  muscle.  This,  however, 
introduces  certain  errors  in  the  movements  of  the  lever,  which  some- 
what deform  what  would  otherwise  be  the  normal  curve.  If  the 
weight  be  attached,  not  opposite  to  the  muscle  attachment,  but  close 


Moist  Chamber. 


to  the  axis  of  the  lever,  the  undesirable  acceleration  of  the  lever  move- 
ment, during  both  contraction  and  relaxation,  is  largely  prevented. 
The  lever  may  be  a  straw,  a  strip  of  celluloid  or  aluminium.  It  should 
be  as  light  as  possible.  The  writing  point  may  be  made  of  stiff  paper, 
a  piece  of  tinsel,  glass  or  aluminium.  It  should  have  sufficient  elas- 
ticity to  keep  it  in  contact  with  the  cylinder  during  the  excursion  of  the 
lever.  The  writing  point  should  be  placed  as  nearly  parallel  as  possible 
to  the  surface  of  the  cylinder. 

Normal  Saline  Solution. — To  prevent  drying  and  a  loss  of  irrita- 
bility the  tissue  under  investigation  should  be  kept  moist  with  the  nor- 
mal saline  solution  (NaCl  0.6  per  cent.).     This  solution  very  largely 
47 


TEXT-BOOK  OF   PHYSIOLOGY. 


prevents  either  absorption  or  extraction  of  water  from  the  tissues  and 
thus  retards  chemic  changes  in  their  composition. 

Ringer's  solution,  largely  used  for  the  same  purpose,  is  made  by 
saturating  0.65  per  cent.  NaCl  solution  with  calcium  phosphate  and 
then  adding  2  c.c.  of  a  1  per  cent,  solution  of  potassium  chlorid  to 
each  100  c.c. 

The  Galvanometer  and  Capillary  Electrometer. — In  the  de- 
tection and  investigation  of  the  electric  currents  of  muscles,  nerves, 
and  other  tissues,  the  physiologist  is  limited  to  the  galvanometer  and 
capillary  electrometer.  The  principle  of  the  galvanometer  is  based 
on  the  fact  that  an  electric  current  flowing  through  a  wire  parallel  in 
direction  with  a  magnetic   needle  will  tend  to  set  the  needle  at  right 

angles  to  the  direc- 
tion of  the  current. 
The  essential  requi- 
site of  any  galvano- 
meter used  for  physi- 
ologic purposes  is 
that  it  will  respond 
quickly  to  the  influ- 
ence of  extremely 
weak  currents.  This 
is  realized  by  the  use 
of  small  light  needles, 
the  adoption  of  the 
astatic  system,  or 
some  similar  device 
by  which  the  direc- 
tive influence  of  the 
earth's  magnetism  is 
eliminated,  and  the 
multiplication  of  the 
number  of  turns  of  the  wire  in  the  coils  which  surround  the  needle. 

The  tangent  galvanometer,  or  boussole,  as  constructed  by  Wiede- 
mann, is  the  form  most  frequently  employed  in  physiologic  investi- 
gations (Fig.  379).  It  consists  primarily  of  a  thick  copper  cylinder, 
through  which  a  tunnel  has  been  bored.  Within  this  tunnel  is  sus- 
pended  a  magnetized  ring,  just  large  enough  to  swing  clear  of  the  sides 
of  the  chamber.  The  object  of  making  the  magnet  ring-shaped  is 
to  increase  its  strength  in  proportion  to  its  size,  and  to  get  rid  of  the 
central  inactive  part.  Connected  with  and  passing  upward  from  the 
magnetized  ring  through  the  copper  cylinder  is  an  aluminium  rod, 
surmounted  by  a  circular  plane  mirror.  Above  the  mirror  rises  a 
glass  tube,  which  carries  on  top,  on  an  ebonite  support,  a  little  wind- 
lass, capable  of  being  centered  by  three  small  screws.  On  the  wind- 
lass is  wound  a  single  filament  of  silk,  which  passes  down  the  tube 
and  is  attached  to  the  mirror.     The  magnet  can,  by  this  contrivance, 


Fig.  379. — Wiedemann's  Boussole. 


PHYSIOLOGIC  APPARATUS.  739 

be  raised  or  lowered  and  centered  in  the  copper  chamber.  Deflections 
of  the  mirror  from  currents  of  air  are  prevented  by  inclosing  it  with  a 
brass  cover  provided  with  a  glass  window.  The  coils  are  placed  on 
each  side  of  the  copper  chamber,  and  supported  by  a  rod,  on  which 
they  slide.  By  this  arrangement  they  can  be  approximated  until 
they  meet  and  completely  conceal  the  cylinder.  By  varying  the 
position  of  the  coils  the  influence  of  the  current  upon  the  needle  can  be 
increased  or  diminished.  An  advantage  which  this  galvanometer 
possesses  is  the  damping  of  the  oscillation  of  the  needle,  so  that  it 
quickly  comes  to  rest  after  deflection.  This  is  accomplished  by  the 
development  of  induction  currents  in  the  copper  cylinder,  the  direction 
of  which  is  opposite  to  that  of  the  movement  of  the  needle.  The 
instrument,  therefore,  is  aperiodic — that  is  to  say,  when  the  needle 
is  influenced  by  a  current  it  moves  comparatively  slowly  until  the 
maximum  deflection  is  reached,  when  it  comes  to  rest  without  oscilla- 
tions. When  the  circuit  is  broken  the  needle  swings  slowly  back  to 
zero,  and  again  comes  to  rest  without  oscillations. 

Inasmuch  as  the  needle  is  not  astatic,  it  is  rendered  so  by  the  use 
of  an  accessory  magnet — the  so-called  Hauy's  bar.  This  magnet, 
supported  by  a  rod  directed  perpendicular  to  the  coils,  is  placed  in  the 
magnetic  meridian,  horizontal  to  the  needle,  with  its  north  pole  point- 
ing north.  By  sliding  the  magnet  toward  the  needle  the  directive 
influence  of  the  earth's  magnetism  is  gradually  diminished,  and  when 
it  is  reduced  to  a  minimum  the  needle  acquires  its  highest  degree  of 
instability.  By  means  of  a  pulley  an  angular  movement  can  be  im- 
parted to  the  end  of  the  accessory  magnet  in  the  direction  of  the 
magnetic  meridian,  which  serves  to  keep  the  needle  on  the  zero  of 
the  scale.  The  deflections  of  the  needle  are  observed  by  means  of  an 
astronomic  telescope,  above  which  is  placed  a  scale  divided  into 
centimeters  and  millimeters,  and  distant  from  the  galvanometer  about 
six  or  eight  feet.  As  the  numbers  on  the  scale  are  reversed,  they  will 
be  seen  in  the  mirror  in  their  natural  position,  and  with  the  deflection 
of  the  needle  the  numbers  will  appear  as  if  drawn  across  the  mirror. 
The  extent  of  the  deflection  is  readily  determined  when  the  needle 
comes  to  rest. 

The  reflecting  galvanometer  of  Sir  William  Thompson  is  also 
used  for  the  same  purposes. 

The  Capillary  Electrometer. — Notwithstanding  the  extreme 
sensitiveness  of  the  modern  galvanometer,  it  has  been  found  desirable, 
in  the  investigation  of  many  physiologic  processes,  to  possess  some 
means  which  will  respond  even  more  promptly  to  slight  variations  'in 
electro-motive  force.  This  has  been  realized  in  the  construction  bv 
Lippmann  of  the  capillary  electrometer.  The  principle  of  this  appa- 
ratus rests  upon  the  fact  that  the  capillary  constant  or  the  surface- 
tension  of  mercury  undergoes  a  change  upon  the  passage  of  an  electric 
current,  in  consequence  of  a  polarization  by  hydrogen  taking  place 
at  its  surface.     If  a  capillary  glass  tube  be  filled  with  mercury  and 


74o 


TEXT-BOOK  OF  PHYSIOLOGY. 


its  lower  end  inserted  into  a  solution  of  sulphuric  acid,  and  the 
former  connected  with  the  positive  and  the  latter  with  the  nega- 
tive electrode,  it  will  be  observed,  upon  the  passage  of  the  current, 
that  a  definite  movement  of  the  mercury  takes  place,  in  the  direction 
of  the  negative  electrode,  in  consequence  of  the  diminution  of  its 
capillary  constant  or  the  tension  of  its  surface  in  contact  with  the  acid. 
As  a  reverse  movement  follows  a  cessation  of  the  current,  a  series  of 
oscillations  will  follow  a  rapid  making  and  breaking  of  the  current. 
If  the  direction  of  the  current  is  reversed,  the  capillary  constant  is 
increased  and  the  mercury  ascends  the  tube  toward  the  negative 
pole.     From  facts  such  as  these  Lippmann  constructed  the  capillary 

electrometer,  a  con- 
venient modification  of 
which  devised  by  M.  v. 
Frey,  is  shown  in  Fig. 
380.  This  consists  of  a 
glass  tube,  A,  forty 
millimeters  in  length, 
three  millimeters  in 
diameter,  the.  lower  end 
of  which  is  drawn  out  to 
a  fine  capillary  point. 
The  tube  is  filled  with 
mercury  and  its  capillary 
point  immersed  in  a  10 
per  cent,  solution  of  sul- 
phuric acid.  The  vessel 
containing  the  acid  is 
filled  to  the  extent  of 
several  millimeters  with 
mercury  also.  The  mer- 
cury in  the  tube  is  put 
in  connection  with  a 
platinum  wire  (a),  and 
the  acid  in  the  vessel 
with  a  second  wire  (b).  When  a  constant  current  passes  into  the 
apparatus  in  the  direction  from  b  to  a  the  mercury  is  pushed  up  the 
tube,  and,  upon  the  breaking  of  the  current,  it  may  or  may  not  return 
to  the  zero-point.  For  the  purpose  of  measuring  in  millimeters  of 
mercury  the  pressure  necessary  to  compensate  this  change  in  the 
capillary  constant  produced  by  the  electro-motive  force  of  polarization, 
the  apparatus  is  provided  with  a  pressure-vessel,  H,  and  a  manometer, 
B.  This  electrometer  can  be  applied  to  any  microscope  having  a 
reversible  stage.  The  oscillations  of  the  mercury  can  then  be  observed 
with  the  microscope  provided  with  an  ocular  micrometer  (Fig.  381). 
The  special  advantage  of  the  electrometer  is,  that  it  will  respond 
instantly  to  any  variation  in  the  electro-motive  force,  and  indicate  a 


Fig.-  380. — Von  Frey's  Capillary  Electrometer. 


PHYSIOLOGIC  APPARATUS. 


74i 


difference  of  potential,  according  to  Lippmann's  observation,  as 
slight  as  the  totts  o"  °f  a  Daniell.  These  rapid  oscillations  can  be  re- 
corded by  photographic  methods. 

In  using  either  the  galvanometer  or  the  electrometer  for  detecting 
the  existence  of  electric  currents  or  differences  of  potential  in  living 
tissues,  it  is  absolutely  essential  that  non-polarizable  electrodes  be 
employed  in  connection  with  it. 

DISSECTION  OF  THE  HIND-LEG  OF  THE  FROG. 


Much  of  our  knowledge  of  the  physiologic  properties  of  muscles 
and  nerves  has  been  derived  from  the  study  of  the  muscles  and  nerves 
of  the  cold-blooded  animals,  especially  of  the  frog,  for  the  reason  that 
in  these  animals  the  tissues  retain  their  vitality 
under  appropriate  conditions  for  a  considerable 
period  of  time  after  death  or  removal  from  the 
body.  The  muscles  generally  employed  for  ex- 
perimental purposes  are  the  gastrocnemius,  the 
sartorius,  the  semimembranosus,  the  gracilis,  and 
the  hyoglossus.  The  nerve  generally  employed 
is  the  sciatic.  Both  muscle  and  nerve  may  be 
studied  independently  of  each  other,  or  they  may 
be  studied  together,  as  when  in  their  usual  physi- 
ologic relation.  For  this  latter  purpose  the  gas- 
trocnemius muscle  and  sciatic  nerve  are  em- 
ployed, constituting  the  so-called  "nerve-muscle 
preparation." 

For  these,  and  many  other  reasons,  the  stu- 
dent should  familiarize  himself  with  the  general 
anatomy  of  the  frog,  and  especially  with  the 
anatomy  of  the  posterior  extremities. 

Preparation  of  the  Frog. — Destroy  the  frog 
by  plunging  a  pin  through  the  skin  and  soft 
tissues  covering  the  space  between  the  occipital 
bone  and  the  first  vertebra  until  the  point  is 
stopped  by  the  vertebra.  Turn  the  pin  toward 
the  head  and  push  it  into  the  brain  cavity; 
move  it  from  side  to  side  and  destroy  the  brain.  Pass  the  pin  into  the 
spinal  canal  and  destroy  the  spinal  cord.  With  a  stout  pair  of  scissors 
cut  off  the  body  behind  the  fore-limbs.  Remove  the  viscera  and  the 
abdominal  walls.  Draw  the  hind-legs  out  of  the  skin.  Place  the 
legs  on  a  glass  plate,  back  uppermost,  and  moisten  them  freely  with 
normal  saline  solution. 

Observe  on  the  outer  side  of  the  dorsal  surface  of  the  thigh  the  follow- 
ing muscles  (Figs.  382,  383).  The  triceps  femoris  (tr),  made  up  of  the 
rectus  anticus  (ra),  the  vastus  externus  (ve),  and  the  vastus  internus 
(vi),  not  seen  from  behind;  on  the  inner  side,  the  semimembranosus 


Fig.  381. — Capillary 
Electrometer.  R. 
Mercury  in  tube;  capil- 
lary tube.  5.  Sulphuric 
acid,  q.  Hg.  B.  Ob- 
server.    M.  Microscope. 


742 


TEXT-BOOK  OF  PHYSIOLOGY. 


(sm)  and  the  rectus  interims  minor  or  gracilis  (ri").  Between  these 
two  groups,  note  the  biceps  femoris  (b).  Above  the  thigh  observe 
the  gluteus  (gl),  the  ileococcygeus  (ci),  and  the  pyriformis  (p). 

In  the  leg  observe  the  gastrocnemius  (g)  with  its  tendon  (the  tendo 
Achillis),  the  tibialis  anticus  (ta),  and  the  peroneus  (pe). 

Turn  the  frog  on  its  back  and  note  the  muscles  on  the  ventral  surface 
of  the  thigh,  the  rectus  internus  major  (ri'),  and  minor  (ri"),  the  ad- 
ductor magnus  (ad"),  the  sartorius  (s),  the  adductor  longus  (ad'), 
and  the  vastus  internus  (vi).  In  the  leg,  in  addition  to  those  already 
seen  from  behind,  note  the  tibialis  posticus  (tp)  and  the  extensor 
cruris  (ec). 


ec 


Fig.  382. — Leg  Muscles  of  the  Frog. 
Ventral  Surface. — (Ecker.) 


Fig.  383. — Leg  Muscles  of  the  Frog. 
Dorsal  Surface. — (Ecker.) 


Note  in  the  abdominal  cavity  the  three  large  spinal  nerves,  the 
seventh,  eighth,  and  ninth. 

Dissection  of  the  Sciatic  Nerve. — The  sciatic  nerve  is  composed 
of  the  seventh,  eighth,  and  ninth  spinal  nerves.  After  its  emergence 
from  the  pelvic  cavity,  it  passes  down  the  thigh  between  the  semi- 
membranosus and  the  biceps  muscles,  in  company  with  the  femoral 
blood-vessels.  Below  the  knee  it  divides  into  the  tibialis  and  peroneus 
nerves;  the  former  sending  branches  into  the  gastrocnemius.  In  its 
course,  the  sciatic  sends  branches  to  the  muscles  of  the  entire  leg. 

Carefully  separate  the  biceps  and  semimembranosus  by  tearing 
the  connective  tissue  uniting  them.  The  sciatic  nerve  and  femoral 
blood-vessels  come  into  view;  with  a  bent  glass  rod  gently  separate  the 


PHYSIOLOGIC  APPARATUS.  743 

nerve  from  its  surroundings  from  the  knee  to  the  thigh.  Begin  at  the 
knee.  In  order  to  expose  the  nerve  at  the  pelvis,  it  will  be  necessary 
to  divide  the  pyriformis  and  the  ileo-coccygeus  muscles.  Care  must 
here  be  exercised,  so  as  not  to  injure  the  nerve  which  lies  immediately 
beneath.  Lift  up  the  uro-style  with  the  forceps  and  separate  it  from 
the  last  vertebra.  With  the  scissors  cut  off  the  vertebral  column  above 
the  seventh  vertebra.  Place  the  legs  on  the  dorsal  surface  and  then 
divide  the  seventh,  eighth,  and  ninth  vertebrae  lengthwise.  With  the 
forceps  lift  up  one  lateral  half  of  the  vertebras  and  free  the  nerve  as  far 
as  the  knee  by  dividing  connective  tissue  and  nerve  branches.  Be 
careful  not  to  injure  the  nerve  with  scissors  or  forceps. 

The  Nerve-Muscle  Preparation.— Divide  the  tendo  Achillis 
just  below  its  nbro-cartilaginous  thickening  at  the  heel,  and  detach  the 
gastrocnemius  up  to  the  knee.  Cut  through  the  tibio-fibular  bone 
just  below  the  knee-joint.  Cut  the  femur  transversely  near  its  middle 
and  remove  the  muscles  from  the  lower  end,  carefully  avoiding  injury 
to  the  nerve.  The  completed  preparation  consists  of  the  gastroc- 
nemius muscle,  the  sciatic  nerve,  with  half  of  the  seventh,  eighth, 
and  ninth  vertebrae  and  the  lower  half  of  the  femur. 


INDEX. 


Abducens  nerve,  597 
Aberration,  chromatic,  676 

spheric,  676 
Absorption,  213 

by  epithelium  of  villi,  224 
of  foods,  223 
of  fat,  227 
of  proteins,  226 
of  sugar,  225 
of  water,  225 
of  lymph,  222 
spectra  of  blood,  256 
Accommodation  of  the  eye,  667 

convergence  of  eyes  during,  672 
force  of,  671 
mechanism  of,  668 
range,  670 
Action  currents  of  muscles,  86 
of  nerves,  115 
reflex,  123 

of  medulla  oblongata,  ^32 
of  spinal  cord,  506 
Adrenal  bodies,  467 
Agraphia,  561 
Albuminoids,  17 
Albumins,  16 
Alcohol,  effects  of,  134 
Alimentary  canal,  146 
Allantois,  712 
Animo-acids,  14 
Amnion,  712 
Amylopsin,  197 
Amyloses,  8 
Animal  body,  structure  of,  3 

heat,  437 
Ankle  clonus,  511 

jerk,  511 
Aphasia,  560 
ataxic,  561 
amnesic,  561 
Apnea,  425 
Arterial  circulation,  326 

pressure,  341 
Arteries,  structure  and  properties  of,  326 
Articulate  speech,  635 
Asphyxia,  426 

Association  centers  of  cerebrum,  561 
Astigmatism,  675 
Auditory  area,  557 
nerve,  603 

Basal  ganglia,  525 
Bile,  201 

composition  of,  202 


Bile,  mode  of  secretion,  204 
physiologic  action,  205 
pigments,  203 
salts,  203 
Bilirubin,  203 
Biliverdin,  203 
Bioplasm,  30 

physiologic  properties,  4 
Blastodermic  membranes,  711 
Blind  spot,  678 
Blood,  236 

changes  in,  during  respiration,  409 
circulation  of,  271 
coagulation  of,  238 
chemistry  of,  267 
extravascular,  268 
intravascular,  269 
constituents  of,  236 
corpuscles,  242,  2.61,  265 
defibrinated,  240 
general  composition  of,  267 
physical  properties  of,  237 
plates,  265 
pressure,  339 
arterial,  341 
capillary,  345 
causes  of,  347 

determination  of,  in  man,  352 
methods  of  estimation,  344,  352 
variations  in,  34S 
venous,  346 
quantity  of,  266 
serum,  240 

velocity  of,  in  arteries,  358 
of,  in  capillaries,  360 
of,  in  veins,  361 
Burdach,  column  of,  466 

Calcium  salts  of  the  body,  22 
Calorimeter,  441 
Capillary  blood-vessels,  32 S 
functions  of,  329 

circulation,  367 

electrometer,  730 
Capsule,  internal.  525 

functions  of,  536 
Carbohydrates,  8 
Carbon  monoxid  hemoglobin,  269 
Cardiac  cycle,  285 
Cardio-accelerator  center,  320 

factors  which  determine  its  activity,  320 
Cardio-inhibitor  center,  321 

factors  which  determine  its  activity,  321 
Cardio-pulmonarv   vessels.    27, 


745 


746 


INDEX. 


Caseinogen,  iS,  451 
Caudate  nucleus,  525 
Cells,  structure  of,  26 

chemic  composition,  27 

manifestatations  of  life  by,  28 

reproduction  of,  31 
Central  organs  of  the  nerve  system,  491 
Cerebellar  tract,  405 
Cerebellum,  569 

functions  of,  571 

results  of  experimental  lesions,  572 
Cerebrum,  537 

convolutions  of,  539 

fissures  of,  537 

functions  of,  545 

localization  of  function  in,  547 

motor  area  of  the   chimpanzee   brain, 

554 

motor  area  of  the  human  brain,  558 
motor  area  of  the  monkey's  brain,  551 
sensor  areas  of  the  human  brain,  555 
sensor  areas  of  the  monkey's  brain,  549 
structure  of  the  gray  matter,  541 
structure  of  the  white  matter,  543 

Chemic  composition  of  the  body,  7 

Chimpanzee  brain,  motor  area  of,  554 

Cholesterin,  203 

Chorda  tympani  nerve,  161 

Chorion,  713 

Chromo-proteins,  19 

Chyle,  227 

Ciliary  movement,  94 
muscle,  625 

function  of,  669 

Circulation  of  blood,  271 

forces  concerned,  370 

Clark's  vesicular  column,  455 

Classification  of  food  principles,":^© 

Coagulated  proteins,  20 

Cochlea,  693 

functions  of,  699 

Colostrum,  453 

Commutator,  727 

Complemental  air,  404 

Conjugated  proteins,  18 

Connective  tissues,  35 

physical  and  physiologic  properties  of, 

4i 
Corpora  quadrigemina,  524 

functions  of,  533 
striata,  525 

functions  of,  534 
Corpus  luteum,  705 
Cranial  nerves,  577 
Crura  cerebri,  523 

functions  of,  533 

origins  of,  577 
Crystalline  lens,  658 

Daily  ration  of  U.  S.  soldier,  144 
Decidual  membrane,  710 
Defecation,  2 it 

nerve  mechanism  of,  211 
Deglutition,  163 

nerve  mechanism  of,  170 


Demarcation  current,  85 

Depressor  nerve,  323,  378,  380 

Dextrin,  9 

Dextroses,  9 

Diabetes,  460 

Diapedesis  of  leucocytes,  369 

Diaphragm,  389 

Dietaries,  143 

Diffusion,  230 

Digestion,  145 

Digestive  apparatus,  145 

Dilatator  pupillae  muscle,  652 

Direct  cerebellar  tract,  500 

pyramidal  tract,  498 
Ductless  glands,  462 
Ductus  arteriosus,  718 

venosus,  717 
Dyspnea,  425 

Electrodes,  non-polarizable,  724 
Electrotonic  alterations  in  excitability    of 
nerves,  117 

current,  116 
Electrotonus,  116 
Encephalo-spinal  fluid,  492 
Endocardium,  274 
Enterokinose,  200 
Epidermis,  487 
Epididymis,  706 
Epinephrin,  468 

Epithelial  tissues,  functions  of,  ^^,  34 
Equilibration,  mechanism  of,  574 
Erepsin,  199 

Erlanger's  sphygmomanometer,  356 
Erythrocytes,  242 
Eupnea,  424 

Eustachian  tube,  689,  698 
Excretion,  472 

Expiratory  forces  and  muscles,  398 
Expired  air,  composition  of,  407 
Eye,  cardinal  points  of,  660 

dioptric  apparatus  of,  658 

muscles  of,  683 

physiologic  anatomy  of,  650 

reduced,  663 

schematic,  662 

Facial  nerve,  598 

paralysis  of,  601 
Fallopian  tube,  702 
Fat,  12 

absorption  of,  227 

digestion  of,  199 

emulsification  of,  13 

saponification  of,  13 
Feces,  210 
Fecundation,  708 
Fchling's  solution,  9 
Fetal  circulation,  715 

membranes,  712 

structures,  712 
Fibrin,  20 
Fibrinogen,  241 
Fillet,  521 
Filtration,  234 


INDEX. 


747 


Follicle,  Graafian,  701 
Food,  127 

animal,  139 

cereal,  141 

composition  of,  139 

disposition  of,  130 

heat  value  of,  134 

principles,  130 

quantities  required  daily,  12S 

vegetable,  142 
Forces    aiding    the.   movement    of    lymph 

and  chyle,  228 
Fovea,  653,  656 

Galactose,  n 

Gall-bladder,  201 

Galvanic  current,  effect  of,  on  nerves,  116 

Galvanometer,  738 

Ganglia,  cephalic,  624 

Gaseous  exchange  in  lungs,  407 

in  tissues,  415 
Gases  of  blood,  relation  of,  410 

tension  of,  414 

carbon  dioxid,  413 

oxygen,  412 
Gastric  digestion,  171 
fistula?,  176 
glands,  174 
juice,  177 

mode  of  secretion,  178 

physiologic  action  of,  182 
Globulins,  17 

Glossopharyngeal  nerve,  605 
Glycogen,  10,  458 

Glycogenic  function  of  the  liver,  457 
Gluco-proteins,  19 
Gmelin's  test  for  bile  pigments,  204 
Goll,  columns  of,  501 
Gowers'  antero-lateral  tract,  500 
Graafian  follicle,  701 
Graphic  method,  733 
Green  vegetables,  142 

Hairs,  489 

Hearing,  sense  of,  689 

Heart,  271 

action  of  sympathetic  nerve  on,  3 1 2 ,  3 1 6 

of  vagus  nerve  on,  314,  317 
auriculo-ventricular  bundle,  280 
beat,  nature  of  the  stimulus,  302 

action  of  inorganic  salts,  303 

frequency  of,  285 

of  the  excised  heart,  296 
blood-supply,  294 
causation  of,  305 
causes  of  the  variations  of,  322 
course  of  blood  through,  276 
cycle  of,  285 
intracardiac  nerve-cells,  310 

pressure,  289 
intraventricular  pressure  curve,  290 
mechanics  of,  282 
modifications  of  beat  due  to  the  action 

of  drugs,  323 
muscle-band  of  His,  280 


Heart,  muscle-fibers  of,  279 
negative  pressure  ofr  292 
nerve,  mechanism  of,  308 
orifices  and  valves,  277,  278 
origin  and  distribution  of  the  sympa- 
thetic nerves  to,  310 
origin   and   distribution  of  the   vagus 

nerve  to,  311 
physiologic  anatomy  of,  271 
relative  function  of  auricles  and  ven- 
tricles, 288 
sounds,  293 

synchronism  of  the  two  sides,  288 
valves,  action  of,  286 
work  done  by,  371 
Heart-muscle,  properties  of,  296 
automaticity,  302 
conductivity,  297 
irritability,  296 
response  to  action  of  a  stimulus, 

306 
rhythmicity,  301 
tonicity,  301 
Heat  dissipation,  442 
income,  438 
relation  to  work,  444 
rigor,  70 
Helmholtz's  theory  of  color  perception,  685 
Hematin,  260 
Hemianopsia,  584 
Hemoglobin,  252 

absorption  spectra,  256 
chemic  composition  of,  253 
compounds  of,  258 
quantity  of,  254 
Hemoglobinometer,  Gowers',  255 
Hemometer,  v.  Fleischl's,  256 
Hering's  theory  of  color  perception,  686 
Histons,  16 
Horopter,  681 
Hypermetropia,  674 
Hyperpnea,  424 
Hypoglossal  nerve,  615 

Incus,  691 

Induced  currents,  730,  731 
Inductorium,  729 
Infra-proteins,  20 
Insalivation,  151 

nerve  mechanism  of,  155 
Inspiration,  395 

movements  of  thorax,  391 

muscles,  395 
Insula,  541 

Intercostal  muscles,  389. 
Internal  capsule,  525 

functions  of,  536 

secretion,  462 
Intestinal  digestion,  191 

fermentation,  210 

juice,  193 

physiologic  action  of,  200 

movements,  206 

nerve  mechanism  of,  20S 
Intracardiac  presssure,  289 


748 


INDEX. 


Intracranial  circulation,  563 

mechanism  of,  564 
Intrapulmonary  pressure,  392 
Intrathoracic  pressure,  392 
Intravascular  coagulation,  269 
Invertin,  200 
Iris,  652 

functions  of,  672 

nerve  mechanism  of,  588,  673 
Iron  of  the  body,  24,  254 
Irritability  of  muscles,  59 

of  nerves,  109 
Island  of  Langerhans,  195 

of  Reil,  541 
Isometric  myogram,  72 
Isotonic  myogram,  67 
Isthmus  of  encephalan,  521 
functions  of,  527 

Jacobsen's  nerve,  605 
Joints,  49 

classification  of,  49 

Kidney,  476 

histology  of,  477 
Knee-jerk,  475 
Kymograph,  734,  735 

Labyrinth  of  ear,  691 
Lacrimal  glands,  687 
Lactation,  718 
Lacteals,  227 
Lactose,  n 
Language,  559 
Large  intestine,  208 
Larynx,  626 

nerve  mechanism  of,  635 

structure  of,  627 
Lateral  columns  of  the  spinal  cord,  499 
Law  of  contraction,  119 
Lecithin,  204 
Lemniscus,  521 
Lens,  crystalline,  658 
Lenticular  nucleus,  526 
Leukocytes,  261 

chemic  composition  of,  262 

classification  of,  263 

functions  of,  265 

number  of,  262 

origin  of,  265 

physiologic  properties,  263 
Levers,  88 
Levulose,  10 
Limbic  lobe;  506 
Liver,  201,  483 

formation  of  urea  in,  462 

functions  of,  455 

influence  of  the  nerve  system  on,  459 

production  of  glycogen,  457 

secretion  of  bile,  456 
Localization  of  functions  in  cerebrum,  547 
Lungs,  structure  of  the,  384 
Lymph,  218 

absorption  of,  222 

1  omposition  of,  219 


Lymph,  functions  of,  221 

movement  of,  228 

physical  properties,  219 

production  of,  220 
Lvmph  capillaries,  214 
Lymph-glands,  215 
Lymph-vessels,  214 
Lymphocytes,  219,  264 

Macula  lutea,  653 
Malleus,  691 
Maltose,  n 
Mammary  gland,  449 
Mastication,  147 
muscles  of,  149 
nerve  mechanism  of,  150 
Meats,  composition  of,  139 
Medulla  oblongata,  519 

reflex  activities  of,  532 
Meibomian  glands,  687 
Membrana  tympani,  690 

functions  of,  697 
Menstruation,  704 
Metabolism  on  protein  diet,  138 

on  fat  and  carbohydrate  diet,  139 
Methemoglobin,  260 
Migration  of  leukocytes,  264,  369 
Milk,  451 

composition  of,  139,  452 
mechanism  of  secretion,  452 
Moist  chamber,  736 
Mosso's  plethysmograph,  366 

spygmomanometer,  353 
Motor  area  of  chimpanzee  brain,  554 
of  human  brain,  558 
of  monkey  brain,  549 
oculi  nerve,  585 
Mouth  digestion,  147 
Movements  of  the  eyeball,  6S2 
of  the  intestines,  206 
of  the  lower  jaw,  149 
of  the  lungs,  400 
of  the  stomach,  186 
Muscle  action  currents,  86 
contraction,  65 

chemic  phenomena  of,  80 
electric  phenomena  of,  83 
graphic  record  of,  66 
modifying  influences  of,  68 
physical  phenomena  of,  63 
rigor  mortis,  89 
summation  effect,  74 
tetanus,  75 

thermic  phenomena  of,  82 
electric  currents  from,  85 
electric    currents,     negative    variation 

of,  85 
energy,  source  of,  81 
fatigue,  70 

groups,  special  action  of,  '87 
sense,  644 
sound,  80 
spindle,  645 
stimuli,  60 
tissue,  53 


INDEX. 


749 


Muscle  tissue,  chemic  composition  of,  56 

elasticity,  58,  64 

histology  of,  54,  91 

irritability,  59 

physical  properties  of,  57 

physiologic  properties  of,  60 

tonicity,  59 
Myopia,  673 
Myosinogen,  17,  57 
Myxedema,  427 

Nerve,  abducens,  597 

auditory,  603 

facial,  598 

glossopharyngeal,  605 

hypoglossal,  615 

irritability,  109 

motor  oculi,  585 

olfactory,  579 

optic,  582 

patheticus,  591 

pneumogastric,  606 ' 

spinal  accessory,  612 

stimuli,  no 

tissue,  histology  of,  96 

trigeminal,  592 
Nerve  impulse,  no 
Nerve-muscle  preparation,  112,  743 
Nerve  system,  functions  of,  493 
Nerve  tissue,  96 

histology  of,  96 
Nerves,    chemic    composition   and   metab- 
olism of,  101 

classification  of,  107 

degeneration  of,  105 

development  of,  104 

effects  of  galvanic  current  on,  116 

electric  currents  of,  114 

electric  currents  of,  negative  varia- 
tion of,  113 

electric  excitation  of,  112 

electric  phenomena  of,  113 
action  currents,  115 
diphasic  action  currents,  115 

peripheral  endings  of,  103 

physiologic  properties  of,  109 

pilo-motor,  108 

polar  stimulation  of,  119,  121 

relation  of,  to  central  nerve  system, 
102 

stimuli  of,  109 
Neuron,  96 

Nicotin,  actions  of,  324 
Nucleo-proteins,  19 
Nucleus  caudatus,  525 

cuneatus,  501 

gracilis,  501 

lenticularis,  526 
Nutrition  of  the  embryo,  713 

Oculo-motor  nerve,  585 
Ohm's  law,  722 
Olein,  13 

Olfactory  nerve,  579 
Oncograph,  483 


Oncometer,  483 
Ophthalmic  ganglion,  624 
Optic  constants,  659 

thalamus,  527 

functions  of,  535 
Optic  nerve,  582 
Optogram,  680 
Organ  of  Corti,  694 
Osazones,  12 
Osmometer,  231 
Osmosis,  230 
Osmotic  pressure,  231 
Ossicles  of  ear,  691 
Otic  ganglion,  625 
Ovary,  701 
Ovulation,  703 
Ovum,  702 
Oxygen  in  blood,  412 

in  tissues,  415 

quantity  absorbed  daily,  422 
Oxyhemoglobin,  258 

Pacinian  corpuscle,  640 
Palmitin,  13 
Pancreas,  194 
Pancreatic  juice,  196 

mode  of  secretion,  196 

physiologic  action  of,  197 
Parathyroids,  465 
Partial  pressure  of  gases,  411 
Parturition,  717 
Pathetic  nerve,  591 
Pepsin,  178 
Peptones,  184 
Perspiration,  486 
Peripheral  organs  of  the  nerve  system, 

100,  491 
Petrosal  nerves,  600 
Pettenkofer-Voit  respiration  apparatus, 

420 
Pexin,  178 
Phagocytosis,  265 
Phloridzin  diabetes,  461 
Phonation,  626 

mechanism  of,  632 
Phospho-proteins,  18 
Physiology  of  the  cell,  26 

of  movement,  43 
Pilo-motor  nerves,  454 
Pituitary  body,  466 
Placenta,  714 

Plasma  of  blood,  composition  of    240 
Pleura,  390 
Pneumatograph,  405 
Pneumogastric  nerve,  606 
Pneumograph,  403 
Polar  stimulation,  119 

of  human  nerves,  121 
Pons  varolii,  522 

functions  of,  523 
Portal  vein,  227 
Postures,  89 
Presbyopia,  673 
Prosecretin,  197 
Protamins,  16 


75° 


INDEX. 


Proteins,  14 

cheraic  composition,  14 

color  reactions,  21 

physical  properties,  15 

precipitation  tests,  21 

structure  of,  14 
Ptyalin,  159 
Pulmonary  vascular  apparatus,  370 

ventilation,  409 
Pulse,  362 

frequency,  363 

wave,  velocity  of,  364 
Punctum  proximuni,  671 

remotum,  671 
Pyramidal  tracts  of  spinal  cord,  498,  501 

Reaction  of  degeneration,  124 
Red  corpuscles,  242 

chemic  composition  of,  252 
effects  of  reagents,  248 
function  of,  251 
life  history  of,  251 
number  of,  246 
of  vertebrated  animals,  249 
Reduced  hemoglobin,  258 
Reflex  action,  125,  506 

laws  of,  509 
Refractory  period  of  the  heart,  307 
Regnault's  and  Reisset's     respiration     ap- 
paratus, 421 
Relation  of  gases  in  the  blood,  410 
Rennin,  178 
Reproduction,  701 

Reproductive  organs  of  the  female,  701 
Reproductive  organs  of  the  male,  706 
Reserve  air,  404  . 
Residual  air,  404 
Respiration,  382 

changes  in  composition  of  air  during, 

406 
changes  in  composition  of  blood,  409 
changes  in  tissues,  415 
chemistry  of,  406 

expiratory  forces  and  muscles,  398 
frequency  of,  402 

mechanism  of  gaseous  exchange,  417 
nerve  mechanism  of,  427 
Respiration,  total  respiratory  exchange,  419 

volumes  of  air  breathed,  403 
Respiratory  apparatus,  382 
movements,  395 
muscles,  395 

effects  of,  on  arterial  pressure, 

434 
effects  of,  on   the    flow   of   blood 
through     the     thoracic     vessel, 

434 
of  upper  air  passages,  401 
I-!'   .-.ures,  392 
quotient,  408,  422 
rhythm,  402 

modifii  ation  of,  429 
Chcyne-Stokcs,  427 
sounds,  405 
i>  pes,  402 


Retina,  653 

functions  of,  677 
Retinal  image,  658 

size  of,  664 
Rheocord,  726 
Rigor  mortis,  80 
Rima  glottidis,  626 

respiratoria,  632 

vocalis,  632 
Routes  of  the  absorbed  food,  227 

Saccharose,  n 
Saliva,  154 

physiologic  action  of,  158 
Salivary  glands,  152 

histologic  changes  in,  during  secre- 
tion, 157 

nerve  mechanism  of,  160 
Sebaceous  glands,  489 
Sclero-proteins,  17 
Sebum,  489 
Secretin,  197 
Secretion,  446 

internal,  462 
Semen,  707 

Semicircular  canals,  575 
Sensor  areas  of  human  brain,  555 

of  monkey  brain,  549 
Serum,  240 

Setchenow's  center,  477 
Sight,  sense  of,  650 
Skeleton,  physiology  of,  48 
Skin,  486 

nerve  endings  in,  603 
reflexes,  509 
Sleep,  565 
Smell,  sense  of,  648 
Sodium  glycocholate,  203 
Sodium  taurocholate,  203 
Spectroscope,  115 
Speech,  635 
Spermatozoa,  708 
Spheno-palatine  ganglion,  624 
Sphygmograph,  364 
Sphygmomanometer,  355,  356 
Spinal  accessory  nerve,  612 
cord,  494 

encephalo-spinal  conduction,  515 

functions  of,  503 

as  a  conductor,  513 

as    an    independent     center, 

5°4 
nerve-cells,  classification  of,  497 
nerve  fibres  of,  498 

classification  of,  463,  498 
reflex  actions  of,  506 
reflex  irritability  of,  511 
relation  of  spinal  nerves  to,  501 
segmentation  of,  503 
spinal    nerve  roots,  functions  of, 

502 
spino-encephalic  conduction,  514 
structure  of  gray  matter,  495 
structure  of  white  matter,  498 
tracts  of,  499 


INDEX. 


75i 


Spirometer,  404 
Splanchnic  nerves,  622 
Spleen,  468 

functions  of,  469 
Stanton's  sphygmomanometer,  355 
Stapes,  691 
Starch,  8 

digestion  of,  158 
Starvation,  136 
Stearin,  12 

Stereognostic  area,  557 
Stomach,  171 

movements  of,  186 
nerve  mechanism  of,  189 
Suprarenal  capsules,  467 
Sweat-glands,  487 

Sweat,  influence  of  nerve  system  on  pro- 
duction of,  488 
Sympathetic  nerve  system,  616 

cephalic  ganglia  of,  624 
functions  of  the  cervical  por- 
tions, 621 
functions  of  the  lumbosacral 

portions,  623 
functions  of  the  thoracic  por- 
tion, 622 

Taste  buds,  647 

nerve  of,  646 

sense  of,  646 
Teeth,  147 
Tegmentum,  488 
Temperature  of  body,  437 

regulation  of,  443 

sense,  643 
Tendon  reflexes,  510 
Tension  of  gases  in  blood,  414 

tissues,  417 
Tensor  tympani  muscle,  691 

functions  of,  697 
Testicles,  706 
Tetanus,  75 

experimental,  79 

pathologic,  79 

pharmacologic,  79 

physiologic,  78 
Thoracic  duct,  217 
Thorax,  388 

dynamic  condition  of,  394 

mechanic  movements  of,  391 

static  condition  of,  392 
Thyroid  gland,  463 

functions  of,  463 
Tidal  air,  404 
Tissue  spaces,  213 
Tongue,  646 
Total  carbon-dioxid  exhaled,  422 

oxygen  absorbed,  422 

respiratory  exchange,  410 
Touch,  sense  of,  639 
Trachea,  384 
Tracts  of  spinal  cord,  465 
Traube-Hering  waves,  435 
Trigeminal  nerve,  592 
Trypsin,  198 


Ttirck,  column  of,  49S 
Tympanum,  689 

Umbilical  cord,  713 
Upper  air-passages,  respiratory  move- 
ments of,  401 
Urea,  473 

seat  of  formation,  462,  474 
Uric  acid,  474 
Urine,  472 

composition  of,  473 
mechanism  of  secretion,  479 

influence  of  blood  composition, 

484 
influence  of  nerve  system,  483 
relation  of  blood-pressure  to,  481 
Urination,  484 

nerve  mechanism  of,  485 
Uterus,  702 

Vagus  nerve,  606 

influence  on  heart,  314,  317 
Valves  of  heart,  2 78 
Vasa  deferentia,  706 
Vascular  apparatus,  326 

glands,  462 

hydrodynamic  considerations,  330, 

333 

stream  bed,  336 

nerve  mechanism  of,  372 
Vaso-motor  center,  376 

direct  stimulation,  377 

nerves,  373 

reflex  stimulation,  378 
Veins,  313 

structure  and  function,  329 
Velocity  of  blood,  357,  359 
Venous  circulation,  369 
Vertebral  column,  51 
Vesiculae  seminales,  706 
Villi,  223 

functions  of,  224 
Visceral  muscle,  90 

functions  of,  93 

properties  of,  91 
Vision,  650 

accommodation,  667 

astigmatism,  675 

binocular,  6S0 

color  perception,  6S4 

functions  of  retina,  677 

hypermetropia,  674 

myopia,  673 

presbyopia,  673 
Visual  angle,  664 
Vital  capacity  of  lungs,  404 
Vocal  bands,  630 

sounds,  633 
Voice  and  speech,  635 
Volume  pulse,  366 

Walking,  90 

Wallerian  degeneration,  106 
Water,  amount  of,  in  the  body,  22 
Watery  vapor  in  breath,  40S 


752  INDEX. 

Wernicke's  pupillary  reaction,  590  Yellow  spot,  653 

White  blood-corpuscles,  261 

function  of,  265  Zona  pellucida,  710 

migration  of,  369  Zymogen,  178 
origin  of,  265  pepsinogen,  159,  173 

varieties  of,  263  ptyalogen,  159 

Wrisberg,  nerve  of,  600  trypsinogen,  200 


QP3+  3  83 


